Stall recovery technique
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Stall recovery technique
Hi all,
I have been reading a few accident cases and an underlying theme seems to be incorrect stall recovery.
More precisely, the PF applying pulling the control column aft/ sidestick back; increasing the AOA of the aircraft; instead of the required decrease of the AOA.
Any thoughts as to why this recurrent behaviour of trying to pull back in spite of repeated training in proper stall recovery training?
Case in point: Air France Flight 447 2009, Colgan Flight 3407 2009
I have been reading a few accident cases and an underlying theme seems to be incorrect stall recovery.
More precisely, the PF applying pulling the control column aft/ sidestick back; increasing the AOA of the aircraft; instead of the required decrease of the AOA.
Any thoughts as to why this recurrent behaviour of trying to pull back in spite of repeated training in proper stall recovery training?
Case in point: Air France Flight 447 2009, Colgan Flight 3407 2009
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in spite of repeated training in proper stall recovery training?
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And despite the lousy performance initially, the AF447 guys didn't get much help from the aircraft when the stall warning - as per design - stopped below a certain speed and then started as they were lowering the nose to recover from the stall. That could have given them the idea that lowering the nose was not the right thing to do.
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Perhaps a reminder to flight crew of certain aircraft types that you should allow the stick pusher to do its thing. Perhaps add a reminder of just what the darned thing does. It would not surprise me in the least to see that there would be a percentage of pilots who would react to keep the aircraft stalled not understanding what the stick pusher was doing.
These events (Air France and Colgan) made me think about the way that stall recovery has been taught over the years. During initial training in light aircraft (in the early 1970s) I was taught that the important thing during recovery from a full stall was to unstall the wings first (by pushing forward to reduce AoA) then to use power to help accelerate back to a normal speed and to reduce the height loss.
As I moved up through larger and heavier aircraft types, this technique seemed to work just as well, until I checked out on the Boeing 737 in 1987. During simulator training we concentrated on recoveries at the stick-shaker (i.e. before the full stall) and the technique here was to slam the thrust levers fully forward and let the plane fly itself out with the attitude held at 5° nose up. This seemed fair enough, given that we hadn't actually stalled.
In 1997 on the Boeing 747 however, this technique was even used to recover from a full stall. A typical exercise would have us stalled in a holding pattern at about 8,000ft with the autopilot engaged. I remember being shaken quite strongly by the buffeting for 10-20 seconds as we waited for the thrust to accelerate us out of the stall with the nose at about 10° and asking, 'why don't we just take the autopilot out and lower the nose?' The answer was that the emphasis then was on minimising the height loss.
Why the emphasis on minimising height loss? I believe that the FAA Practical Test Standards (PTS) may hold the answer. From memory, these were quite prescriptive in specifying a maximum loss of height during a demonstration of a stall recovery. If this loss of height was exceeded, the manoeuvre was failed. So, most pilots' training for the 'Check Ride', emphasised the technique for minimising loss of height (rather than getting the wings flying again first).
Three accidents helped to change things: the Turkish Airlines 737 at Amsterdam, the Air France A.330 over the Atlantic and the Colgan Air Dash 8 at Buffalo. In all cases the aircraft stalled at a reasonable altitude and never recovered before ground/sea impact. Analysis shows that the aircraft could have recovered in the height remaining if only the pilot flying had taken positive steps to reduce the AoA and unstall the wings. There was plenty of 'height to lose' for these aircraft but it's a fair bet that (for Turkish and Colgan) both pilots' training resulted in them hanging on grimly with the stick-shaker rattling away, waiting for the engines to 'fly them out'.
(In the case of Colgan, it is possible that the PF thought that there was tailplane icing, in which case pulling back on the stick was a reasonable response).
Boeing and Airbus and FlightSafety reissued their guidance on stall recovery and, guess what? The emphasis is now on reducing the AoA, even if this involves losing height! We have come full circle.
