A Deep
Stall is the blanking of the horizontal stabilizer by wake off the wing
at high AOAs, resulting in loss of elevator effectiveness. When
this situation develops, the airplane becomes trapped in a high AOA -
high sink rate, sometimes called a "super stall" - unable to
recover using normal pitch controls or engine power. It is a
nasty by-product of airplanes with T-Tails.
The Deep Stall entered the aviation world, as I recall, with the advent
of the first T-Tail jet transports, starting with the Trident and
BAC-111 in the early 60s. A number of fatal accidents ensued,
involving the Trident, BAC-111 and Canadair Challenger, despite the
presence of professional test pilots. One accident I recall
happened despite the fitment of a special tail parachute intended to
pitch the nose over while the phenomenon was being explored. The
Piper Tomahawk, a T-Tailed two-seat trainer, also has a checkered
career, with a series of accidents bearing Deep Stall
characteristics. Lew Wallick told a story of flying the 727
during cert testing with a certain FAA pilot, that resulted in an
"exciting event and recovery", believed to be a brush with Deep Stall,
although he never elaborated in public just what the details
were. Legend around here has it that Boeing forswore ever again
using a T-Tail configuration on another airplane - although the YC-14
did have that config in the 70s. And then.......
A very interesting T-Tail aircraft Deep Stall Accident involving an MD-82
In 2005, an accident occurred to an MD-82 that apparently bore the characteristics of a Deep Stall. The report, released in 2010, proved an interesting read and motivated me to contact my friends. Their comments created an interesting history of the Deep Stall on the DC-9 and 727 series airplanes, which is provided below.
The accident involved a West Caribbean MD82 at Machiquez, Venezuela on Aug 16th 2005, which failed to recover from high altitude stall. An excellent report on this was created by Simon Hradecky, Sep 5th 2010, and can be found here.
In this accident, the pilot attempted to climb from FL310 to FL330, too high for the aircraft's weight. Complications arose due to reduced power from the use of engine anti-icing. The stab was continuously retrimmed by the autopilot until it reached 10.8 units nose up at an AOA of +7.7. It remained there for the rest of the flight. The autopilot was trimming nose up to maintain FL330 while the autothrottles were trying to maintain M 0.75. When the engines couldn’t maintain the speed, it decayed - requiring the autopilot to trim more and more nose up to maintain the altitude. The speed decayed to M 0.60 and the airplane started descending, unable to maintain the selected altitude. The pilot then disconnected the autopilot with the severe mistrim now in place. He never corrected it. 40 seconds later, with the autothrottle still engaged, the stick shaker activated and the stall warning went off. The airplane entered a deep stall from which it never recovered.
The crash site (Photo: JIAAC):
DEEP STALL
I asked my colleagues, many line or test pilots, how they would have tried to recover from the bad situation. Some interesting history emerged.
My DC-9 / MD-80 knowledge is limited, but a number of Mad Dog drivers are on my correspondence list - so I asked whether they could elucidate me and the rest. Specifically, I wondered whether Douglas explored or experienced the phenomenon on the DC-9 series, and whether special pilot training was used. I know Stick Pushers were developed for a number of airplanes to deal with the problem.
In the accident case involved, it seemed deceptively simple for the unwitting pilot to slide into the situation where suddenly, insidiously - "he was there." The actions involved changing altitude and configuring the autopilot systems to perform an altitude hold or Mach number hold, seemingly like the chores performed daily. Perhaps he wasn't paying the kind of attention he should have, and the arrival of his crew meal at a critical point was a distraction. Still, the sequence seemed like a "There, but for the grace of God" situation. Once there, it was quite clear what was happening - with the co-pilot calling out "Full Stall". But, what to do? If anything................ The engines were losing power confusing the analysis and leading the Capt to believe he had experienced a double engine loss.
With the airplane seemingly locked into a deadly configuration, and the elevators useless, the engines doing strange things, and the altimeter unwinding rapidly (300 fps) - what, I pondered, could be done to recover and break the stall. I got to thinking about what still worked.
