In-flight upset, Northwest Airlines, Inc., Boeing 727-251

In-flight upset, Northwest Airlines, Inc., Boeing 727-251

In-flight upset, Northwest Airlines, Inc., Boeing 727-251, N274US, Near
Thiells, New York, December 1, 1974
Micro-summary: Following a high-altitude stall, this Boeing 727 crashed.
Event Date: 1974-12-01 at 1926 EST
Investigative Body: National Transportation Safety Board (NTSB), USA
Investigative Body's Web Site: http://www.ntsb.gov/
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TRANSPORTATION
BOARD
WASHINGTON, D.C. 20594
IDENT REPORT
i
N~ORTHWEST
AIRLINES, INC'.,
,
,
BOEING 727-251, ~ 2 7 4 u . s ;
NEAR THIELLS, NEW YORK~:
'
DECEMBER'I,
1974
UNITED STATES GOVERNMENT
^&
TABLE OF CONTENTS
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Synopsis
Investigation
History of the Flight
Injuries to Persons
Damage to Aircraft
Other Damage
Crew Information
Aircraft Information
Meteorological Information
Aids to Navigation
Communications
Aerodrome and Ground Facilities
Flight Recorders
Aircraft Wreckage
Medical and Pathological Information
Fire
Survival Aspects
Tests and Research
Pitot Head Examination and Icing ~ests
Aircraft Performance Analysis
Other Information
Pretakeoff Checklist
Airspeed Measuring System
B-727 Stall Characteristics
Analysis and Conclusions
Analysis
Conclusions
(a) Findings
(b) Probable Cause
Recommendations
Appendixes
Appendix
Appendix
Appendix
Appendix
A
B
C
D
.Investigation and Hearing . . . .
.Crew Information . . . . . . . . .
.Aircraft Information . . . . . . .
.Recommendations A-75 25-27 to FAA
F i l e No. 1-0031
NATIONAL TRANSPORTATION SAFETY BOARD
WASHINGTON, D .C
20594
.
AIRCRAFT ACCIDENT REPORT
Adopted August 13, 1975
NORTHWEST AIRLINES, INC.
BOEING 727-251, N274US
NEAR THIELLS, NEW YORK
DECEMBER 1, 1974
SYNOPSIS
About 1926 e . s . t . o n December 1, 1974, Northwest Airlines F l i g h t
6231, a Boeing 727-251, crashed about 3.2 mi west of T h i e l l s , New York.
F l i g h t 6231 was a f e r r y f l i g h t t o Buffalo, New York. The accident occurred about 12 minutes a f t e r t h e f l i g h t had departed John F. Kennedy I n t e r national Airport, Jamaica, New York. Three crewmembers, the only persons
aboard the a i r c r a f t , died i n the crash. The a i r c r a f t was destroyed.
and entered an uncontrolThe a i r c r a f t s t a l l e d a t 24,800 f e e t m.s.1.
l e d , s p i r a l l i n g descent t o t h e ground. Throughout t h e s t a l l and descent
the flightcrew did not recognize the a c t u a l condition of the a i r c r a f t and
did not take the c o r r e c t measures necessary t o r e t u r n the a i r c r a f t t o
l e v e l f l i g h t . Near 3,500 f e e t m . s . l . ,
a large portion of t h e l e f t horizontal s t a b i l i z e r separated from the a i r c r a f t , which made c o n t r o l of the
a i r c r a f t impossible.
The National Transportation Safety Board determines t h a t the probable
cause of t h i s accident was the l o s s of c o n t r o l of t h e a i r c r a f t . because the
flightcrew f a i l e d t o recognize and c o r r e c t t h e a i r c r a f t ' s high-angle-ofa t t a c k , low-speed s t a l l and i t s descending s p i r a l . The s t a l l was precipitated by t h e flightcrew's improper r e a c t i o n t o erroneous airspeed and
Mach indications which had resulted from a blockage of t h e p i t o t heads by
atmospheric icing. Contrary t o standard operational procedures, the
flightcrew had not activated t h e p i t o t head heaters.
1. INVESTIGATION
1.1 History of F l i g h t
On December 1, 1974, Northwest A i r l i n e s , Inc., F l i g h t 6231, a Boeing
727-251, N274US, was a f e r r y f l i g h t from John F. Kennedy I n t e r n a t i o n a l
Airport (JFK) , Jamaica, New York, t o Buffalo, New York. Three crewmembers were the only persons aboard the a i r c r a f t .
F l i g h t 6 2 3 1 d e p a r t e d J F K a b o u t 1 9 1 4 I / o n a standard instrument departure.
Af t e r takeoff, Kennedy departure c o n t r o l cleared the f l i g h t t o climb t o
14,000 f e e t . 21 A t 1920:21, New York a i r route t r a f f i c c o n t r o l center
(ZNY) assumedradar c o n t r o l of the f l i g h t , and a t 1921:07, ZNY cleared
the f l i g h t t o climb t o f l i g h t l e v e l 310. 31
F l i g h t 6231 proceeded without reported d i f f i c u l t y u n t i l 1924:42,
" on ZNY frequency.
when a crewmember transmitted, "Mayday, mayday
go ahead," and che crewmember s a i d ,
The ZNY c o n t r o l l e r responded,
"Roger, we're out of c o n t r o l , descending through 20,000 feet.''
"...
...
After giving interim a l t i t u d e clearances, a t 1925:21, t h e ZNY cont r o l l e r asked F l i g h t 6231 what t h e i r problemwas, and a crewmember responded, "We're descending through 12, we're i n a s t a l l . " The sound of
an a c t i v e radio transmitter was recorded a t 1925:38. There were no
f u r t h e r transmissions from F l i g h t 6231.
A t 1925:57, F l i g h t 6231 crashed i n a f o r e s t i n the Harriman S t a t e
Park, about 3.2 n m i west of T h i e l l s , New York. No one witnessed t h e crash.
The accident occurred during hours of darkness.
The geographic coordinates of the accident s i t e a r e 41Â 12' 53" N.
l a t i t u d e and 74O 5 ' 40" W. longitude.
1.2
1.4
I n j u r i e s t o Persons
Injuries
Crew
-
Fatal
Nonfatal
None
3
0
0
Passengers
Other
-
Damage t o A i r c r a f t
The a i r c r a f t was destroyed.
1.4
Other Damage
Trees and bushes were e i t h e r damaged or destroyed.
1.5
Crew Information
The crewmenbers were qualified and c e r t i f i c a t e d f o r the f l i g h t . The
t h r e e crewmembers had off-duty periods of 15 hours 31 minutes during the
24-hour period preceding t h e f l i g h t . (See Appendix B.)
16
7.1
-'31
A l l times herein a r e e a s t e r n standard, based on the 24-hour clock.
A l l a l t i t u d e s herein a r e mean sea l e v e l , unless otherwise indicated.
An a l t i t u d e of 31,000 f e e t which i s maintained with an altimeter
s e t t i n g of 29.92 inches.
I n October 1974, the f i r s t o f f i c e r advanced from second o f f i c e r i n
B-707 a i r c r a f t t o f i r s t o f f i c e r i n B-727 a i r c r a f t ; he had flown about 46
hours i n the l a t t e r capacity.
1.6
A i r c r a f t Information
N274US was owned and operated by Northwest A i r l i n e s , Inc. It was
c e r t i f i c a t e d and maintained i n accordance with Federal Aviation Administ r a t i o n (FAA) regulations and requirements.
(See Appendix C.)
N274US was loaded with 48,500 lbs. of J e t A f u e l . The gross weight
a t takeoff was about147,OOO lbs. The weight and center of g r a v i t y (c.g.)
were within prescribed l i m i t s . The a i r c r a f t was i n compliance with a l l
pertinent airworthiness d i r e c t i v e s .
I n t h e Boeing 727 a i r c r a f t , t h e p i t o t - s t a t i c instruments on t h e capt a i n ' s panel, the p i t o t - s t a t i c instruments on the f i r s t o f f i c e r ' s panel,
and t h e p i t o t - s t a t i c instrumentation i n the f l i g h t data recorder (FDR) a r e
connected t o separate p i t o t and s t a t i c sources. The t h r e e p i t o t systems
have no common elements and a r e completely independent. The t h r e e s t a t i c
systems a r e a l s o independent except f o r manual s e l e c t o r valves i n both t h e
captain's and f i r s t o f f i c e r ' s systems which provide f o r s e l e c t i o n of t h e
FDR s t a t i c system a s an a l t e r n a t e pressure source i f e i t h e r primary source
malfunctions.
The f i r s t o f f i c e r ' s p i t o t and s t a t i c systems a r e connected t o a Mach
airspeed warning switch. The switch a c t i v a t e s a warning horn when i t
senses a d i f f e r e n t i a l pressure which i n d i c a t e s t h a t the a i r c r a f t 's speed is exceeding Vmo o r &, ?/ depending on the a i r c r a f t ' s
a l t i t u d e . A redundant Mach airspeed warning system i s incorporated i n
the FDR p i t o t and s t a t i c systems.
The p i t o t head f o r the captain's p i t o t system i s located on t h e l e f t
s i d e of t h e a i r c r a f t ' s fuselage; the p i t o t heads f o r the f i r s t o f f i c e r ' s
system .and t h e FDR system a r e located on t h e r i g h t s i d e of t h e fuselage.
Each of these heads incorporates a heating element and a small d r a i n hole,
f o r exhausting moisture, a f t of the t o t a l pressure sensing i n l e t . The
three s t a t i c systems each have a s t a t i c port located on e i t h e r s i d e of the
fuselage. The l e f t s t a t i c port i s connected t o t h e r i g h t s t a t i c port t o
o f f s e t s i d e s l i p e f f e c t s by balancing t h e pressures within the systems.
Each of t h e p o r t s is equipped with a heating element.