As I moved up through larger and heavier aircraft types, this technique seemed to work just as well, until I checked out on the Boeing 737 in 1987. During simulator training we concentrated on recoveries at the stick-shaker (i.e. before the full stall) and the technique here was to slam the thrust levers fully forward and let the plane fly itself out with the attitude held at 5° nose up. This seemed fair enough, given that we hadn't actually stalled.
In 1997 on the Boeing 747 however, this technique was even used to recover from a full stall. A typical exercise would have us stalled in a holding pattern at about 8,000ft with the autopilot engaged. I remember being shaken quite strongly by the buffeting for 10-20 seconds as we waited for the thrust to accelerate us out of the stall with the nose at about 10° and asking, 'why don't we just take the autopilot out and lower the nose?' The answer was that the emphasis then was on minimising the height loss.
Why the emphasis on minimising height loss? I believe that the FAA Practical Test Standards (PTS) may hold the answer. From memory, these were quite prescriptive in specifying a maximum loss of height during a demonstration of a stall recovery. If this loss of height was exceeded, the manoeuvre was failed. So, most pilots' training for the 'Check Ride', emphasised the technique for minimising loss of height (rather than getting the wings flying again first).
Three accidents helped to change things: the Turkish Airlines 737 at Amsterdam, the Air France A.330 over the Atlantic and the Colgan Air Dash 8 at Buffalo. In all cases the aircraft stalled at a reasonable altitude and never recovered before ground/sea impact. Analysis shows that the aircraft could have recovered in the height remaining if only the pilot flying had taken positive steps to reduce the AoA and unstall the wings. There was plenty of 'height to lose' for these aircraft but it's a fair bet that (for Turkish and Colgan) both pilots' training resulted in them hanging on grimly with the stick-shaker rattling away, waiting for the engines to 'fly them out'.
(In the case of Colgan, it is possible that the PF thought that there was tailplane icing, in which case pulling back on the stick was a reasonable response).
Boeing and Airbus and FlightSafety reissued their guidance on stall recovery and, guess what? The emphasis is now on reducing the AoA, even if this involves losing height! We have come full circle.
But stall recovery is not to do with attitude (specifically).
People say "Move the control column (centrally) forward" but how far?
I teach (on basic) "Move the control column (centrally) forward until the stall identification ceases" (this would include, amongst others, stick shaker ceasing to operate).
+TSRA, did AF447 and Colgan have a stick pusher?
People say "Move the control column (centrally) forward" but how far?
I teach (on basic) "Move the control column (centrally) forward until the stall identification ceases" (this would include, amongst others, stick shaker ceasing to operate).
Perhaps a reminder to flight crew of certain aircraft types that you should allow the stick pusher to do its thing. Perhaps add a reminder of just what the darned thing does. It would not surprise me in the least to see that there would be a percentage of pilots who would react to keep the aircraft stalled not understanding what the stick pusher was doing.
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(In the case of Colgan, it is possible that the PF thought that there was tailplane icing, in which case pulling back on the stick was a reasonable response).
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eckhard
I agree with you 100percent.
Part of the philosophy too was to train for stall recovery and windshear/microburst recovery too.
But they are really different things .
PUSH FORWARD and you are out of the stall (unless you are doing something nutty aerobatically).
Windshear recovery should be taught differently.
The douglas stall recovery (at least takeoff stalls) also included firewall power, flaps fifteen. Configuring you for flying in case you had forgotten the flaps (and slats) on takeoff!
I agree with you 100percent.
Part of the philosophy too was to train for stall recovery and windshear/microburst recovery too.
But they are really different things .
PUSH FORWARD and you are out of the stall (unless you are doing something nutty aerobatically).
Windshear recovery should be taught differently.
The douglas stall recovery (at least takeoff stalls) also included firewall power, flaps fifteen. Configuring you for flying in case you had forgotten the flaps (and slats) on takeoff!
Too many people start with the conclusion that the stall condition has been identified, thus the focus on recovery.
The initial replies clearly identify the problem, awareness; however it is also necessary to consider many preceding factors which can influence a pilots mind set, and even with awareness bias the choice of action.
