Air France 447 and the Colgan Air crash at Buffalo have injected new vigour into a debate that has been going on quietly in the industry for some years now.
It's about how to teach airline pilots to handle the aircraft when it's close to the edge of the flight envelope.
The debate about airline training for upset recovery (recovery from unusual attitudes) is a part of this, but it's stall recovery specifically that I want to concentrate on here.
For brevity, I am going to assume that readers know that stall is ultimately about angle of attack and that they understand the aerodynamics of a stall, so I can concentrate on existing perceptions about recovery technique, because technique is what the industry debate is about.
Some of stall recovery's finer points are in dispute. Let me set out what I think the case is, and then please feel free to tear my perceptions to shreds; the objective is to discover the truth, if there is a single truth.
During basic training, most pilots are taught to recover from a stall by dealing with the attitude first, accepting some height loss as inevitable, then applying power.
However line pilots, who spend most of their time in controlled airspace, have been taught for decades that they should apply power first, then adjust attitude if necessary to minimise height loss.
I suspect that this technique, blessed by the FAA, was was taught because it was assumed pilots would be reacting to a stall warning, rather than waiting for the actual stall. And the controlled airspace factor means that any loss of altitude would mean loss of cleared vertical separation.
Buffalo has caused a lot of heart-searching at the FAA and National Transportation Safety Board, leading to a re-discovery that the "power-then-attitude" stall recovery technique was not the one that the manufacturers' test pilots were required to demonstrate to win certification for the aeroplane type in the first place: test pilots use the classic "attitude-then-power" recovery, accepting height loss as inevitable. So the FAA, seemingly without noticing, had authorised a line training technique different than the one they required for type certification.
When and why the "power-then-attitude" technique change was accepted is not completely clear, but I have a personal theory. Please feel free to shoot it down:
In the 1950s when transport aeroplanes were powered by big piston engines and props, the power response was instant and gave you lift-producing propwash over the wings, which could assist stall recovery and restore airspeed. Also the thrust line and drag line were almost the same, so there was no pitch-up with power application as there is with today's underwing jet engines. And wings were not supercritical, so the stall was less dramatic.
My theory is that the FAA approved the power-then-attitude system for propliners and then forgot to review it for jets.
I have put this to many worthy senior airline pilots, but have not been able to confirm it. Maybe it's lost in the mists of time.
Finally, has the "power-then-attitude" technique got anything to do with the apparent fact, as demonstrated over the Atlantic and at Buffalo, that pilots both sides of the Atlantic - in big jets and turboprops (Colgan was a Q400) - are at risk of reacting wrongly to stalling?
on June 1, 2011 1:41 PM | Reply
David
According to page 113 of the 2nd edition of "Handling the Big Jets", traditional piston-engined transports exhibit a substantially straight nose drop at stall.
What does an A330 do?
on June 1, 2011 2:11 PM | Reply
It all depends on the altitude when the aircraft was stalled. At high cruising altitudes in a jet transport, the pilot must forget about recovering with minimum loss of height. In the simulator (737) the technique is to immediately lower the nose to just below the horizon and increase power at the same time. The pitch up that follows use of power must not be permitted to occur and a deliberate "dive" for want of a better description, to lower the angle of attack is maintained until a safe speed is attained for level flight. This usually means a loss of 3000 ft is anticipated. The 737 figure to reach is Vref 40 plus 100 knots which works out around 230 knots IAS.
On the other hand, a low level stall below 1000 ft with flaps in landing setting is critical for ground contact reasons. To lower the nose well below the horizon will cause a high rate of descent which could be dangerous near the ground. To prevent this, the nose should be adjusted to around five degrees nose up and power applied at the same time. Again it is vital to prevent the strong pitch up that is characteristic of aircraft with under-wing engines and forward elevator combined with judicious use of forward stabiliser trim is needed. The stick shaker must be respected and flaps or landing gear not raised until a safe climb speed is attained.
That is the main difference between the two situations. At high altitude a deliberate height loss is needed to attain an unstalled condition until a safe cruise airspeed is attained. Near the ground it takes careful handling to "claw" for altitude while adjusting the nose attitude to minimise height loss yet slowly attain a safe climb speed with flaps down. Retracting the flaps at extremely low airspeed and altitude must be avoided until a safe airspeed is attained.
on June 1, 2011 4:43 PM | Reply
Agree with the previous post.
The problem with AF447 was much more complex than a simple stall recovery. A B737 doesn't auto-trim with AP-OFF. When handling the aircraft one has a feed-back feeling from the yoke and that is fundamental for the recovery itself. When one applies forward stick inputs to recover from stall, one "recognizes" the amount of force needed and also the need/or not, for trimming the aircraft. Airbus Aircraft (like the one I fly, A330+A340) will switch from "Normal Law" to "Alternate Law" (otherwise in theory, it won't enter the Stall) and eventually to "Direct Law". So, the pilot, in step of a straight forward reaction to a "Stall, Stall" auto-warning has to "think" about all the reversions, has to cancel the AP/ATHR disengagement aural warnings, and has to recover from the Stall. A Stall in a FBW aircraft means always that you're in a degraded mode. Without feed-back on the stick, with thrust frozen in a certain set, with your mind occupied on unreliable speed indications and a never ending distracting flow of ECAM Warnings...