How about dropping the gear? Or extending the flaps? Both should produce nose down pitching moments.
Using reverse thrust (assuming that is possible while airborne in an MD-80)? Would that help....or hinder.
I thought of what else still worked. The ailerons still worked. How about rolling inverted? Or better yet, rolling 90 degrees with the expectation and hope that the nose would fall through? Maybe kicking in same side rudder to force the nose down? The 727 was reported to have had a Deep Stall encounter during its flight test program. A recent magazine article about test pilot Lew Wallick and this event. talkled about it, but without the specific details I was looking for.. Perhaps my Boeing pals would know more?
So, there you have it. Maybe this is an exercise in futility. Deep Stalls, I thought, were something from the aviation historical past, until - until that is - I read this recently released report on a 2005 accident. To my experienced pilot brethren out there - what would you do? And would it work?
Remember - here's the situation: A simple climb from FL310 to FL330. A difficulty making the climb due to airplane weight and engine limitations. The interaction of Alt Hold, Mach Hold, Eng Anti-Ice. The airplane enters a stall. The stall warning goes off. The stick shaker is activated. The co-pilot is yelling "Full Stall". Your meal tray is on your lap. The AOA is high and the engines seem to be spontaneously losing and recovering power. Airspeed falls to 150 KIAS at FL240 (M.38). You think the engines have flamed out. The elevators have become useless and the altimeter is unwinding rapidly. THE VSI is pegged and the sink rate increases to over 17,000 fpm. 159 other people depend on you for their survival. Oh! And it's dark. You have less than two minutes......
Imagine the investigators dilemma if there had not be fully functioning FDR/CVR recorders.
Some interesting answers.......
I received a number of really interesting responses which documented
the history of Deep Stall activities, involving the 727 and DC-9.
Some of these answers are shown below.
I dealt with the problem of deep stall in configurating both the YC-14 and the B-757.
I wasn't directly involved with the aerodynamics of the 727, so all my
observation are from my memory of viewing the program from the outside.
(I was Chief of Aerodynamic Research at the time.)
As I recall, the wind tunnel data on the 727 showed a strong pitch down
as the airplane entered stall (with the flaps down), and this moment
continued up to the 15-20 degrees angle of attack that the test was
run. It wasn't until after the fatal accidents of the BAC 111 and the
DH 121 (Trident) that tests were carried out to extremely high angles
of attack, about 45 deg., I think, and these data showed a strong
pitch-up loop that barely remained nose-down throughout the alpha
range. A strong nose-down moment showed at the highest angles of
attack, as these were beyond the pitch-up loop. I understood that a
flight test was done, and that the airplane barely came out of the deep
stall. Everybody agreed never to try it again. Again, my memory tells
me that the wind tunnel data showed that a small slideslip produce a
large nose-down pitching moment, allowing deep-stall recovery. However
at such high angle of attack, coupled with the high sweepback of the
vertical tail made it very difficult to induce a sideslip. I think that
flaps up, the 727 did not have a locked in stall problem. All the guys
that really would know, like Dave Norton or Jerry Bowes are no longer
around to ask. Bill Cook might remember.
On the YC-14, we considered putting the tail on the body where
locked-in stall would not be a problem. However, the YC-14 approached
landing at a very high lift coefficient with corresponding high down
wash. Ground effect markedly reduced this downwash and the download on
the tail. The changes were so large and quick, that the stabilizer trim
system couldn't keep up with it, and the elevator wasn't powerful
enough to compensate and still leave enough travel to provide control.
NASA data showed that if the tail were high enough and not too far aft,
locked in stall could be avoided, and that is the position we chose.
Wind tunnel data, even at high thrust, showed we were OK. The airplane
never was stalled at the thrust levels we used for landing approach
because the safety policy was to never fly the airplane below its
single-engine-out control speed. The YC-14 had no appreciable sweepback
on the wing and that fact eased the potential for locked in stall as
there was less tendency for tip stall than on a sweptback wing, and any
tip stall that did occur did not induce high pitching moments on the
wing.
I was not aboard during the 727 episode you described but a mutual
friend of ours, an S&C weenie, told me he was on that flight.