Many analyses work backwards from effect to cause, hindsight bias. However, considering accidents from an alternative perspective that ‘both the contribution of latent system states, and the complexity of conditions that could end in an incorrectly performed human action - even leading to the extreme notion of "error forcing" conditions’ and where ‘the "sharp" end, which often are the initiating events, and actions at the "blunt" end, which create the conditions that either make an action failure near inevitable or turn minor mishaps into major disasters’. (Accident Analysis and "Human Error"), it is probable that a large range of factors contributed to the accident e.g. education, training, experience, dynamic awareness – thinking ahead, avoiding hazardous situations.
The initial replies clearly identify the problem, awareness; however it is also necessary to consider many preceding factors which can influence a pilots mind set, and even with awareness bias the choice of action.
Many analyses work backwards from effect to cause, hindsight bias. However, considering accidents from an alternative perspective that ‘both the contribution of latent system states, and the complexity of conditions that could end in an incorrectly performed human action - even leading to the extreme notion of "error forcing" conditions’ and where ‘the "sharp" end, which often are the initiating events, and actions at the "blunt" end, which create the conditions that either make an action failure near inevitable or turn minor mishaps into major disasters’. (Accident Analysis and "Human Error"), it is probable that a large range of factors contributed to the accident e.g. education, training, experience, dynamic awareness – thinking ahead, avoiding hazardous situations.
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Originally Posted by eckhard
Air France and Colgan made me think about the way that stall recovery has been taught over the years. During initial training in light aircraft (in the early 1970s) I was taught that the important thing during recovery from a full stall was to unstall the wings first (by pushing forward to reduce AoA) then to use power to help accelerate back to a normal speed and to reduce the height loss.
Originally Posted by eckhard
As I moved up through larger and heavier aircraft types, this technique seemed to work just as well, until I checked out on the Boeing 737 in 1987. During simulator training we concentrated on recoveries at the stick-shaker (i.e. before the full stall) and the technique here was to slam the thrust levers fully forward and let the plane fly itself out with the attitude held at 5° nose up. This seemed fair enough, given that we hadn't actually stalled.
In 1997 on the Boeing 747 however, this technique was even used to recover from a full stall. A typical exercise would have us stalled in a holding pattern at about 8,000ft with the autopilot engaged. I remember being shaken quite strongly by the buffeting for 10-20 seconds as we waited for the thrust to accelerate us out of the stall with the nose at about 10° and asking, 'why don't we just take the autopilot out and lower the nose?' The answer was that the emphasis then was on minimising the height loss.
In 1997 on the Boeing 747 however, this technique was even used to recover from a full stall. A typical exercise would have us stalled in a holding pattern at about 8,000ft with the autopilot engaged. I remember being shaken quite strongly by the buffeting for 10-20 seconds as we waited for the thrust to accelerate us out of the stall with the nose at about 10° and asking, 'why don't we just take the autopilot out and lower the nose?' The answer was that the emphasis then was on minimising the height loss.
Of course recoveries from some fully developed aerodynamic stalls can be achieved with merely advancing power, BUT (note the significance!) BUT, that would be predicated on the fact that with the airplane already having stalled, that would have resulted in the airplane, because of its no longer being able to “fly,” would have pitched over – even if one wing or the other was further down … the important factor was thought to exist was that the nose would have been lower, and the AoA effectively reduced. The only problem is that someone neglected to notify all of the airplanes that that they were expected to experience a lowering of the nose when the wings no longer maintained flying speed.
How many ways can you spell D U M B ?
Originally Posted by eckhard
Why the emphasis on minimising height loss? I believe that the FAA Practical Test Standards (PTS) may hold the answer. From memory, these were quite prescriptive in specifying a maximum loss of height during a demonstration of a stall recovery. If this loss of height was exceeded, the manoeuvre was failed. So, most pilots' training for the 'Check Ride', emphasised the technique for minimising loss of height (rather than getting the wings flying again first).
Therefore, the loss of altitude was invariably found to be almost a negative performance. And that was the way the “grades” were distributed – some organizations actually had a table of what grade was given based on how much altitude was lost! Yeah, I know … no longer just Dumb … now we were full-on practicing S T U P I D I T Y.