I n addition t o t h e above systems, two independent p i t o t - s t a t i c
systems a r e connected t o a mechanism i n t h e a i r c r a f t ' s longitudinal cont r o l system. The f o r c e which the p i l o t must exert t o move t h e elevator
control surfaces v a r i e s a s a function of the dynamic pressure measured
by these systems. The two p i t o t heads f o r these systems a r e mounted one
on each s i d e of the v e r t i c a l s t a b i l i z e r , and t h e i r design i s s i m i l a r t o
the other p i t o t heads.
-4 1
Maximum operating l i m i t speed or maximum operating l i m i t Mach.
1.7
Meteorological Information
Northwest Airlines' meteorology department supplied the weather information for Flight 6231. This information included a synopsis of surface conditions, terminal forecasts, a tropopause and wind forecast for
the 300-millibar level, appropriate surface observations, and turbulence
plots. For the period 1700 to 2300, Northwest meteorologists forecasted
moderate to heavy snowshowers from Lake Michigan to the Appalachian
Mountains and moderate to heavy rainshowers and scattered thunderstorms
east of the Appalachians.
Northwest's turbulence plot (TP) No. East 2 was in effect and available to the flightcrew on the day of the accident. TP East 2 was a triangular area defined by lines connecting Pittsburgh, Pennsylvania, New
York City, New York, and Richmond, Virginia. Thunderstorm cells with
maximum tops to 28,000 feet were located in this area.
, S1GMF.T 51 Delta 2, issued at 1755 and valid 1755 to 2200, predicted
frequent moderate icing in clouds, locally severe in precipitation above
the freezing level, which was at the surface in southwestern New York and
which sloped to 6,000 feet eastward to the Atlantic coast.
The surface weather observations at Newburgh, New York, about 17
miles north of the accident site, were:
- Estimated
ceiling -- 2,500 feet broken, 5,000 feet overcast, visibility -- 12 miles, temperature -- 34"F., dew
point -- 229.,wind-- 070Â at 14 kn, gusts -- 24 kn,
altimeter setting -- 29.98 in.
2000 - Similar conditions to those reported at 1900 except
that very light ice pellets were falling.
1900
Another Northwest flight was on a similar route behind Flight 6231.
The captain of that flight stated that he encountered icing and light
turbulence in his climb. He was in instrument conditions from 1,500 feet
to 23,000 feet, except for a few minutes between cloud layers at an
intermediate altitude.
1.8 Aids to Navigation
There were no problems with navigational aids.
1.9
Communications
There were no problems with air-to-ground communications.
-5/
A S1GMF.T is an advisory of weather severe enough to be potentially
hazardous to all aircraft. It is broadcast on navigational and voice
frequencies and by flight service stations. It is also transmitted on
Service-A weather teletype circuits.
1.10
Aerodrome and Ground F a c i l i t i e s
Not applicable.
1.11 F l i g h t Recorders
N274US was equipped with a Fairchild Model 5424 f l i g h t d a t a recorder
(FDR), s e r i a l No. 5146, and a Fairchild A-100 cockpit voice recorder
(CVR), s e r i a l No. 1640. Both recorders sustained s u p e r f i c i a l mechanical
damage, but the recording tapes were i n t a c t and undamaged. A l l of t h e
FDR t r a c e s and the CVR channels were c l e a r l y recorded.
The readout of t h e FDR t r a c e s involved 11 minutes 54.6 seconds of
f l i g h t , beginning 15seconds before l i f t o f f .
Pertinent portions of the CVR tape were transcribed, beginning with
the flightcrew's execution of the pretakeoff c h e c k l i s t and ending with the
sounds of impact. The following t r a n s c r i p t was made of the flightcrew's
a c t i v i t i e s between 1906:36 and 1906:51:
F i r s t Officer:
Zero, zero and thirty-one, f i f t e e n , f i f t e e n
Second Officer:
Bug.
Second Officer:
P i t o t heat.
F i r s t Officer:
Captain:
.... blue.
Off and on.
One forty-two is t h e bug.
... do you want
F i r s t Officer:
Or
F i r s t Officer:
Huh!
the engine heat on?
Sound of f i v e c l i c k s .
Air-to-ground communications, cockpit conversations, and other sounds
recorded on t h e CVR were c o r r e l a t e d t o the FDR a l t i t u d e , airspeed, heading, and v e r t i c a l acceleration t r a c e s by matching t h e r a d i o transmission
time indications on both the CVR and FDR.
The FDR t o CVR c o r r e l a t i o n showed t h a t a f t e r .takeoff, the a i r c r a f t
climbed t o 13,500 f e e t and remained a t that a l t i t u d e f o r about 50 seconds,
during which time t h e airspeed 51 increased from 264 kn t o 304 kn. During
t h a t 50 seconds, t h e airspeed t r a c e showed two aberrations i n a 27-second
period; each aberration was characterized by a sudden reduction i n airspeed.
These reductions were 40 kn and 140 kn and l a s t e d f o r 7 and 5 seconds,
respectively.
-61
A l l airspeeds a r e indicated airspeeds, unless otherwise noted.
The a i r c r a f t then began t o climb 2,500 f e e t per minute while maintaining an airspeed of about 305 kn. A s t h e a l t i t u d e increased above
16,000 f e e t , the recorded airspeed began t o increase. Subsequently, both
the r a t e of climb and t h e r a t e of change i n airspeed increased. About
t h i s same time, the f i r s t o f f i c e r commented, "Do you r e a l i z e we're going
340 kn and I ' m climbing 5,000 f e e t a minute?"
The flightcrew discussed t h e implications of t h e high airspeed and
high r a t e of climb. The second o f f i c e r commented, "That's because we're
l i g h t , " a f t e r which t h e c a p t a i n s a i d , "It gives up real f a s t , " and "I
wish I had my shoulder harness on, it's going t o give up p r e t t y soon."
The r a t e of climb eventually exceeded 6,500 f e e t per minute.
The sound of an overspeed warning horn was recorded a s t h e a l t i t u d e
reached 23,000 f e e t . A t t h a t time, t h e recorded airspeed was 405 Un and
t h e following conversation took place:
Captain:
"Would you believe t h a t #.It
F i r s t Officer:
Captain:
"I believe i t , I j u s t can't do anything about it."
"No, j u s t p u l l her back, l e t her climb."
This l a s t comment was followed by t h e sound of a second overspeed warning
horn.
The sound of t h e s t a l l warning s t i c k shaker was recorded i n t e r m i t t e n t l y l e s s than 10 seconds a f t e r t h e onset of t h e overspeed warning. Five
seconds l a t e r , v e r t i c a l a c c e l e r a t i o n reduced t o 0.8g, and t h e a l t i t u d e
leveled a t 24,800 f e e t . The recorded airspeed was 420 kn.
The s t a l l warning began again and continued while the f i r s t o f f i c e r
commented, "There's t h a t Mach b u f f e t , 7 1 guess we'll have t o p u l l i t up."
followed by the captain's comnand, " ~ u i li t up," and t h e sound of t h e
landing gear warning horn. The FDR readout shows t h e following:
Two seconds l a t e r (about 13 seconds a f t e r t h e a i r c r a f t arrived
a t 24,800 f e e t ) , t h e v e r t i c a l acceleration t r a c e again declined t o
0.8g and the a l t i t u d e t r a c e began todescend a t a r a t e of 15,000
f e e t per minute. The airspeed t r a c e decreased simultaneously at a
r a t e of 4 kn per second and t h e magnetic heading t r a c e changed from
290Â t o 080Â within 10 seconds, which indicated t h a t the a i r c r a f t
was turning rapidly t o the r i g h t .
-7 1
A s l i g h t b u f f e t t h a t occurs when-an a i r c r a f t exceeds i t s c r i t i c a l
Mach number. The b u f f e t is caused by the formation of a shock wave
on t h e a i r f o i l surfaces and a separation of airflow a f t of t h e shock
wave. The change from laminar flow t o turbulent flow a f t of t h e
shock wave causes a high frequency v i b r a t i o n i n t h e c o n t r o l surfaces
which is described a s "buffet" or "buzz."
A s the a i r c r a f t continued t o descend, t h e v e r t i c a l acceleration
t r a c e increased t o 1.5g. The a i r c r a f t ' s magnetic heading t r a c e
fluctuated, but moved b a s i c a l l y t o t h e r i g h t . About 10 seconds
a f t e r the descent began, the "Mayday" was transmitted.
Thirty-three seconds l a t e r t h e crew reported, "We're descending
through 12, we're i n a s t a l l . " About 5 seconds a f t e r t h a t transmis,'I
and a sound similar t o
sion, the captain commanded, "Flaps two..
movement of the f l a p handle was recorded. There was no apparent
change i n t h e r a t e of descent; however, t h e v e r t i c a l acceleration
t r a c e increased immediately,.with peaks t o +3g. The recorded a i r speed decreased t o zero, and the sound of t h e s t a l l warning became
intermittent.
..
Five seconds a f t e r the captain's command f o r f l a p s , t h e f i r s t
p u l l , t h a t ' s it." Ten seconds l a t e r ,
o f f i c e r s a i d , "Pull now
the peak values f o r v e r t i c a l acceleration increased t o +5g. THe
r a t e of descent decreased s l i g h t l y ; however, t h e a l t i t u d e continued
t o decrease t o 1,090 f e e t
the elevation of the t e r r a i n a t t h e accident s i t e . The a i r c r a f t had descended from 24,800 f e e t i n 83
seconds.
...
--
1.12
A i r c r a f t Wreckage
The a i r c r a f t struck t h e ground i n a s l i g h t l y nosedown and r i g h t wingdown a t t i t u d e i n an area where the t e r r a i n sloped downward about lo0. The
a i r c r a f t s t r u c t u r e had disintegrated and ruptured and was d i s t o r t e d extensively. There was no evidence of a preexisting malfunction i n any of
t h e a i r c r a f t ' s systems.