According to his account, there was some lateral control effectiveness
still available and Lew used what there was to rock the airplane to
increasingly higher bank angles until the nose finally fell through and
normal control response was recovered. Unfortunately, Lew is no longer
with us and I have heard nothing from our mutual friend for many years
so I have no way of confirming my recollections. Obviously there must
have been several guys from Flight Test involved and you may hear the
story first-hand.
****************************
One
of my S&C friends, J.M., was on that flight and I remember his
description of the event very clearly. Apparently it required
several oscillations before Lew built up enough amplitude ( bank angle
that is ) to make the nose fall through and Jimmy reported that the
swish-swish sound as they went through bottom dead center on each cycle
was positively eerie. All aboard were mighty lucky that the
incomparable Samuel Lewis Wallick was at the controls because the guy
had nerves of steel and a very quick mind.
[RAB Note: I was glad to hear that my proposal to use what works
- namely the ailerons - to roll to a high bank angle and let the nose
fall through, actually was a viable solution, and, apparently, was used
by Lew.]
Update April 2011: At USAF test pilot Guy Townsend's funeral on April 18th, Bill Cook was in attendance. Bill is an amazing 98 and still sharp as a tack. So - here was my chance - after the service, I asked him, "How did Lew get out of the Deep Stall?" Without a moment's hesitation, he said; "Rolled her on her side and the nose fell through."
So -- there you have it!
Well - not so quick; more responses
were obtained - including the following DEFINITIVE one. This is an
interchange with Boeing pilot and engineer Jess Wallick, who was Lew's
brother and also was sitting behind him during the deep stall flight in
question. The 727 Deep Stall occured during a flight with an FAA
pilot aboard. It was made with the speed brakes up. I went
to a Flight Test reunion and asked and they assured me they never did
stalls with the speed brakes up - it was not a cert requirement and was
not a flight test item.
| Hello Bob, Some more information on the 727 deep stall. I don’t know how many deep stalls Lew was exposed to, but I rode thru one. It was with flaps down (15 or 25 degrees) and speed brakes up. It was an FAA cert flight with FAA pilot (deleted) who got us into the stall, Lew got us out. He did not roll on the side, he did his best to keep wings level. We rolled from side to side, approximately 30 degrees, like doing a falling leaf. The flight test data showed zero airspeed for approximately 16 seconds (seemed much longer). There is a good article about this stall in the “FLIGHT JOURNAL” of December 2008, written by Lew’s daughter Rebecca. All the best, Jess Wallick Hi Bob, More on the deep stall, I did talk to Lew, he said he did his best to keep the blue side up. He was concerned that if the roll increased and the nose dropped we would be in a spin and be hard to recover. I was there, in the first jump seat. He never tried to roll the airplane. He took control, dropped the speed brakes , retracted the flaps to 5 degrees, pushed the thrust levers a bunch, and recovered control. Lew said that he could see that the nose was dropping, very slow at first. All the best, Jesse Wallick |
After
some BAC111 and Trident accidents, the UK CAA became very sensitive to
t-tail stall characteristics. It was I think 1974 or thereabouts
when Dan-air bought some used 727-100's; prior to then neither the
727-100 nor the -200 had been UK-certified. The CAA required the
retrofit of a stick pusher [the cert flights must have been really
scary] before they would approve the airplane. They also required
installation of -200-type exits aft of the wing.
During
1973-74 I was in S and C during initial development of the 727-300.
Boeing could not get the stall characteristics to work in the wind
tunnel. Then E002 was outfitted with extended [fixed] main gear and
[get this] inverted [also fixed] stab leading edge flaps to increase
stab down-force effectiveness for takeoff rotation and low-speed
flight. It still didn’t work. The E002 mods were not
reversible; the airplane was scrapped soon after.
So the 727-300 became a long-body 727 with two underwing engines and a
t-tail. Then the T-tail became a conventional tail. Then
the 767 flight deck was grafted to the nose, and the 757 was
born.