If you add turbulent environment conditions to that, you will add an extra difficulty which is instrument read-outs and the ability to bring the aircraft to the right attitude that, by the way, may be difficult to attain in a "coffin corner" environment, where really small speed/attitude changes will get you into a high speed stall or a low speed one.
I'm an aerobatic pilot. I'm used to fly upside-down. But let me tell you that my best advice on stall recovery in a big jet, would be to avoid it. And that's what we are there for. To be prepared and never complacent.
on June 1, 2011 4:59 PM | Reply
Blacknavigator, I hear what you say about trim, but the essential question to answer is why pilots pull up when there is a stall warning in the first place. A 737 wouldn't like that either.
on June 1, 2011 5:45 PM | Reply
Stalling an airliner is something that a commercial pilot should never ever allow to happen so when it does their immediate instinct is to follow a course of action that will minimize height loss to prevent ATC from noticing.
Secondly, once the pilot has applied max power then confirmation bias takes over and the pilot starts looking for evidence that the stall has been recovered. In AF447 this was likely the stall warning horn shutting off (because the measured speeds were
The Colgan crash shows the exact same desire by the PF to not lose altitude but recover using power alone and thus not make his mistake in allowing speed to decay become evident to ATC.
In theory the PNF should be a safeguard against these destructive behaviors but the PNF will be very reluctant to challenge the PF because if they are wrong they know this will cause a significant strain in the relationship between the two pilots and maybe even get them a negative reputation in the company. This is less of an issue when a more experienced Captain is present who will feel a lot more comfortable overriding a more junior FO. However in Colgan the PF was the Captain and in AF447 two peers were in the driving seats.
Fundamentally pilots need to feel they can recover from upsets without fear of excessive punishment and secondly PNFs have got to accept that it's their responsibility to confront PFs in the cockpit.
on June 1, 2011 5:58 PM | Reply
Doesn't this problem come down to the philosophy of the manufacturer? Airbus relies on the computer to communicate information to the pilot differently depending mode it is in or on how healthy the computer systems are. How many accidents have happened when the pilot thought he was in a different computer mode? Even their test pilots have crashed from this.
Every pilot knows how to recover from a stall. Every pilot might not remember what mode he is in or know there is a problem with the computer.
The analogy that seems to explain this is one of a person falling. Airbus wants you to look at your feet and then step onto the floor with the one that is closest to the floor. When an accident happens you don't take the time to look at your feet first you just instinctively step onto the floor. Their computer systems add a step into the process that increases the chance of a problem. Training will hone a pilots instincts but a computer system can go against this with mode confusion
on June 1, 2011 6:13 PM | Reply
I have taught stall recovery in a wide range of aircraft from Tiger Moths to Hunters, AHHH, and it was always the same, and it worked every time. At the on-set of the buffet, or when the stall warning goes off, REDUCE the BACK PRESSURE, move the stick forward. This reduces the angle of attack and unstalls the wing. It's that simple folks. I agree with Blacknavigator, take action at the onset of the stall, unload the aeroplane and be home for supper. Do NOT let a stall develop unless you are in an aerobatic aircraft and know what you are doing. If you want to die pull the stick back.
on June 1, 2011 7:22 PM | Reply
Looks like my 3rd paragraph got chopped off probably because I included a less than sign. It should have read:-
Secondly, once the pilot has applied max power then confirmation bias takes over and the pilot starts looking for evidence that the stall has been recovered. In AF447 this was likely the stall warning horn shutting off (because the measured speeds were less than 60kt and thus AOA values were invalid). The pilots then subsequently discounted the attitude indicator and rapidly unwinding altitude because again they didn't want to believe they were stalled (I suspect their internal model of the world at that time centered around some sort of large scale systems failure affecting all instruments as opposed to just blocked pitots).
on June 1, 2011 8:18 PM | Reply
However in this case the pilots showed no sign of being confused by the airplane mode. The BEA report contains this line:-
At 2 h 10 min 16, the PNF said "so, we’ve lost the speeds" then "alternate law […]".
This shows clear evidence that the PNF understands that they are in alternate law. A pilot that cannot remember that alternate law means that AOA protection has been lost shouldn't be up at the pointy end. Even if the PF didn't hear him or see the visual annunciation the stall warning horn would be unmistakable.