Except f o r both elevator t i p s , the l e f t horizontal s t a b i l i z e r , and
three pieces of l i g h t s t r u c t u r e from t h e l e f t s t a b i l i z e r , t h e e n t i r e a i r c r a f t was located within an area 180 f e e t long and 100 f e e t wide. The
above components were located between 375 f e e t and 4,200 f e e t from the
main wreckage.
The horizontal s t a b i l i z e r trim s e t t i n g was 1.2 u n i t s of t r i m a i r c r a f t noseup. The landing gear and s p o i l e r s were r e t r a c t e d . The wing
t r a i l i n g edge f l a p s were extended t o t h e 2O position, and t h e Nos. 2 , 3,
6, and 7 leading edge s l a t s were f u l l y extended, which corresponded t o a
t r a i l i n g edge f l a p s e l e c t i o n of 2'.
The No. 1 and No. 3 engines were separated from t h e i r respective
pylons. The No. 2 engine remained i n i t s mounting i n t h e empennage. The
engines exhibited impact damage but l i t t l e r o t a t i o n a l damage. The speed
servo cams i n a l l t h r e e f u e l c o n t r o l u n i t s were a t o r near t h e i r high
speed detents.
The outboard s e c t i o n of t h e l e f t horizontal s t a b i l i z e r had separated
between s t a t i o n s 50 and 60. The inboard s e c t i o n remained attached t o the
v e r t i c a l s t a b i l i z e r . The l e f t elevator between s t a t i o n s 78 and 223 remained attached to the separated section. The r i g h t horizontal s t a b i l i z e r
was attached t o the v e r t i c a l s t a b i l i z e r except f o r the t i p s e c t i o n from
s t a t i o n 188 outboard. The r i g h t elevator, from s t a t i o n 188 inboard, remained attached t o t h e horizontal s t a b i l i z e r .
The t h r e e a t t i t u d e i n d i c a t o r s were damaged on impact. The i n d i c a t o r s
20' nosedown, with the wings almost
showed similar a t t i t u d e information
level.
--
The two p i t o t head heater switches were i n t h e "off" p o s i t i o n and t h e
switches' toggle levers were bent a f t . The damage t o t h e switch levers
and the debris deposited on them was t h a t which would be expected i f they
had been i n t h e "off" p o s i t i o n a t impact. A new switch with i t s toggle
lever i n the "off" p o s i t i o n , when struck with a heavy o b j e c t , exhibited
i n t e r n a l damage s i m i l a r t o t h e damage found i n t h e i n t e r n a l portions of
the r i g h t p i t o t heater switch.
Four of the f i v e p i t o t head heater c i r c u i t breakers were operable and
were e l e c t r i c a l l y closed. The a u x i l i a r y p i t o t head heater c i r c u i t breaker
was jammed i n t o i t s mounting s t r u c t u r e , and i t was e l e c t r i c a l l y open.
The l e f t elevator p i t o t head was lying on the frozen ground; when retrieved, a t l e a s t eight drops of water dripped from t h e pressure i n l e t
port. After exposure t o sunlight, more water drained from t h e port. The
captain's p i t o t head was retrieved and cleared of frozen mud. The pressure i n l e t p o r t was f i l l e d with dry wood f i b e r s . After exposure t o sunl i g h t , wet wood f i b e r s were removed from t h e i n t e r i o r of t h e i n l e t p o r t ,
and moisture was present on the inner surface of the port. The c o p i l o t ' s
p i t o t head and t h e auxilary p i t o t head were crushed and damaged severely;
they could not be checked f o r water content. The r i g h t elevator p i t o t
head remained attached t o t h e v e r t i c a l s t a b i l i z e r . The head was i n good
condition and contained no water o r i c e .
The engine a n t i - i c e switches f o r t h e Nos. 1 and 2 engines were i n the
"open" position. The switch f o r t h e No. 3 engine was i n t h e "closed"
p o s i t i o n and the switch handle was bent a f t . Tests of the bulb filaments
of t h e engine a n t i - i c e indicator l i g h t s showed t h a t a l l t h r e e l i g h t s were
on a t impact.
1.13
Medical and Pathological Information
The t h r e e crewmembers were k i l l e d i n t h e crash. Toxicological t e s t s
disclosed no evidence of carbon monoxide, hydrogen cyanide, alcohol, or
drugs i n any of the crewmembers.
There was no f i r e , e i t h e r during f l i g h t or a f t e r impact.
1.15
Survival Aspects
The accident was not survivable.
1.16
1.16.1
Tests and Research
P i t o t Head Examination and Icing Tests
A metallurgical examination of t h e separated heater conductor wire i n
the p i t o t head from t h e f i r s t o f f i c e r ' s p i t o t system showed t h a t t h e c i r cumference of the wire was reduced before the wire broke. The metal i n
t h e wire had not melted, and t h e r e were no signs of e l e c t r i c a l current
arcing or shorting.
A p i t o t head of t h e same type t h a t provided p i t o t pressure t o t h e
f i r s t o f f i c e r ' s airspeed/Mach indicator was exposed t o icing conditions
i n a wind tunnel. With the p i t o t heater inoperative, 1 t o 2 inches of
i c e formed over t h e pressure i n l e t port. During t h e exposure, a t h i n
f i l m of water flowed i n t o t h e pressure p o r t , some-of which flowed out of
the d r a i n hole.
Blockage of the d r a i n hole by i c e seemed t o depend on t h e length of
time required f o r i c e t o form and block t h e t o t a l pressure i n l e t port.
The longer i t took f o r i c e t o form and block t h e t o t a l pressure p o r t , t h e
more l i k e l y i t became t h a t t h e d r a i n hole would be blocked by i c e . Also,
the g r e a t e r t h e angle between the longitudinal a x i s of the p i t o t head and
t h e r e l a t i v e wind, t h e g r e a t e r the likelihood t h a t t h e d r a i n hole would
become blocked with i c e .
Constant a l t i t u d e pressure measurements showed t h a t when t h e t o t a l
pressure i n l e t port was blocked by i c e and the d r a i n hole remained open,
pressure changes occurred t h a t would cause a reduction of indicated a i r speed. However, when both the t o t a l pressure p o r t and d r a i n hole were
blocked, t h e t o t a l pressure remained constant, which would cause indicated
airspeed t o remain fixed. Also, abrupt and small pressure f l u c t u a t i o n s
occurred s h o r t l y before e i t h e r t h e pressure port o r d r a i n hole became
blocked by i c e .
I n an e f f o r t t o reproduce t h e apparent inconsistencies between t h e
airspeed and a l t i t u d e values on t h e FDR t r a c e s , t e s t s were conducted with
an airspeed indicator and an a l t i m e t e r connected t o vacuum and pressure
sources. By a l t e r i n g t h e vacuum t o t h e a l t i m e t e r and t o t h e airspeed
i n d i c a t o r , t h e a l t i t u d e t r a c e could be reproduced. However, following
ascent above 16,000 f e e t , the FDR airspeed and a l t i t u d e values could be
simultaneously duplicated only when t h e t o t a l pressure t o t h e airspeed
indicator was fixed a t i t s FDR value f o r an a l t i m e t e r reading of about
15,675 f e e t and an indicated airspeed of about 302 kn.
1.16.2
A i r c r a f t Performance Analysis
'Following t h e accident, t h e Safety Board requested t h a t t h e a i r c r a f t
mailufacturer analyze t h e d a t a from t h e CVR and FDR t o determine:
(1) The
consistency of these d a t a , p a r t i c u l a r l y t h e airspeed and a l t i t u d e values,
with t h e t h e o r e t i c a l performance of t h e a i r c r a f t ; (2) t h e significance and
possible reason f o r a simultaneous a c t i v a t i o n of t h e overspeedand s t a l l
warning systems; and (3) the body a t t i t u d e of t h e a i r c r a f t during i t s
f i n a l ascent and descent. The following a r e some r e s u l t s of the manuf a c t u r e r ' s performance analysis:
The airspeed and a l t i t u d e values which were recorded were consistent
with the a i r c r a f t ' s predicted climb performance u n t i l t h e a i r c r a f t reached
16,000 f e e t . The simultaneous increases i n both airspeed and r a t e of ascent which were recorded t h e r e a f t e r exceeded t h e t h e o r e t i c a l performance
c a p a b i l i t y of a B-727-200 s e r i e s a i r c r a f t of the same weight as N27AUS.
Consequently, t h e recorded airspeed values were suspected t o be erroneous,
and i t appeared t h a t they varied d i r e c t l y with the change i n recorded a l t i tude. The recorded airspeeds correlated within 5 percent with t h e theor e t i c a l airspeeds which would be expected i f t h e pressure measured i n t h e
p i t o t system had remained constant a f t e r t h e a i r c r a f t ' s climb through
16,000 f e e t .
The indicated airspeed of t h e a i r c r a f t when the s t i c k shaker was
f i r s t activated was calculated t o be 165 kn a s compared t o t h e 412 kn
recorded by t h e FDR. The decrease i n airspeed from 305 kn t o 165 kn a s
t h e a i r c r a f t climbed from 16,000 f e e t t o 24,000 f e e t (within 116 seconds)
i s within t h e a i r c r a f t ' s t h e o r e t i c a l climb power performance. The a i r c r a f t ' s p i t c h a t t i t u d e would have been about 30Â noseup a s s t i c k shaker
speed was approached. The s t a l l warning s t i c k shaker i s activated by
angle of a t t a c k instrumentation which i s completely independent o f , and
therefore not affected by e r r o r s i n , t h e a i r c r a f t ' s airspeed measuring
systems.