Additionally, someone must have lost sight of the fact that when taking a transport category airplane up on a training flight – there were very few times when the gross weight would be at or greater than the weight at which the airplane would be normally operated, even the weight at landing at a distant airport. Therefore, the pilot attempting to maintain the attitude at which the stick shaker initiated, often found it necessary when they added power, they very slightly moved the controls forward to keep the airplane from climbing – or so they thought. Sometimes, this actually reduced AoA and allowed the recovery to occur somewhat more quickly.
Still maintaining the S T U P I D I T Y level!
Originally Posted by eckhard
Three accidents helped to change things: the Turkish Airlines 737 at Amsterdam, the Air France A.330 over the Atlantic and the Colgan Air Dash 8 at Buffalo. In all cases the aircraft stalled at a reasonable altitude and never recovered before ground/sea impact. Analysis shows that the aircraft could have recovered in the height remaining if only the pilot flying had taken positive steps to reduce the AoA and unstall the wings. There was plenty of 'height to lose' for these aircraft but it's a fair bet that (for Turkish and Colgan) both pilots' training resulted in them hanging on grimly with the stick-shaker rattling away, waiting for the engines to 'fly them out'.
(In the case of Colgan, it is possible that the PF thought that there was tailplane icing, in which case pulling back on the stick was a reasonable response).
Boeing and Airbus and FlightSafety reissued their guidance on stall recovery and, guess what? The emphasis is now on reducing the AoA, even if this involves losing height! We have come full circle.
(In the case of Colgan, it is possible that the PF thought that there was tailplane icing, in which case pulling back on the stick was a reasonable response).
Boeing and Airbus and FlightSafety reissued their guidance on stall recovery and, guess what? The emphasis is now on reducing the AoA, even if this involves losing height! We have come full circle.
Turkish Airlines 737 at Amsterdam, Feb 25th 2009
Air France A.330 over the Atlantic, Jul 5, 2012
Actually, there was an accident that preceded these 3 tragedies.
ABX Air (Airborne Express) Douglas DC-8-63, Narrows, Virginia, December 22, 1996.
The NTSB actually said: “The National Transportation Safety Board determines that the probable causes of this accident were the inappropriate control inputs applied by the flying pilot during a stall recovery attempt, the failure of the nonflying pilot-in-command to recognize, address, and correct these inappropriate control inputs… Contributing to the causes of the accident were the inoperative stick shaker stall warning system and the ABX DC-8 flight training simulator’s inadequate fidelity in reproducing the airplane’s stall characteristics.”
Basically, what happened was that this crew had trained on a simulator that would “recover” from an approach to stall, even a fully developed aerodynamic stall, with simple power advancement. What the simulator failed to provide was the disrupted airflow into the engines when the airplane was at such a positive pitch attitude. In fact, each time they attempted to simply add power, they experienced varying degrees of compressor stalls on one or more of the engines. What they didn’t realize was that the airflow was disrupted – due to the angle of the engine intake with respect to the airflow into that intake. This was exacerbated with the power advancement … needing more airflow that was not there … the result, compressor stalls.
Unfortunately, the simulator’s engines had no such programming. Those engines responded (and the programming followed suit) based on throttle position … making it a veritable “piece of cake” to “fly out of the stall.” Unfortunately, when they attempted that in the airplane – the engines performed as they were designed and simply could not generate much power at all, let alone the power needed to recover from the stall. The result was a crash, killing all on board.
Unfortunately – the actual facts were not made widely known and simulator programming was not pinned as the culprit. Also, unfortunately, the concept of minimum altitude loss during approach to stall/stall recovery was still the expected standard … and by the way, THAT has just been recently changed.
For your reading DISCOMFORT – here is a very cursory commentary of the pitch oscillations during that DC-8 attempted recovery:
Pitch attitude initially maintained at +10degrees until initiation of stick shaker;
when it dropped to approximately -10 degrees,
it then oscillated, first between minus 20degrees and plus 10degrees;
then between minus 20degrees and 0degrees;
then between minus 5degrees and positive 18degrees,
then very slight oscillations around plus 10degrees;
before plummeting to minus 70degrees!)