When
I got aboard the 757 program, the sale had just been made to BA and
Eastern and the configuration still included the 727 T-tail. I spent a
lot of time trying to define pitch stability criteria that would be
satisfactory and still accommodate the pitch-up sure to come with the
T-tail. I was trying to allow x seconds from the time a pilot would
recognize pitch-up and then he could pitch-out using the elevator. Ken
Holtby, who was tracking both the 757 and 767, wouldn't buy it (smart
guy!), finally telling me to stop trying to write my criteria around
the configuration. I then created the following criteria. "With the
stabilizer in position to trim the airplane in final approach, the
pilot can push out of any angle attack he can pull into using the
elevator". Ken accepted this criteria, and that forced the tail onto
the body. I think that criteria is a good one, and I think something
like is still in use today. The program managers on the 757 didn't like
giving up the 727 tail because they thought they could save money
thereby, but they really didn't have any choice. I remember late one
afternoon, T Wilson was hunched over a drafting board looking at the
configuration and remarked (and this is a pretty direct quote), "Don't
bust your ass trying to save the 727 tail". (Wasn't it great when the
top management knew something about what had to be done to get an
airplane designed!) With the horizontal tail on the body, the vertical
tail now could be made from the upper part of the 767 vertical tail
(again at Ken's insistence), so we saved some money doing that. I was
glad to get rid of the T-tail for lots of reasons, including ground
de-icing during snowstorms and jackscrew and elevator actuator
maintenance.
Incidentally, body-mounted tails have less
trim drag than do T-tails. The downwash behind the wing tilts the usual
down load vector of the horizontal tail forward, decreasing the trim
drag. (It so happens that it just compensates for the induced drag of
the the extra lift carried by the wing because of the down load on the
tail.)The T-tail is in a reduced downwash field, so gets less of this
gain. But that's another story.
As to what the MD-82 pilot could have done to break out of the stall, I received the following comment and analysis:
After
reading the stuff you sent out on the MD-80 deep stall, I drew a pile
of pitching moment curves that would result from trimming into the
stall, like apparently was done in that case, rather than using the
elevator, to see if my criteria would still work. I believe it would.
All the pilot would have to do would be to retrim into the normal
flight condition, and he then could pitch out with the elevator.
Some Recollections of the DC-9 Deep Stall
Jerry Lundry
October 19, 2010
Revised October 21, 2010
During the early development of the DC-9, I worked at Douglas Aircraft in Long Beach, CA and in the Aerodynamics Fluid Mechanics Group, an applied R&D unit. One of my roommates worked in the Wind Tunnel Testing Group, and some of what follows is based on comments he made at the time.
In the timing of the early small jet transport market, the DC-9 program was preceded by the BAC-111 program, and, in turn, preceded the 737 program. During flight test, two BAC-111s experienced Deep Stalls and crashed. The flight test crews were killed in these accidents in which the airplanes hit the ground essentially level in pitch attitude, at little or no forward speed, and at a high rate of descent.
My memory of the press accounts at the time indicated each accident started with the airplane in low pitch attitude and low or idle power. As the airplane slowed, drag increased, but the airplane remained level. Rate of descent increased gradually and substantially, even though the airplane fuselage was approximately parallel with the horizon. The BAC-111 pilots did not immediately notice the increasing and ultimately high rate of descent. The combination of high rate of descent and level pitch attitude produced a high angle of attack, well past stall.
At the time of the second accident, the DC-9 program was either about ready for first flight or had just accomplished it. The flight test program focused on the first model, the DC-9-10. When the source of the BAC-111 accident was identified as "Deep Stall," the question at Douglas Aircraft came promptly: "Does the DC-9 have it, too?" Both airplanes had T-tails.
The Chief of Aerodynamics at the time, Dick Shevelle, told the DC-9 aerodynamicists and the Wind Tunnel Testing Group to "Get something in a wind tunnel --- fast!"
The only immediately available wind tunnel was a small research facility that had been used for some fundamental boundary layer experiments conducted for the Office of Naval Research by Darwin Clutter and A. M. O. Smith on surface roughness and boundary layer transition. Someone had made the fortunate decision to move this facility from the El Segundo Plant to the Long Beach plant when the former was closed in 1962.