V e r t i c a l acceleratipn reduced s l i g h t l y a s t h e a i r c r a f t leveled a t
24,800 f e e t probably because the p i l o t relaxed t h e back pressure being
applied t o t h e c o n t r o l column. The s t i c k shaker-ceased momentarily; however, the a i r c r a f t continued t o d e c e l e r a t e because of t h e drag induced by
t h e high body a t t i t u d e , and t h e s t i c k shaker reactivated. Boeing personn e l interpreted the sound of t h e landing gear warning horn on t h e CVR t o
i n d i c a t e t h a t the t h r u s t l e v e r s had been retarded t o i d l e . The second ret o 0.8g which was coincident with a
duction i n v e r t i c a l a c c e l e r a t i o n
was probably caused
sudden descent and a rapid magnetic heading change
by an aerodynamic s t a l l with a probable l o s s of l a t e r a l control.
--
-
Theoretical r e l a t i o n s h i p s of angle of a t t a c k , v e l o c i t y , and drag were
compared t o the recorded r a t e of descent and load f a c t o r t o determine t h e
a t t i t u d e of t h e a i r c r a f t a f t e r t h e s t a l l . The comparison showed t h a t t h e
a i r c r a f t a t t a i n e d an angle of a t t a c k of 22O, or g r e a t e r , during t h e
descent. Transient nosedown a t t i t u d e s of more than 60Â would have been
required t o achieve t h e measured descent r a t e with an angle-of a t t a c k of
22O. The v a r i a t i o n s i n load f a c t o r s , which averaged about +1.5g, were
a t t r i b u t e d t o v a r i a t i o n s i n the a i r c r a f t ' s angle of. bank.
The a i r c r a f t was probably exceeding 230 kn, with a nosedowi a t t i t u d e
of about SO0 a s i t descended below 11,000 f e e t , when t h e f l a p s were extended t o 2O. The momentary cessation of t h e s t i c k shaker indicated t h a t
t h e angle of a t t a c k had been reduced t o l e s s than 13O. The increase i n
v e r t i c a l acceleration t o 2.5g was a t t r i b u t e d t o t h e a i r c r a f t ' s being i n
a t i g h t nosedown s p i r a l with a bank angle between 70 and 80°
With a normally operating elevator f e e l system, and a s t a b i l i z e r t r i m
s e t t i n g of 1.2 u n i t s a i r c r a f t noseup, t h e p i l o t would have t o e x e r t a p u l l
f o r c e of between 45 and 50 1 b s . t o achieve a 2.5g load f a c t o r a t 5,000
f e e t and 250 kn.. I f , however, t h e elevator p i t o t systemwas blocked so
t h a t t h e system sensed a zero indicated airspeed, a p u l l f o r c e of l e s s .
than 30 l b s . woqld have produced t h e same load f a c t o r . After the a i r c r a f t
had descended through 5,000 f e e t , the load f a c t o r reached peak values of
+5g.
The manufacturer's engineers s t a t e d t h a t t h e a i r c r a f t ' s s t r u c t u r a l
l i m i t s would have been exceeded a t high angles of s i d e s l i p and load fact o r s approaching +5g. They s t a t e d t h a t a consequent f a i l u r e of the
elevator assemblies could have produced an aerodynamic f l u t t e r which
could have, i n turn, caused t h e elevator spar t o f a i l and t h e l e f t horizontal s t a b i l i z e r t o separate. With the a i r c r a f t a t a s t a l l angle of
a t t a c k when t h e horizontal s t a b i l i z e r separated, an uncontrollable noseup
pitching moment would have been produced, which could have r e s u l t e d i n
an angle of a t t a c k of 40' or more.
1.17
1.17.1
Other Information
Pretakeoff Checklist
Northwest A i r l i n e s ' operational procedures r e q u i r e t h a t t h e f l i g h t crew make a pretakeoff check of c e r t a i n items.' The second o f f i c e r i s required t o read t h e c h e c k l i s t items, and t h e f i r s t o f f i c e r must check t h e
items and respond t o the second o f f i c e r ' s challenge. Included on t h e
c h e c k l i s t are:
Second Officer
Flaps
Marked Bug
I c e Protection
P i t o t Heat
Pressurization
F i r s t Officer
K
15, 15 (25,25) Blue
(C, FO) Numbers Set
OFF (ON)
ON
(C, FO) Zero,
Normal Flags
09
Company p i l o t s s t a t e d t h a t the c h e c k l i s t i s used only t o check t h a t
the required a c t i o n has already been performed; i t i s not used a s a l i s t
of items t o be'accomplished. With regard t o t h e a c t i v a t i o n of p i t o t head
h e a t e r s , i t was the f i r s t o f f i c e r ' s duty t o turn the two switches t o t h e
' 0 n " " p o s i t i o n s h o r t l y a f t e r the engines had been s t a r t e d and t o check t h e
ammeter readings on t h e various heaters t o confirm t h e i r proper operation.
After checking these items, he was supposed t o leave the p i t o t heater
switches on and t o check t h a t they were on during the pretakeoff check.
1.17.2
Airspeed Measuring System
Whenanaircraft moves t h r o u g h a n a i r mass, ~ r e s s u r e i s c r e a t e dahead of
the a i r c r a f t , which adds t o the e x i s t i n g s t a t i c pressure w i t h i n t h e a i r
mass. The added pressure, dynamic pressure, i s d i r e c t l y proportional t o
t h e velocity of the a i r c r a f t . When a symmetrically shaped object, such a s
a p i t o t head, i s placed i n t o the moving airstream, the flow of a i r w i l l
separate around t h e nose of the object so t h a t t h e l o c a l velocity a t t h e
nose is zero. A t t h e zero velocity point, t h e airstream dynamic pressure
i s converted i n t o an increase i n the l o c a l s t a t i c pressure. Thus, t h e
pressure measuredat t h e n o s e o f t h e object i s c a l l e d t o t a l pressure, and i t
i s equal t o t h e sum o f t h e dynamic pressureand t h e ambient s t a t i c pressure.
I n an a i r c r a f t airspeed measuring system, the t o t a l pressure i s
measured by t h e p i t o t head and is transmitted through t h e p i t o t system
plumbing t o one s i d e of a d i f f e r e n t i a l pressure measuring instrument (airspeed i n d i c a t o r ) . The ambient s t a t i c pressure i s measured a t s t a t i c
p o r t s which a r e mounted i n an area t h a t ' i s not s i g n i f i c a n t l y influenced
by t h e moving airstream. The s t a t i c pressure measured a t these p o r t s i s
transmitted t o t h e opposite s i d e of the d i f f e r e n t i a l pressure measuring
instrument. I n e f f e c t , t h e d i f f e r e n t i a l pressure instrument (whether i t
be an airspeed i n d i c a t o r gage, a f l i g h t data recorder pressure transmitt e r , or a component within an a i r data computer) s u b t r a c t s the ambient
s t a t i c pressure measured by the s t a t i c system from the t o t a l pressure
measured by t h e p i t o t system. The r e s u l t a n t dynamic pressure i s a
d i r e c t measurement of indicated airspeed.
Since the ambient s t a t i c pressure i s a component p a r t of t o t a l pressure, any change i n s t a t i c pressure would normally r e s u l t i n an equal
change i n both the p i t o t and s t a t i c pressure systems. Therefore, a change
i n ambient s t a t i c pressure, such a s t h a t encountered during a change i n
a l t i t u d e , would normally have no e f f e c t on airspeed measurement. Only a
change i n dynamic pressure produced by a change i n t h e a i r c r a f t ' s velocity
would cause a change i n the indicated airspeed. I f , however, only one
s i d e of the airspeed i n d i c a t o r sensed a change i n t h e ambient s t a t i c pressure, an erroneous change i n indicated airspeed would r e s u l t , even though
the a c t u a l dynamic pressure remained unchanged. Such a condition would
occur i f e i t h e r t h e p i t o t or s t a t i c systemwas blocked or was otherwise
rendered i n s e n s i t i v e t o external pressure changes.
I n t h e event of a blocked p i t o t o r s t a t i c system, the d i r e c t i o n of
t h e indicated airspeed e r r o r would depend on which of the systems was
blocked and the d i r e c t i o n of change i n t h e anbient s t a t i c pressure.
Under conditions where t h e pressure i n t h e s t a t i c system increases with
respect t o t h e pressure i n t h e p i t o t system, t h e indicated airspeed w i l l
read low erroneously. For the opposite condition, where t h e pressure i n
t h e s t a t i c system decreases with respect t o t h e pressure i n t h e p i t o t
system, t h e indicated airspeed w i l l read high erroneously. The l a t t e r
would e x i s t i f the p i t o t head was blocked so t h a t a constant pressure
was trapped i n t h e p i t o t system while the a i r c r a f t was ascending. This
i s because the s t a t i c system pressure would decrease and t h e r e s u l t a n t
d i f f e r e n t i a l pressure would appear a s an increase i n dynamic pressure.
Indicated airspeed e r r o r may a l s o occur when t h e p i t o t system becomes i n s e n s i t i v e t o changes i n t o t a l pressure i n such a manner t h a t t h e
system vents t o an ambient s t a t i c pressure source. The pressure measured
by the p i t o t system w i l l equalize with t h e pressure i n the s t a t i c system,
and t h e dynamic pressure (indicated airspeed) w i l l decrease t o zero. The
vent source i n a p i t o t head which can produce t h i s kind of e r r o r i s the
moisture d r a i n hole which i s located downstream from a blocked t o t a l
pressure sensing i n l e t .