If you have a morbid curiosity - obtain a copy of the "official" report - and read the CVR transcripts and look at the FDR readouts - its an "old" FDR, but the effect is clearly seen!
Last edited by AirRabbit; 5th Jul 2014 at 18:44.
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The story is hidden within the previous posts.
(a) basic certification stall and recovery (as in the AFM) is not concerned with ground contact ergo it is all about unstalling (ie alpha reduction), getting the speed to a comfortable margin above stall (1.2Vs is normally quoted) and then recovering back to normal flight.
(b) in the real world (excluding real world military operations and like higher risk civil activities), the risk of stalling principally is in the circuit while manoeuvring for a landing, especially the IFR bad weather circuit case whilst hooking around to final and overbanking to adjust a misjudged base.
Hence, the operational Regulatory process traditionally was concerned with minimum height loss during the recovery. The result was that the typical recovery trained and demonstrated in endorsement and recurrent training had scant resemblance to the certification animal .. this could, on occasion, result in unexpected frights.
One notes that there have been numerous deltas between the design/certification and operational sides of the table over the years ...
Mishaps, such as AF447, clearly demonstrated that the system possibly had missed the point of the difference between design/certification (ie what the aircraft should reasonably be presumed capable of doing) and operational (ie what was going to kill us most of the time in short order) reality.
As an Industry we are in the process of recalibrating our thinking .. so far this has evolved at airline level and progressively will filter down to the GA area in due course.
The aim now is to unstall FIRST .. then worry about recovering to normally controlled flight and avoiding ground contact.
As in all things (other than death and, possibly, taxes) there are no guarantees .. only probabilities .. and the aim is to load the dice in favour of survival. That might not work ALL the time but it will work MOST of the time.
(a) basic certification stall and recovery (as in the AFM) is not concerned with ground contact ergo it is all about unstalling (ie alpha reduction), getting the speed to a comfortable margin above stall (1.2Vs is normally quoted) and then recovering back to normal flight.
(b) in the real world (excluding real world military operations and like higher risk civil activities), the risk of stalling principally is in the circuit while manoeuvring for a landing, especially the IFR bad weather circuit case whilst hooking around to final and overbanking to adjust a misjudged base.
Hence, the operational Regulatory process traditionally was concerned with minimum height loss during the recovery. The result was that the typical recovery trained and demonstrated in endorsement and recurrent training had scant resemblance to the certification animal .. this could, on occasion, result in unexpected frights.
One notes that there have been numerous deltas between the design/certification and operational sides of the table over the years ...
Mishaps, such as AF447, clearly demonstrated that the system possibly had missed the point of the difference between design/certification (ie what the aircraft should reasonably be presumed capable of doing) and operational (ie what was going to kill us most of the time in short order) reality.
As an Industry we are in the process of recalibrating our thinking .. so far this has evolved at airline level and progressively will filter down to the GA area in due course.
The aim now is to unstall FIRST .. then worry about recovering to normally controlled flight and avoiding ground contact.
As in all things (other than death and, possibly, taxes) there are no guarantees .. only probabilities .. and the aim is to load the dice in favour of survival. That might not work ALL the time but it will work MOST of the time.
My last recurrent stall series, the instructor had us level off, in the sim, at MDA to a runway and let the speed decay until the stall indications and continue until the stick pusher fired. Recovery action was then " allowed". Well, a windshield full of runway! If you let the pusher work and release, then recover from the " resulting dive", ithe ground was I damaged. Try to fight the pusher, as the instructor demonstrated, and crash was inevitable. The view, no doubt, would terrify any pilot without low flying experience.
fireflybob I don't know about the the A330 but in the Colgan accident the captain pulled back when he got the stick shaker, this induced the stick pusher, he then proceeded to pull back against the stick pusher all the way to the ground. The sad thing is that they were initially not at a dangerously low speed. The de-ice had been left on and that biases the stick shaker to come on at a higher speed than normal. When the shaker shook, the aeroplane was flying just fine and all the captain had to do was advance the power levers to increase the airspeed a bit, instead he heaved back on the column and doomed them all to a rapid contact with the ground. The FO didn't help by retracting the flaps.