This facility had a maximum test section speed of about 100 feet per second of extremely low turbulence air. There was no return leg --- just the nozzle, the test section, and the diffuser. There were several different turbulence screens, used in various combinations, upstream of the test section. The drive system was at the end of the diffuser. Some members of the Wind Tunnel Testing Group referred to this facility in jest and somewhat sarcastically as "the flow generator," deeming it too small and under-powered to be a proper "wind tunnel."
Nevertheless, it was available immediately. Members of the Wind Tunnel Testing Group went to Dick Shevelle's office and confiscated his large, beautiful display model of the DC-9. It was over-painted suitably for flow visualization, and rigged with a mount. In less then a week, the answer to the question came back: "Yes, we have Deep Stall, too."
The flight test DC-9s was placarded against stalls, with a generous margin, and flight tests proceeded according to schedule, except for stalls. Meanwhile, an intense effort focused on finding a "fix." However, someone found time to define a Deep Stall. My paraphrasing of this definition is: "A high-angle of attack condition, stable in pitch, well past the stall angle of attack, for which longitudinal control is inadequate to effect recovery from stall."
Wind tunnel flow field visualization confirmed the horizontal tail was immersed in the wake of the stalled wing over a range of high angles of attack. Local flow speeds near the aft fuselage and empennage were a modest fraction of flight speed at these conditions, greatly reducing pitch control effectiveness.
Many configuration changes were developed and tested (possibly in the GALCIT Wind Tunnel at Cal Tech or at NASA-Ames). No single change was completely effective.
A larger horizontal tail (focused on increased span) provided significant, but inadequate, improvement.
A vortex-generating pylon on the undersurface of the wing and just aft of the leading edge also provided significant, but inadequate, improvement. At cruise, the pylon was attached to the lower wing just aft of the local wing leading edge stagnation line, where local flow speed was low, so that its cruise drag was less than nominal. At high angles of attack, the stagnation streamline moved well aft of the pylon leading edge, with local flow moving forward past the pylon. In this locally-reversed flow field, the pylon produced a vortex that swept forward along the lower wing surface, up past the wing leading edge, aft over the wing, and into the horizontal tail flow field. The vortex was thought to improve both the separation pattern on the wing and downwash at the tail, and possibly provide an increase in local flow speeds near the tail. This pylon extended aft for more than half of the local wing chord.
A short-chord fence was also used outboard of the vortex-generating pylon. This fence was conventional, as it extended around the wing leading edge on both upper and lower surfaces.
There might have been other fixes but I do not recall them. In the end, the only change that proved adequate was a combination of fixes, including the three described above.
A meeting had been called to report on Deep Stall progress with the President of Commercial Airplane Division, Donald W. Douglas, Jr. Costs estimates had been prepared for each of the individual changes.
There was a lot of nervousness about "Junior's" reaction to spending as much as $10M on a combination of fixes, a large amount of money in the 1960s. He had not been President very long, and was about to make a huge decision for the future of the Company. Some anticipated a decision to delay, with direction to find a lower-cost fix, resulting in probable overall program delay, and all of the expense that would entail.
The meeting with Mr. Douglas was short. When the options had been presented, he directed immediate implementation of the combined changes. He was quoted as saying, "We have no other choice." A lot of people in Engineering gained a huge respect for Mr. Douglas on that day.
Someone did not like the name, "vortex-generating pylon." It became the "vortilon." I believe this was the origin of both the name and the device, which is used on business jets today.
The vortilon was used on all of the DC-9 models, and the MD-80. I do not know if it was used on either the MD-90 or the MD-95.
The original flight test program had focused on the DC-9-10, which had the shortest fuselage, and hence, the shortest tail arm for effecting pitch stability and control. The DC-9-15 and DC-9-20 also had this same fuselage length. However, these later models did not have the wing fence, according to: http://www.airlinercafe.com/page.php?id=396. I do not know the reason(s) for eliminating it.