1.17.3
B-727 S t a l l C h a r a c t e r i s t i c s
During i t s type c e r t i f i c a t i o n process, t h e B-727-200 s e r i e s a i r c r a f t
demonstrated s t a l l c h a r a c t e r i s t i c s which met the requirements of t h e C i v i l
A i r Regulations, p a r t s 4b. 160-162. The s i g n i f i c a n t requirements defined
t h e r e i n are: (1) That, a t a n angle of a t t a c k measurably g r e a t e r than t h a t
of maximum l i f t , t h e inherent f l i g h t c h a r a c t e r i s t i c s g i v e a c l e a r indicat y p i c a l indications a r e
t i o n t o the p i l o t t h a t the a i r c r a f t is s t a l l e d
a nosedown p i t c h o r a r o l l which cannot be r e a d i l y a r r e s t e d ; (2) t h a t recovery from t h e s t a l l can be effected by normal recovery techniques s t a r t ing a s soon a s t h e a i r c r a f t i s s t a l l e d ; (3) t h a t t h e r e i s no abnormal
noseup pitching and t h a t the longitudinal c o n t r o l f o r c e be p o s i t i v e , up
t o an including t h e s t a l l ; (4) t h a t a s a f e recovery from a s t a l l can be
effected with t h e c r i t i c a l engine inoperative; and (5) t h a t a c l e a r and
d i s t i n c t i v e s t a l l warning be apparent t o t h e p i l o t a t an airspeed a t
l e a s t 7 percent above the s t a l l i n g airspeed.
-
The c e r t i f i c a t i o n s t a l l tests, conducted with t h e a i r c r a f t i n a l l
operating configurations and with the most adverse weight and c.g. condit i o n s , demonstrated t h a t a s the a i r c r a f t was slowed and i t s wing angle of
a t t a c k was increased, t h e b u f f e t produced by airflow separation from t h e
wing provided a n a t u r a l warning of impending s t a l l . With the landing
f l a p s extended, however, t h e airspeed margin provided by t h e b u f f e t warning was considered t o be i n s u f f i c i e n t . Consequently, a s t i c k shaker system was i n s t a l l e d t o provide an a r t i f i c i a l warning f o r a l l configurations.
I n t h e clean configuration, 81 t h e s t i c k shaker activated when t h e
When the a i r c r a f t was slowed f u r t h e r ,
angle of a t t a c k reached 13'.
n a t u r a l buffeting occurred a t an angle of a t t a c k between 16O and la0.
The b u f f e t was described a s "quite heavy" when t h e speed was reduced t o
within 2 t o 3 kn of t h e speed associated with maximum l i f t . When the
angle of a t t a c k f o r maximum l i f t (about 22O) was reached, t h e r e was a
tendency f o r t h e nose t o drop i f t h e p i l o t relaxed pressure on the cont r o l column. Also, l a t e r a l s t a b i l i t y was reduced noticeably, which increased the p i l o t ' s workload i n maintaining wings-level f l i g h t .
During c e r t i f i c a t i o n f l i g h t t e s t s , the angle of a t t a c k was increased
t o 25O, a f t e r which recovery was effected by relaxing t h e p u l l force on
the c o n t r o l column. With the use of engine t h r u s t during recovery, t h e
a l t i t u d e l o s t was r e s t r i c t e d t o about 2,000 f e e t .
Up t o t h e o n s e t of s t a l l b u f f e t , t h e longitudinal c o n t r o l forces
needed t o e f f e c t s t a l l entry increased a s the angle of a t t a c k increased.
A t higher angles of a t t a c k , up t o and beyond the angle f o r maximum l i f t ,
the p u l l force required t o maintain a noseup pitching moment decreased.
The forces did not reverse, however, and, with normal t r i m , a reduction
i n p u l l force resulted i n a decreased angle of attack.
The B-727 longitudinal c o n t r o l system i s capable of developing the noseup
pitching moments needed t o obtain angles of a t t a c k much higher than those
associated with s t a l l . For an a i r c r a f t having t h e same weight, c.g. locat i o n , and s t a b i l i z e r t r i m s e t t i n g a s N274US, t h e manufacturer's a n a l y s i s
showed t h a t an angle of a t t a c k of approximately 37O could be a t t a i n e d i f
a continuous p u l l force was exerted t o hold t h e c o n t r o l column a f t .
Like other a i r c r a f t which have horizontal s t a b i l i z e r s located near or
on top of t h e i r v e r t i c a l s t a b i l i z e r s , the B-727 does pass through a range
of high angles of a t t a c k where longitudinal i n s t a b i l i t y occurs. This ins t a b i l i t y causes the a i r c r a f t , when no c o n t r o l force i s applied, t o p i t c h
t o even higher angles of a t t a c k . Longitudinal i n s t a b i l i t y i s caused by
degraded horizontal s t a b i l i z e r effectiveness when t h e a i r c r a f t ' s a t t i t u d e
i s such t h a t t h e horizontal s t a b i l i z e r i s enveloped by the low-energy turbulent a i r i n the wake from t h e wings. When these high angles of a t t a c k
a r e reached, a push f o r c e on t h e c o n t r o l column i s required t o reduce t h e
angle of attack. For a B-727 with an a f t c.g. location and s t a b i l i z e r
t r i m i n the c r u i s e range, wind tunnel d a t a show t h a t a nosedown pitching
moment w i l l decrease the angle of a t t a c k and s t a l l recovery can be attained
by applying push forces t o the c o n t r o l column.
A s t i c k pusher i s a device which w i l l apply a force t o move the cont r o l column forward whenthe angle of a t t a c k f o r maximum l i f t i s exceeded.
The usefulness of a s t i c k pusher i s controversial since i t can e f f e c t p r i mary c o n t r o l of the a i r c r a f t . However, a s t i c k pusher i s required on
B-727 and other a i r c r a f t r e g i s t e r e d by the United Kingdom. That s t i c k
pusher is designed so t h a t i t s a c t i o n can be overpowered by a p u l l force
of about 80 lbs. on the p i l o t ' s c o n t r o l column.
-81
Without landing gear, f l a p s , or s p o i l e r s extended.
2.
2.1
ANALYSIS AND CONCLUSIONS
Analysis
The a i r c r a f t was c e r t i f i c a t e d , equipped, and maintained i n accordance with regulations and approved procedures. The a i r c r a f t weighed subs t a n t i a l l y less than i t s authorized maximum weight f o r takeoff.
Although t h e speed servo cams i h a l l t h r e e engine f u e l c o n t r o l l e r s
were positioned f o r high engine revolutions per minute, t h e engines were
producing very l i t t l e t h r u s t a t impact a s evidenced by t h e absence of sign i f i c a n t r o t a t i o n a l damage t o t h e engines. Probably, t h e t h r o t t l e s had
been advanced shortly-before impact, but t h e r e was e i t h e r i n s u f f i c i e n t
time for the engines t o a c c e l e r a t e , o r a c c e l e r a t i o n was limited because
airflow i n t o the engine i n l e t s had been d i s t o r t e d by t h e extreme angle of
a t t a c k and probable s i d e s l i p .
The flightcrew was properly c e r t i f i c a t e d and each crewmember had received the t r a i n i n g and off-duty time prescribed by regulations. There
was no evidence of medical o r physiological problems t h a t might have
affected t h e i r performance.
The conversations recorded on t h e CVR revealed t h a t , following ascent
above 13,500 f e e t , t h e flightcrew became concerned and puzzled by t h e apparent performance of the a i r c r a f t because of the indicated airspeed and
the indicated r a t e of ascent. The FDR airspeed and a l t i t u d e t r a c e s provided i n v e s t i g a t o r s an i n s i g h t regarding these conversations. The a i r speed t r a c e increased rapidly a f t e r t h e a i r c r a f t ascended above 16,000
f e e t while t h e r a t e of climb continued t o increase and eventually reached
a peak value of 6,500 f e e t per minute. The Boeing Company's a n a l y s i s of
t h e airspeed and r a t e s of climb values t h a t r e g i s t e r e d above 16,000 f e e t
showed t h a t these values were incompatible with t h e a i r c r a f t ' s performance
capabilities.
Analysis showed t h a t t h e r e was a d i r e c t r e l a t i o n s h i p between t h e a i r speed and a l t i t u d e values. This r e l a t i o n s h i p was based on the assumptions
t h a t (1) the t o t a l pressure measured by t h e FDR p i t o t system remained cons t a n t a f t e r t h e a i r c r a f t ascended above 16,000 f e e t , and (2) t h e pressure
measured by t h e FOR s t a t i c system varied according t o the recorded a l t i tude values. These assumptions were substantiated by t h e t e s t s which
determined t h a t t h e FDR airspeed and a l t i t u d e t r a c e s could be reproduced
only i f the t o t a l pressure t o the airspeed indicator was held constant
during ascent above 16,000 f e e t .
Although the p i t o t systems for t h e captain's and f i r s t o f f i c e r ' s a i r speed Mach i n d i c a t o r s and the FDR airspeed instrumentation a r e t h r e e separ a t e and completely independent systems, i t i s reasonable t o conclude t h a t
a l l t h r e e systems were sensing nearly i d e n t i c a l and erroneous t o t a l pressures. This can be concluded because the flightcrew made no reference t o
any d i f f e r e n c e between the airspeed readings on t h e captain's and f i r s t
o f f i c e r ' s i n d i c a t o r s , and t h e f i r s t o f f i c e r ' s reference t o "...going 340
kn
corresponded closely t o t h e airspeed value recorded on t h e FDR a t
that time. Additionally, t h e near simultaneous a c t i v a t i o n of t h e overspeed warnl'ng systems tends t o prove t h a t t h e f i r s t o f f i c e r ' s airspeed was
c l o s e t o t h e value recorded on t h e FDR when t h e a i r c r a f t neared i t s peak
altitude.
..."
The erroneously high airspeed indications were caused by a complete
and nearly simultaneous blockage of a l l t h r e e p i t o t pressure systems.
Moreover, since t h e only common elements among t h e systems were the
design f e a t u r e s of t h e p i t o t heads and t h e environment t o which they were
exposed, t h e Safety Board concludes t h a t the p i t o t heads were blocked by
i c e which formed around t h e heads and closed t h e d r a i n holes and t h e press u r e i n l e t ports. The conclusion is supported by t h e airspeed aberrations
t h a t were recorded while t h e a i r c r a f t was f l y i n g l e v e l a t 13,500 f e e t and
by t h e moisture which was found i n t h e p i t o t heads when they were ecovered
and examined. Additionally, i t is known t h a t icing conditions e x i ted i n
t h e area through which F l i g h t 6231 was f l y i n g , and it i s unlikely t h a t
any other type of blockage o r malfunction would simultaneously a f f e c t t h e
t h r e e independent systems.