Interesting comment about 'filter down to GA', JT. I'm not sure it needs to filter quite that far (apart from maybe a few GA places that have adopted the discredited FAA/airline mantra). Throughout my GA career, stall recovery has always been to pitch to reduce alpha then add power or thrust to minimise height loss.
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There exist some areas where the minimum height loss approach is adopted to the exclusion of other considerations. However it is achieved, a more balanced approach to stall recovery can only help avoid problems.
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Any thoughts as to why this recurrent behaviour of trying to pull back in spite of repeated training in proper stall recovery training?
Case in point: Air France Flight 447 2009,
Case in point: Air France Flight 447 2009,
but of course we'll never know.
if the fuselage had accreted a few tons of ice in the flight it all melted in the sea leaving no evidence.
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Originally Posted by dubbleyew eight
in the case of the air france flight I have often wondered if the aircraft didn't accrete a few tons of ice on the fuselage. if the pilot flying was convinced that the tailplane was stalled then pulling up elevator, which would reduce the elevator angle of attack, would be an entirely logical move to try to get it flying again.
but of course we'll never know.
if the fuselage had accreted a few tons of ice in the flight it all melted in the sea leaving no evidence.
but of course we'll never know.
if the fuselage had accreted a few tons of ice in the flight it all melted in the sea leaving no evidence.
Therefore, with any aircraft condition approaching an aerodynamic stall, should the pilot initiate pulling back on the controls (i.e., pulling the elevator “up” with respect to the horizontal tail surface) there would be an increase the “lift” generated on the underside of the horizontal tail, thereby moving the tail downward and the nose upward, increasing the AoA, and compounding the problem. With an ice accretion already on the airplane, likely already having decreased the stall AoA, any successful pulling-back on the controls would result in further raising the nose, and moving the airplane closer to the stall or moving it more deeply into an already present stall. How could that be even close to logical?
Also, I think we need to keep the estimates we throw around just a bit more within the realm of possibility, even when realism is totally disregarded. With little doubt, there has been a serious misunderstanding about the weight of ice accretion on an airplane. The idea that an airplane can accumulate “several tons” of additional weight (where several tons ... 4 tons? ... 8 tons? ... would be 8,000? pounds ... 16,000? pounds) is, at best, extremely unlikely.
In fact, recent regulatory/industry working groups focusing on the operations of airplanes at or beyond the normal flight envelope, have described the additional weight accumulating on an airplane of “average” size, likely being about the same as adding the weight of a couple of additional passengers – and on the largest of airplanes this estimate reaches up to 10 to 30 additional passengers – where the average weight of passengers is routinely estimated to be between 180 and 200 pounds, including their carry-on baggage. And ... this is the accumulation that might occur on the ground prior to takeoff - and much less accumulation when already airborne. Therefore, this means an accumulation of less than 400 pounds of ice for airplanes with a maximum takeoff gross weight a bit under or over 200,000 pounds and as much as 2000 to 6000 pounds for airplanes with a maximum takeoff gross weight approaching (and some now that exceed) a million pounds.
While it is true that ice accumulation has an effect on airflow over and around an airplane, and of course we simply cannot dismiss that fact … we must understand that the actual weight of the ice accumulating on an airplane is relatively insignificant when compared to the airflow disruption it causes.
Last edited by AirRabbit; 6th Jul 2014 at 20:34.
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in the case of the air france flight I have often wondered if the aircraft didn't accrete a few tons of ice on the fuselage. if the pilot flying was convinced that the tailplane was stalled then pulling up elevator, which would reduce the elevator angle of attack, would be an entirely logical move to try to get it flying again.
Pulling back in the case of ICTS is to prevent or resist aerodynamic overbalance of unpowered elevators which is unlikely to be necessary with powered flying controls.
If the tail plane had stalled on AF447 they would have suffered a large nose down pitch moment and the elevator would be effectively ineffective (!) and wouldn't have been able to maintain such a nose up attitude.