These shortest-fuselage models were all able to demonstrate acceptable stall recovery (certifiable) at aft CG locations with the Deep Stall modifications, but such recovery was less than ideal and could not have been described as robust. I presume "acceptable" meant that stalls could be entered and a pitch attitude higher than that for maximum lift could be obtained, followed by a normal, if slow, stall recovery.
The DC-9-30, -40, and -50 each had increasingly longer fuselages than their predecessors. The MD-80 and -90 family fuselages were also longer than that of the DC-9-10, -15, and -20. All thus had longer tail arms than the initially certified DC-9-10, favorably affecting both pitch stability and control, and very possibly producing slightly weaker downwash at the tail, as the tail was further aft from the wing. Tail entry into a stalled-wing wake would occur at a higher angle of attack than for the shorter fuselage models.
Thus, all models of the DC-9 and MD series were, and are, capable of recovery from a stall, provided the pilots identify the initial stalling condition and respond appropriately.
One interesting aspect of the DC-9 Deep Stall effort was the pitch control system. Douglas Aircraft used trim tabs to drive the control surfaces on all large commercial products through the DC-9. This was investigated as a possible contributor to the DC-9 Deep Stall, but was not a factor.
Douglas had used trim tabs to actuate control surfaces since the DC-3, and had not changed this practice, even with the DC-8 and DC-9 jet transports, probably as a result of "technology inertia." This use was questioned again after DC-9 drag had been determined in flight test.
My Fluid Mechanics supervisor, Ed Rutowski, explained the DC-9 drag issue and asked for suggestions to resolve it. The issue, as Ed put it to me, was that "We lucked out on drag. Our basic drag level is high by about 5% but our compressibility drag at cruise is low by about 5%, so we are OK for cruise drag."
Ed then asked for ideas for the source of the high basic drag level. I mentioned several possibilities. All but one had been considered. The exception was gap drag. I asked about the control surface gaps and how they were sealed. Ed told me they were not sealed, as the control surfaces were driven by trim tabs. The control surface gaps needed to be open for aerodynamic balancing, so that trim tabs could provide moments adequate to drive the primary surfaces. I suggested those gaps were a likely source for at least part of the drag, and mentioned that Art Mooney sealed the control surfaces gaps on his later, highly efficient light planes with strips of fabricate to prevent flow through those gaps and the resulting drag. Others people might also have identified this candidate drag source.
Ultimately, a senior aerodynamicist, Frank Lynch, managed two high Reynolds number wind tunnel tests to measure gap drag. A 6% scale DC-9 horizontal tail was tested in the North American Rockwell Tri-sonic Wind Tunnel.
Frank concluded the DC-9 control surface gap drag was about 5% and control surface float drag was about 1%. For the DC-8, these drags were 4% and 2%, respectively.
As a result of these tests, the DC-10 became the first Douglas commercial transport going back to the DC-3 to use powered controls and in all three axes. The DC-10 gap drag was estimated as "less than ½ %."
I still have a copy of Frank's memo in my files.
727 Deep Stall Accident
The 727 was, in fact, involved in a Deep Stall accident -
this occurred to a NW -200 on a ferry flight enroute BUF to pick up an
NFL team after its game (1 Dec 1974.) The airplane was climbing out of
JFK in icing conditions when the pitots iced over; the crew had
forgotten to turn them on (or actually, it is speculated, the low time
co-pilot (46 hours in the airplane) can be heard to apparently turn
them off. Responding inappropriately to the erroneous Air Data
information, showing an increasing airspeed, altitude, and
rate-of-climb, the co-pilot, who was flying the airplane, continued to
raise the nose until the overspeed warning went off at FL230. The
erroneous indications showed an AS of 405 kts and a 6500 fpm rate of
climb. The Capt commented: “...just pull her back, let her
climb.” The stick shaker then activated at 420 kts (as recorded
on the FDR.) The F/O then misinterpreted the buffet as Mach
buffet, as the airspeed was likewise erroneously increasing, and
applied yet more back force to the controls. The stall warning
continued as the F/O said “There’s that Mach buffet, guess we’ll have
to pull it up,” followed by the Capt’s response “Pull it up.” Two
seconds later, the aircraft began descending (in a stall) at 15,000
fpm. 43 seconds later, the crew transmitted a “Mayday -
we’re descending through 12; we’re in a stall”, and extended the flaps
to Position 2. Five seconds later, the final CVR dialog was
spoken by the co-pilot “Pull now....Pull; that’s it.”