2
The formation of i c e on t h e p i t o t heads should have been prevented by
e l e c t r i c a l heating elements which a r e activated by the p i t o t heater
switches located i n t h e cockpit. The Safety Board concludes t h a t ' t h e
heating elements were never activated because the p i t o t heater switches
were not i n the "on" p o s i t i o n during the f l i g h t . This conclusion i s subs t a n t i a t e d by the p o s i t i o n and condition of t h e switches i n t h e wreckage,
t h e i n t e r n a l damage t o t h e r i g h t switch, and t h e lack of evidence t h a t
e l e c t r i c a l current was present i n the heater c i r c u i t t o t h e p i t o t head i n
t h e f i r s t o f f i c e r ' s p i t o t system a t t h e time of impact.
The Safety Board was unable t o determine why the p i t o t head heater
switches were not placed i n t h e "on" p o s i t i o n before departure. It i s
c l e a r t h a t the flightcrew performed t h e pretakeoff checks required by
Northwest's operational procedures. However, t h e proper c h e c k l i s t sequence was not followed, and i t i s possible t h a t t h e f i r s t o f f i c e r positioned the switches improperly because of an omission i n the sequence
and h i s inexperience a s a B-727 c o p i l o t .
While reading t h e c h e c k l i s t , t h e second o f f i c e r c a l l e d "bug" and,
before receiving a response from e i t h e r t h e captain o r f i r s t o f f i c e r , he
omitted t h e "ice protection" c a l l and c a l l e d "pitot heat." The f i r s t
o f f i c e r apparently responded t o both t h e omitted c a l l and t h e " p i t o t
heat" c a l l by saying, "off and on," but following t h e captain's response
t o the "bug" c a l l , the f i r s t o f f i c e r asked whether t h e engine heat was
needed. The c a p t a i n may o r may not have responded with a nod o r hand
s i g n a l , but t h e sound of f i v e c l i c k s was recorded and t h e f i r s t o f f i c e r
returned t o the task of s e t t i n g h i s airspeed bug.
The f i v e c l i c k s may h a v e been t h e movement of t h e p i t o t heater
switches t o t h e "off" p o s i t i o n and t h e movement of t h e engine a n t i - i c e
a r e v e r s a l of t h e i r normal positions..
switches t o t h e "on" p o s i t i o n
This assumption i s supported by t h e p o s i t i o n of t h e engine a n t i - i c e and
p i t o t heater switches i n t h e wreckage, t h e condition of the l i g h t s associated with t h e engine a n t i - i c e switches, and t h e lack of any reference
during the f l i g h t t o t h e need f o r engine anti-ice.
--
Because of t h e flightcrew's comments concerning a i r c r a f t performance
and t h e absence of cormtents about possible instrument e r r o r o r airspeed
system i c i n g , t h e Safety Boardconcludes t h a t the flightcrew a t t r i b u t e d t h e
high airspeed and t h e high r a t e of climb t o the a i r c r a f t ' s r e l a t i v e l y low
gross weight and t o an encounter with unusual weather, which included
strong updrafts. The flightcrew's a n a l y s i s of t h e s i t u a t i o n mast have
been strongly influenced by these f a c t o r s and by t h e f a c t t h a t both a i r speed instruments were indicating e s s e n t i a l l y t h e same values. However,
t h e a i r c r a f t ' s a t t i t u d e a s i t neared the top of i t s ascent should have
warned them t h a t t h e a i r c r a f t ' s performance was abnormal because i t s
nearly 30Â noseup a t t i t u d e was about 25O higher than t h e normal climb
a t t i t u d e , and a t such a high noseup a t t i t u d e i t would have been impossible
f o r t h e airspeed t o continue t o increase even i f influenced by extreme upd r a f t s . Because t h e use of a t t i t u d e references is a fundamental of i n s t r u ment flying, which is s t r e s s e d i n Northwest's flightcrew t r a i n i n g program,
t h e Safety Board concludes t h a t t h e flightcrew improperly r e l i e d on a i r speed indications a s a means of determining a i r c r a f t performance.
Although t h e a c t i v a t i o n of t h e overspeed warning systems probably
reinforced the f l i g h t c r w ' s b e l i e f t h a t they were taking appropriate
a c t i o n , t h e operation of t h e s t a l l warning s t i c k shaker should have
a l e r t e d them t h a t t h e a i r c r a f t a c t u a l l y was approaching a s t a l l . The
f i r s t o f f i c e r apparently misinterpreted t h e c o n t r o l column v i b r a t i o n produced by t h e s t i c k shaker a s Mach b u f f e t because when t h e s t i c k shaker
began, he'commented,
t h e r e ' s t h a t Mach buffet." The c a p t a i n apparentl y agreed with t h i s i n t e r p r e t a t i o n because he then commanded, "Pull it up."
The almost simultaneous a c t i v a t i o n of t h e s t a l l and t h e overspeed warning
systems undoubtedly created some confusion; however, t h e d i f f e r e n c e s between s t a l l b u f f e t and Mach b u f f e t a r e s u b s t a n t i a l and t h e former should
have been e a s i l y recognized. Again, though, i t appears t h a t t h e f l i g h t crew r e l i e d almost exclusively on t h e airspeed i n d i c a t o r s and t h e i r
r e l a t e d warning systems t o a s s e s s the a i r c r a f t ' s performance.
"...
Even a f t e r t h e s t a l l , as manifested by t h e rapid heading change
(banked a t t i t u d e ) and t h e sudden descent, t h e flightcrew f a i l e d t o recognize t h e problem f o r a number of seconds. They continued t o e x e r t back
pressure on t h e c o n t r o l column which kept t h e a i r c r a f t a t a , h i g h angle of
a t t a c k . They probably were having d i f f i c u l t y with l a t e r a l c o n t r o l , and
t h e a i r c r a f t entered i n t o a s p i r a l l i n g descent t o t h e r i g h t , during which
the a c t u a l airspeed of the a i r c r a f t began t o increase rapidly.
The erroneous airspeea indications, the steep nosedown a t t i t u d e , and
t h e proprioceptive sensations associated with the p o s i t i v e v e r t i c a l accele r a t i o n forces undoubtedly contributed t o confusion which prevented t h e
flightcrew from recognizing t h e t r u e condition of t h e a i r c r a f t . Additiona l l y , i t i s probable t h a t the nosedown and banked a t t i t u d e s of t h e a i r c r a f t were so steep t h a t the horizon references i n the a t t i t u d e i n s t r u ments were nearly hidden. This would have made the l a t e r a l a t t i t u d e of
t h e a i r c r a f t d i f f i c u l t t o determine. However, had t h e p i l o t s concentrated
more on t h e a t t i t u d e i n d i c a t o r s , and p a r t i c u l a r l y t h e position of the "sky
pointers", 91 they probably could have returned t h e a i r c r a f t t o l e v e l
f l i g h t had they taken appropriate c o r r e c t i v e a c t i o n within 30 t o 40
seconds a f t e r t h e s t a l l .
Probably because of t h e low airspeed i n d i c a t i o n s , t h e captain decided
that t h e a i r c r a f t was i n a s t a l l . He transmitted: "We're descending
through 12, we're i n a s t a l l , " and he c a l l e d . f o r t h e f l a p s t o b e extended
t o 2'
a proper s t e p i n t h e s t a l l recovery procedure. However, t h e
a c t u a l indicated airspeed a t t h a t time was probably i n excess of 230 kn
and increasing rapidly; consequently, although the s t i c k shaker ceased
operation momentarily, the extension of t h e f l a p s had l i t t l e favorable
effect.
-
Even a f t e r the p i l o t s decided t h a t the a i r c r a f t was s t a l l e d , t h e
Safety Board believes t h a t they continued t o r e a c t primarily t o the high
r a t e of descent indications and proprioceptive sensations because they
continued t o exert a p u l l f o r c e on the c o n t r o l column. This is substantiated by t h e increasing v e r t i c a l acceleration forces a s the descent continued. However, because the wings were not leveled f i r s t , t h e a i r c r a f t
continued t o descend rapidly i n a s p i r a l l i n g , accelerated s t a l l .
Since the p i t o t heads f o r t h e elevator f e e l system were probably
blocked by i c e , the f o r c e required of t h e p i l o t s to move t h e elevators
would have been increased while t h e a i r c r a f t was above 16,000 feet,. However, when the a i r c r a f t descended below t h a t a l t i t u d e , the f o r c e required
would have beendiminished. A s t h e descent continued below 5,000 f e e t , t h e
a c t u a l indicated airspeed probably exceeded 350 kn while the airspeed
sensed by t h e elevator f e e l systemwas probably near zero. Consequently,
conditions were created i n which high v e r t i c a l acceleration forces could
be produced with r e l a t i v e ease. A s evidenced by t h e FDR acceleration
trace, high v e r t i c a l acceleration forces were produced below 5,400 f e e t .
A s the a i r c r a f t continued i t s descent through 3,500 f e e t , t h e high
v e r t i c a l acceleration forces induced were s u f f i c i e n t to cause the f a i l u r e o f
t h e l e f t horizontal s t a b i l i z e r . Thereafter, t h e a i r c r a f t probably r o l l e d
t o a near wings-level a t t i t u d e , pitched up t o an extremely high angle of
a t t a c k s a n d c o n t i n u e d t o d e s c e n d i n a n uncontrollable s t a l l t o the ground.
-91
A t r i a n g u l a r index which is positioned above t h e movable horizon and
which moves i n the opposite d i r e c t i o n from the a i r c r a f t ' s banked
a t t i t u d e t o i n d i c a t e t h e number of degrees of bank.
During t h e Safety Board 's investigation, i n c i d e n t s involving possible
p i t o t - s t a t i c system i c i n g were reviewed. Although none of these incidents r e s u l t e d i n a catastrophic accident, i t became c l e a r t h a t p i t o t o r
s t a t i c system i c i n g during f l i g h t can and does occur. Also, t h e r e s u l t a n t
e f f e c t s on pressure-operated f l i g h t instruments can produce a t least
momentary confusion among t h e crewmembers.
-
While a l l of t h e flightcrews involved i n these incidents reverted t o
a t t i t u d e f l y i n g u n t i l t h e cause of t h e i c i n g could be eliminated o r instrum e n t f l i g h t could be terminated, i t was apparent from these incidents t h a t
some p i l o t s who understood t h e b a s i c p r i n c i p l e s of airspeed measurement
f a i l e d t o analyze t h e possible r e s u l t s of a blockage of t h e p i t o t or
s t a t i c systems. The p i l o t s o f t e n f a i l e d t o determine t h e proper reasons
f o r an increasing airspeed indication; they a t t r i b u t e d such indications
t o unusual weather phenomena.
Although unusual weather phenomena such a s mountain waves, extreme
turbulence, and v e r t i c a l wind shear can produce s i g n i f i c a n t airspeed
deviations, these phenomena usually a r e of s h o r t duration and cause e r r a t i c
o r abruptly changing airspeed indications r a t h e r s t e a d i l y increasing,
s t e a d i l y decreasing, o r fixed airspeed indications. Also, t h e a i r c r a f t 's
a t t i t u d e during encounters with these phenomena i s i q o r t a n t i n determining airspeed trends and possible sources of e r r o r . Consequently, t h e
Safety Board believes t h a t p o t e n t i a l p i t o t - s t a t i c system problems and
a t t i t u d e f l y i n g a s a temporary remedy f o r these problems should be reemphasized i n instrument f l y i n g training programs, and the Safety Board has
made a recommendation t o t h i s e f f e c t t o the Administrator, Federal
Aviation Administration.
2.2
Conclusions
(a)
Findings
A l l members of t h e flightcrew were properly c e r t i f i c a t e d
and were q u a l i f i e d f o r t h e i r respective d u t i e s .
The a i r c r a f t had been properly maintained and was a i r worthy f o r t h e f l i g h t ; i t s gross weight and c.g. were
within t h e prescribed l i m i t s .
There was no evidence of a system malfunction o r f a i l u r e o r
of a s t r u c t u r a l d e f e c t i n t h e a i r c r a f t .
The flightcrew had adequate weather information f o r the
flight.
The F'DR v e r t i c a l acceleration t r a c e i n d i c a t e s t h a t only
l i g h t turbulence was encountered.
The weather conditions encountered during t h e f l i g h t were
conducive t o t h e formation of moderate airframe i c e .
The a i r c r a f t accumulated s u f f i c i e n t i c e during i t s f l i g h t
t o block completely t h e d r a i n holes and t o t a l pressure
i n l e t p o r t s of the p i t o t heads; t h e s t a t i c p o r t s were not
affected by t h e i c e .
The p i t o t heads became blocked a t an a l t i t u d e of about
16,000 f e e t .
The i c e formed on t h e p i t o t heads because t h e p i t o t head
heater switches had not been turned on before F l i g h t 6231
departed JFK
.
The complete blockage of t h e p i t o t heads caused the cockpit
airspeed i n d i c a t o r s t o read erroneously high a s t h e a i r c r a f t
climbed above 16,000 f e e t and t h e s t a t i c pressure decreased.
The flightcrew r e a c t e d t o t h e high airspeed indications by
increasing t h e noseup a t t i t u d e of t h e a i r c r a f t which increased t h e r a t e of climb. While t h i s caused the indicated
airspeed t o increase more rapidly because t h e s t a t i c press u r e decreased more rapidly with the increased r a t e of
climb, the a c t u a l airspeed was decreasing.
The airspeed overspeed warning and s t a l l warning s t i c k
shaker operated simultaneously because of the blocked p i t o t
heads and the high noseup a t t i t u d e of t h e a i r c r a f t .
The flightcrew misconstrued t h e operation of the s t a l l
warning s t i c k shaker a s Mach b u f f e t .
The flightcrew continued t o increase t h e noseup a t t i t u d e of
t h e a i r c r a f t following t h e operation of the s t a l l warning
s t i c k shaker.
The a i r c r a f t s t a l l e d a t an a l t i t u d e of 24,800 f e e t while i n
a noseup a t t i t u d e of about 300.
Following the s t a l l , t h e a i r c r a f t entered i n t o a r i g h t
s p i r a l l i n g dive a t a high r a t e of descent. Throughout t h e
descent, t h e flightcrew reacted primarily t o airspeed and
r a t e of descent indications instead of a t t i t u d e indications,
and thus f a i l e d t o i n i t i a t e proper recovery techniques and
procedures.
17.
(b)
I n an e f f o r t t o recover the a i r c r a f t from a high r a t e of
descent, the flightcrew exerted excessive p u l l forces on
the c o n t r o l columns which induced high v e r t i c a l acceleration
forces and caused t h e l e f t horizontal s t a b i l i z e r t o f a i l .
Probable Cause
The National Transportation Safety Board determines t h a t t h e probable
cause of t h i s accident was t h e l o s s of c o n t r o l of the a i r c r a f t because the
flightcrew f a i l e d t o recognize and c o r r e c t t h e a i r c r a f t ' s high-angle-ofa t t a c k , low-speed s t a l l and i t s descending s p i r a l . The s t a l l was precipated by t h e flightcrew's improper reaction t o erroneous airspeed and
Mach indicatdons which had r e s u l t e d from a blockage of t h e p i t o t heads by
atmospheric icing. Contrary t o standard operational
the
flightcrew had not activated t h e p i t o t head heaters.
3.
RECOMMENDATIONS
A s a r e s u l t of t h i s accident, t h r e e recommendations were made t o t h e
Administrator, Federal Aviation Administration. (See Appendix D.)
BY THE NATIONAL TRANSPORTATION SAFETY BOARD
Is/
FRANCIS H. McADAMS
Member
/S/
LOUIS M. THAYER
Member
/S/
ISABEL A. BURGESS
Member
John H. Reed, Chairman, and William R. Haley, Member, did not p a r t i c i p a t e
i n the adoption of t h i s report.
August 13, 1975
Intentionally Left Blank
in Original Document
- 23
-.
APPENDIX A
Investigation and Hearing
1.
Investigation
The National Transportation Safety Board was notified of the accident
about 1935 on December 1, 1974. The Safety Board immediately dispatched
an investigative team to the scene. The following morning the team
established investigative groups for operations/witnesses, air traffic
control, weather, structures, powerplants, systems, flight data recorder,
maintenance records, and cockpit voice recorder.
Parties to the investigation were: The Federal' Aviation Administration, Northwest Airlines, Inc., The Boeing Company, Air Line Pilots
Association, International Association of Machinists and Aerospace
Workers, and the Pratt and Whitney Division of the United Aircraft
Corporation.
2.
Hearing
A public hearing was held at Bear Mountain, New York, on February
12 and 13, 1975. All of the parties to the investigation except the
Pratt and Whitney Aircraft Division were parties to the hearing.
- 24 APPENDIX B
Aircrew Information
Captain John B. Lagorio
Captain Lagorio, 35, was employed by Northwest Airlines on January
17, 1966. He held Airline Transport P i l o t c e r t i f i c a t e No. 1496609 with
airplane multiengine and single-engine land ratings, counnercial privileges and a type rating i n the B-727. He held Flight Engineer c e r t i f i c a t e No. 1682555 and a valid f i r s t - c l a s s medical c e r t i f i c a t e which was
issued with HO limitations on August 22, 1974.
Captain Lagorio had accumulated about 7,434 flight-hours, of which
about 1,973 w e r e i n the B-727. I n the 30-, 60-, and 90-day periods preceding the accident, he flew about 58, 122, and 185 hours, respectively,
a l l i n the B-727.
Captain Lagorio was advanced from f i r s t o f f i c e r t o captain on August
5, 1969. He completed h i s l a s t general refresher training on January 15,
1974. and h i s l a s t B-727 refresher trainine on November 15. 1974. He
a proficiency f l i g h t check i n the ~ 1 7 2 7simulator on November 15,
1974.
F i r s t Officer Walter A. Zadra
F i r s t Officer Zadra, 32, was employed by Northwest Airlines on
January 8, 1968. He held Commercial P i l o t c e r t i f i c a t e No. 1624729 with
airplane multiengine and single-engine land ratings, and an instrument
rating. He held Flight Engineer c e r t i f i c a t e No. 1834609 and a valid
f i r s t - c l a s s medical c e r t i f i c a t e which was issued with no limitations on
July 9, 1974.
F i r s t Officer Zadra had flown about 1,550 hours a s a p i l o t or f i r s t
o f f i c e r and about 3,152 hours a s a second o f f i c e r ( f l i g h t engineer) of
which about 1,244 hours were i n the B-727. He upgraded from second
o f f i c e r i n B-707 a i r c r a f t t o f i r s t o f f i c e r i n B-727 a i r c r a f t on October
16, 1974, and he had flown about 46 hours i n the l a t t e r capacity. I n
the 30-, 60-, and 90-days periods preceding the accident, he flew,
respectively, about 46 hours as f i r s t o f f i c e r i n the B-727 and 23 and
76 hours a s second o f f i c e r i n the B-707.
F i r s t Officer Zadra completed general refresher training on January
7, 1974, and he passed a f i r s t o f f i c e r proficiency check i n the B-727
on October 16, 1974.