The entire sequence from FL24.8 to impact consumed 83 seconds.
Vertical loads exceeded 5 G’s. The LG and spoilers were retracted,
Flaps at 2 units, and most of the LH horiz stab had separated in
flight. The engines were at Idle thrust. The stab trim was
found at 1.2 units Nose Up. The entire wreckage was found in an
area that exactly matched the dimensions of the aircraft’s planform
(similar to the MD-82.)
Analysis showed the apparent airspeed and climb rate exceeded the
performance capability of the aircraft. Pitch attitude at stick
shaker (controlled by the AOA vane, independent of the Air Data probes)
was 30 degrees nose up with an IAS of 165 kts. Stick shaker goes
off at an AOA of 13 degrees; the actual calculated AOA was 22 degrees
or greater during the descent.
So - was this a Deep Stall accident? The answer is decidedly No,
if you define a Deep Stall as an airplane attitude and flight condition
from which the pilot is unable to recover using pitch and thrust.
During flight tests, the airplane was stalled at AOAs of 25 degrees and
recovered by relaxing the pull force on the control column. With
the use of thrust during recovery, altitude lost was limited to about
2000 ft. Data shows that the AOA can be decreased and stall
recovery effected by pushing on the column.
Was this a Deep Stall accident? The answer is Yes if you
eliminate the “unable to recover” aspect of a “true” Deep Stall.
This airplane entered a pilot induced Deep
Stall condition and remained in that condition from inducement by the
F/O until impact. The crew fixated on the Air Data indications
and ignored their Attitude references, which, at 30 degrees nose up,
showed an attitude about 25 degrees greater than normal. The
co-pilot maintained, with the Capt’s concurrence, heavy back pressure
from before entering the stall, through the entire 25,000 ft descent,
until impact with the ground.
They could have
recovered, according to the NTSB report, for up to 40 seconds after
entering the stall, by merely - as a minimum - relaxing back pressure
on the control column. In this regard, their situation was even
more benign than some other accidents described in other pages of this
section, because the stab was in a near neutral position and not badly
mistrimmed, as, for example, was the MD-82. Even with the MD-82, if the
analysis of my correspondent noted above is correct, the airplane could
have been recovered if the stab had been trimmed nose down accompanied
by nose down elevator. More about this on other parts of this
web-site sub-section.
Although pilots train for “partial panel” flight at an early stage in
their flight training, experience has indicated that commercial pilots
can do poorly in that arena, especially when Air Data information is
involved. Two 757 accidents in Latin America were the result of
perfectly good airplanes crashing into the sea after experiencing
impaired Air Data functions. Northwest’s Paul Soderlind was a
pioneer in this arena, starting with the 720B accident, and was a
staunch advocate of Attitude and Power, but his message and training
did not always get through - at least not in some cases. Not even in
his own company. http://www.avweb.com/news/profiles/182945-1.html
Study of this accident is worthwhile today, not just in the context of
the recent MD-82 Deep Stall accident, but as it relates to the on-going
investigation into the loss of AF 447. This A330, as I have
reported previously, likely experienced pitot probe icing, and
transmitted a stream of ACARS messages that related to Air Data
anomolies, including loss of airspeed calculations (green dots) on the
PFD, rudder travel limits, CAS and Mach values outside certain limits,
and, importantly pitot probes - reflecting changes in recorded AS of
more than 30 kts in 1 second.
While both the NW and AF crews were faced with confusing and erroneous
Air Data indications, one important difference is the effect of bad
data on the A330's highly automated FMC and FBW systems.