Appendix B
Page 2
Second Officer James F. Cox
Second Officer Cox, 33, was employed by Northwest Airlines on
February 2, 1969. He held Commercial P i l o t c e r t i f i c a t e No. 1643627 with
m l t i e n g i n e land and instrument r a t i r g s . He held Flight Engineer (turboj e t powered) c e r t i f i c a t e No. 1920999 and a f i r s t - c l a s s medical c e r t i f i c a t e which was issued with no limitations on March 1,1974.
Second Officer Cox had.acquired about 1,938 hours of flying time
a s a second o f f i c e r with Northwest Airlines, including about 1,611 hours
i n B-727 a i r c r a f t . I n the 30-, 60-, and 90-day periods preceding the
accident, he flew about 45, 113 and 180 hours, respectively, a l l i n
B-727 a i r c r a f t
.
Second Officer Cox completed general refresher training on January
10, 1974, and he passed a second o f f i c e r proficiency-check on April 10,
1974.
APPENDIX C
Aircraft Information
N274US was manufactured by The Boeing Company on December 2, 1969,
and it was assigned serial No. 20295. It had accumulated about 10,289
hours of time in service.
N274US was powered by three Pratt and Whitney JT8D-7 engines.
Pertinent engine data are as follows:
Position
Serial No.
Total Time
Time Since Heavy Maintenance
1
649153
18,641 hours
3,044 hours
2
654070
14,818 hours
2,234 hours
3
648988
17,612 hours
1,193 hours
All, of the required maintenance inspections and checks on the aircraft had been performed in accordance with Northwest Airlines approved
directives.
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27
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NATIONAL TRANSPORTATION SAFETY BOARD
WASHINGTON, D.C.
APPENDIX D
ISSUED: March 20, 1975
.........................................
Forwarded to:
Honorable Alexander P. B u t t e r f i e l d
Administrator
Federal Aviation Administration
Washington, D. C. 20591
The National Transportation Safety Board i s investigating t h e
Northwest Airlines, Inc , Boeing 727, N27bUS, a i r c r a f t crash which
occurred near Thielle, New York, on December 1, ,1974. The Board I s
continuing investigation has revealed that i ~ ~ b l o c k et hde p i t o t heads.
.
A preliminary review of t h e evidence i n t h i s accident suggests
t h e p o s s i b i l i t y t h a t t h e crew concentrated on a i r data instrumentation
t o t h e exclusion of a i r c r a f t a t t i t u d e indications. The timely use of
t h e a t t i t u d e information may have prevented t h e s t a l l and subsequent
crash.
About 5 minutes before t h e rapid descent, t h e f l i g h t data recorder
(FDR) recorded aberrations i n t h e airspeed t r a c e . These aberrations
were caused by t h e closure of t h e ram a i r i n l e t and t h e d r a i n hole of
t h e p i t o t mast. These aberrations were v e r i f i e d by wind-tunnel i c i n g
t e s t s of a p i t o t mast and pneumatic t e s t s of an a l t i m e t e r and airspeed
system. These t e s t s produced airspeed/altitude t r a c e s similar t o those
recorded on t h e FDR.
The Safety Board i s aware of other incidents i n which an a i r c r a f t
encountered d i f f i c u l t i e s while f l y i n g i n freezing p r e c i p i t a t i o n because
of a l a c k of p i t o t heat. I n these incidents, t h e flightcrews recognized
t h e problem and took corrective action.
Evidence i n t h i s case indicates t h a t t h e p i t o t heater control
switches were not on, although t h e heaters were capable of operation.
The a i r c r a f t had been f l y i n g i n clouds and freezing temperatures.
Recently, one air c a r r i e r reported that it i s operating i t s p i t o t
heater system continuously and t h e f a i l u r e r a t e i s minimal, i.e., one
element f a i l u r e per a i r c r a f t per year. Several other a i r c a r r i e r s a r e
a c t i v e l y considering t h e i n s t i t u t i o n of a similar procedure, and they
believe t h e r e would be no adverse a f f e c t on t h e l i f e of t h e p i t o t heater
elements
.
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28
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2
APPENDIX D
Honorable Alexander P. B u t t e r f i e l d
-
-
The National Transportation Safety Board believes t h a t c o r r e c t i v e
a c t i o n i s necessary and recommends t h a t t h e Federal Aviation Administration:
1. Issue an Operations B u l l e t i n t o a l l a i r c a r r i e r and
general a v i a t i o n inspectors t o s t r e s s t h e need f o r
p i l o t s t o use a t t i t u d e information when questionable
information i s presented on instruments t h a t a r e
dependent on t h e a i r d a t a system. The information i n
t h i s B u l l e t i n should be disseminated t o a l l operators
f o r incorporation i n t o t h e i r operations procedures and
t r a i n i n g programs. ( c l a s s 1)
2.
Issue an Airworthiness Directive t o require t h a t a
warning system be i n s t a l l e d on t r a n s p o r t category
a i r c r a f t which w i l l i n d i c a t e , by way of a warning l i g h t ,
when t h e f l i g h t instrument p i t o t heating system is not
operating. The warning l i g h t should operate d i r e c t l y
from t h e heater e l e c t r i c a l current. ( c l a s s 2 )
3.
Amend t h e applicable Federal A i r Regulations t o require
t h e p i t o t heating system t o be on any time e l e c t r i c a l
power i s applied t o an a i r c r a f t . This should a l s o be
incorporated i n t h e o p e r a t o r ' s operations manual. ( c l a s s 2 )
Our s t a f f i s a v a i l a b l e t o assist your personnel i n t h i s matter, i f
desired.
REED, Chairman, McADAMS, THAYER, BURGESS, AHD HALEY, Members, concurred
i n t h e above recommendations.
Chairman
APPENDIX D
THE
SECRETARY OF TRANSPORTATION
WASHINGTON. D.C.
20590
March 13, 1975
Honorable John H. Reed
Chairman, National Transportation
Safety Board
800 Independence Avenue, S.W.
Washington, D. C. 20591
Dear Mr. Chairman:
This is to acknowledge receipt of your letter of March 12 enclosing
a copy of a safety recommendation to the Federal Aviation Administrator
concerning the Board's investigation of the Northwest Airlines, Inc.,
Boeing 727; N274US, aircraft crash which occurred near Thielle, New
York, on December 1, 1974.
The recommendations are receiving attention by the Department's
Assistant Secretary for Environment, Safety and Consumer Affairs,
as well as other appropriate ~e~artmental
officials.
Sincerely,
-\
William T. Coleman, Jr.
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30
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APPENDIX D
DEPARTMENT OF TRANSPORTATION
FEDERAL AVIATION ADMINISTRATION
OFFICE OF
THE ADMINISTRATOR
MAY 2 7 1975
Notation 1481
Honorable John H. Reed
Chairman, National Transportation Safety Board
600 Independence Avenue, So W.
Washington, D. C. 20594
Dear-Mr. Chairmana
This is in response to your letter of March 12 which transmitted
NTSB Safety Recommendations A-75-25 thru 27.
Recommendation No. 1.
Issue an Operations Bulletin to all air carrier and general aviation
inspectors to stress the need for pilots to use attitude information
when questionable information is presented on instruments that are
dependent on the air data system. The information in this Bulletin
should be disseminated to all operators for incorporation into their
operations procedures and training programs. (Class 1)
Air Carrier Operations Alert Bulletin 75-3 dated February 13 covers
this subject. A Part135, Air Taxi Bulletin, is being prepared. We
are also considering the issuance of an advisory circular on the
subject.
Recommendation No. 2.
Issue an Airworthiness Directive to require that a warning system be
installed on transport category aircraft which will indicate, by way
of a warning light, when the flight instrument pitot heating system
is not operating. The warning light should operate directly from the
heater electrical current. (Class 2)
Comment.
We do not concur in this recommendation. Some current aircraft have
cycling types of pitot heaters. These cycle on and off as controlled
by thermostats or timers. Warning lights would flash on and off with
APPENDIX D
t h e cycling. We consider this a s distracting and possibly detrimental
t o safety. Other a i r c r a f t i n which t h e p i t o t heat i s controlled d i r e c t l y
by a simple on-off switch could be modified by adding a power relay and
siarning light. We do not consider this necessary or desirable. Operation
of p i t o t heat i s on cockpit checklists and i s well covered in operations
manuals and crew training. In addition, t h e effectiveness of additional
warning l i g h t s among t h e many warning l i g h t s presently i n s t a l l e d in t h e
cockpit i s of doubtful value.
Recommendation No. 3.
Amend the applicable Federal Air Regulations t o require t h e p i t o t heating
system t o be on any time e l e c t r i c a l power i s applied t o an aircraft.
This should also be incorporated in t h e operator's operations manual.
(Class 2)
Comment.
!l'his recommendation i s considered t o apply t o all. types of a i r c r a f t in
service and t o f u t u r e designs. We propose t o delete from consideration
those a i r c r a f t which a r e limited t o 7FR f l i g h t only since they a r e not
required t o have any deicing capabilities.
Retrofit on existing a i r c r a f t presents many problems and we do not
consider t h e recommendation practical f o r general adoption. Some
cyclic i n s t a l l a t i o n s will not t o l e r a t e continuous heat and would have
t o be completely replaced. Continuous heat would be unsafe in many
circumstances such a s extended parking with e l e c t r i c a l power on. A s
you mentioned, r e l i a b i l i t y would be reduced leading t o more frequent
unsafe conditions in flight. We do not consider r e t r o f i t of existing
a i r c r a f t practical or feasible.
For new designs t h e recommendation may be feasible because t h e i n s t a l l a t i o n s
can be safe and r e l i a b l e by design of interfacing e l e c t r i c a l power systems,
positioning of p i t o t tubes, and construction of p i t o t tubeso A regulatory
project leading t o a Notice of Proposed Rule Making and subsequently a
r u l e requiring an appropriately designed p i t o t heating system i s being
established.
Sincerely,
@P\&/
:
ames E. Dow
V ~ c ing
t Administrator
Intentionally Left Blank
in Original Document