The loss of valid Air Data inputs on the A330, per the BEA Interim
Report, triggered loss of Flt Director Function, Autopilot,
Autothrottle, Rudder travel protection, speed calculations, wind shear
protection, and a switch to Alternate 2 Control Law, which then
resulted in a list of further degradations to normal protections.
Although my primary interest was in air transport airplanes, I received this fascinating email regarding Deep Stalls in high performance fighter type airplanes:
Hello, Bob,
My name is Bob Hoey. I am a retired flight test engineer who worked for the AF from 1955 to 1987 at Edwards AFB. Dave Kerzie sent me your email regarding the analysis of AF447. I am in total agreement. I also looked at your excellent description of the deep stall phenomena where you suggested that the Trident and BAC 111 in the early 60's were the first to encounter deep stall. I believe the first demonstration of a a classical T-tail pitch-up and deep stall was in the XF-104 in 1954 or 55 when Chuck Yeager made the first AF flight in the new airplane. I was the AF flight test project engineer for the AF Stability and Control tests of the F-104A in 1957 and the pitch-up and subsequent deep-stall phenomena were well known.
Lockheed installed stick shakers and stick pushers to prevent high angle of attack flight. (The F-101, another fighter with a low aspect ratio wing and T-tail was also in early testing at the same time and had similar problems.) A spin program was conducted by Lockheed on the F-104A. The F-104 had considerable gyroscopic effect from the single rotating engine. This, combined with the anhedral in the wings, caused a rather violent lateral oscillation in the deep-stall region which eventually resulted in a nose slice bringing the AOA down and allowing recovery (if there was enough altitude!!).
Chuck Yeager's later flight in the NF-104A (1967 with the rocket) demonstrated the true deep stall characteristics of the F-104 configuration. On this zoom mission, where the engine was shut down at high altitude, the RPM had dropped to almost zero thus negating the lateral oscillation normally associated with an F-104 at high AOA. The airplane stabilized in a deep stall and stayed there all the way down. (Chuck ejected at 7,000 ft.)
I believe the early wind tunnel tests on the F-104 went to 20 to 25 degrees - high enough to identify the sharp reversal in the pitching moment curve (and subsequent pitchup typical for T-tails), but not high enough to identify the stable trim point at about 53 degrees AOA. Lockheed's early efforts were merely to avoid the pitchup region with flight control features. I have seen subsequent wind tunnel test of the F-104 all the way to 90 degrees AOA conducted in a low speed tunnel, and the deep stall is plainly evident, even with full aircraft-nose-down stab deflectlion.("Low Speed High Angle of Attack Wind Tunnel Tests of an .0858 Scale Model F-104A With and Without Drag Parachute" Lockheed A.C. Report No. LAL 343, 11 Dec 1958)
The F-16 also exhibits a deep stall characteristic, probably related to its aft cg (about 6% unstable subsonically) since it does not have a T-tail. Spin tests showed that it could be recovered from the deep stall by rocking the airplane fore and aft with the stick.
Fascinating
subject. Of course, as you mentioned, the current fighters (F-22, F-35)
"enter and leave" the deep stall region routinely using thrust
vectoring.
Were you ever involved in model airplanes?
Specifically the competition free flight models? These airplanes
quickly evolved into essentially tandem-wing cofigurations with lifting
horizontal tails of 40-50% wing area, and cg locations at 70 to 80% of
the wing chord. (Made for rapid recovery from stalls). In the late 30's
maybe early 40's someone (?) created a "dethermalizer" to keep from
loosing airplanes in thermals. The horizontal tail was pivoted at the
leading edge and the trailing edge attachment was released with a fuze
or timer. The horizontal tail unloaded and rotated to an angle of about
-70 degrees, the airplane entered a deep stall in a level attitude, and
the airplane descended like a parachute. Wing loadings were very low so
descent rates at landing were fairly gentle. This concept is still in
use by modellers, and works very well.
Burt
Rutan has carried the concept a little farther and used the "deep
stall" idea for a vertical reentry from space with SpaceshipOne.
Bob Hoey
Revised 8 Sept 2010
Revised 20 Apr 2011
Revised 2 Jun 2011
Revised 11 Sept 2016