The fact that 3 pilots apparently stalled a flyable airplane all the way from 38000ft into the ocean strengthens not weakens the case for AOA protection for me.
on June 1, 2011 9:36 PM | Reply
WJLAviator says it well and that should be how a computer lets you recover. The A330 stall recovery procedure at the time and with "working computers" is different. Airbus had said to apply takeoff thrust and full aft stick, relying on the computer to maintain maximum allowable AOA. Here it looks like the computer was working with bad info from the pitot tubes. Another problem with this procedure is that the computer cant control AOA at abnormally low speeds and changes computer modes when this happens.
So as this flight went through different air speeds its my guess that the computer kept changing the flight laws that it was following. This changes how the pilot recovers from the stall to having to manage AOA himself.
Do you want to have a pilot in a emergency situation react counter to basic instincts or guessing what mode the computer is in?
This is all just my guess of the situation but it seems Airbus is going out of their way to say it wasn't the airplane leaving us to assume its the pilots fault. Who is responsable for the pilot airplane interface? It has to be Airbus.
on June 1, 2011 10:49 PM | Reply
However in this case the pilots showed no sign of being confused by the airplane mode. The BEA report contains this line:-
At 2 h 10 min 16, the PNF said "so, we’ve lost the speeds" then "alternate law […]".
This shows clear evidence that the PNF understands that they are in alternate law. A pilot that cannot remember that alternate law means that AOA protection has been lost shouldn't be up at the pointy end. Even if the PF didn't hear him or see the visual annunciation the stall warning horn would be unmistakable.
The fact that 3 pilots apparently stalled a flyable airplane all the way from 38000ft into the ocean strengthens not weakens the case for AOA protection for me.
on June 1, 2011 10:55 PM | Reply
However in this case the pilots showed no sign of being confused by the airplane mode. The BEA report contains this line:-
At 2 h 10 min 16, the PNF said "so, we’ve lost the speeds" then "alternate law […]".
This shows clear evidence that the PNF understands that they are in alternate law. A pilot that cannot remember that alternate law means that AOA protection has been lost shouldn't be up at the pointy end. Even if the PF didn't hear him or see the visual annunciation the stall warning horn would be unmistakable.
The fact that 3 pilots apparently stalled a flyable airplane all the way from 38000ft into the ocean strengthens not weakens the case for AOA protection for me.
on June 2, 2011 11:36 AM | Reply
This was sent to me by a pilot who was having trouble posting his comment to this blog, so I'm putting it up for him:
"I fly the A380 and we are in the (ongoing) process of doing manual handling in sims - stalling the aircraft at FL350 is one of the tricks. And yes - we are doing the pitch first, then power, to avoid the secondary stall so common in under-slung engines.
The best advice comes from 'Handling The Big Jets' (the only information in that lofty tome that most of us can understand):
"... When faced with stalling a big jet or doing something else, consider the latter."
You must NEVER stall. Your aircraft becomes as aerodynamic as a piano falling from a fifty story building. And if you do stall, make sure its only for a second.
on June 2, 2011 9:52 PM | Reply
David,
Yes you're right. The problem is that the CVR transcripts were not released yet, so we don't have a clue of what was presented to the Pilot Flying instruments (assuming it was CM2). Was he first trying to dissipate speed (if faced with a high speed showing PFD)? Remember ISIS gets info from ADIRU 1 and 3, but even so, it got a "less than a minute" delayed speed indication, when compared with PFD1. Was PF "responding" to a somatogravic illusion? I really don't know. What I know is that it is not a normal reaction from any kind of pilot to pull the stick to recover from a Stall (except for an upside-down vrille, maybe)...
WJLAviator,
You're also right: "REDUCE the BACK PRESSURE, move the stick forward".
I agree with you but, the problem with Airbus FBW aircraft is that the stick doesn't give you any feed-back forces to "reduce the back pressure"...it never gives you any feed-back forces, period. It even doesn't give you the feed-back of the other pilot's inputs! (And inputs from both side-sticks are added - if moved at the same time - which is, IMHO one of the most dangerous issues of the FBW family). This is to say that very important clues about the behavior of our aircraft were taken away from us, by Airbus designers. If one adds to all of this, the fact that throttles stay fixed in the Climb detent for more than 90% of the flight, one may think: how do they fly it? How do they "feel" it?
Let me tell you, it takes a hard work and perseverance to stay in the loop and fight against complacency.
on July 16, 2011 5:26 AM | Reply
I am an aero engineer, not a pilot, but I believe that you will find that the pull up response to a stall warning only started after the crash of Delta Airlines Flight 193 at Dallas in 1985, in a severe microburst on final approach. Shortly after that, I was talking with an airline executive who himself was an ex-pilot. He said that when the airline became aware of the circumstances of the crash, they simulated the conditions in the simulator and the first 15 pilots (including him) crashed because they pitched down in response to the stick shaker. A crash could be averted only by pulling back to give the engines those extra few seconds to spool up. My recollection is that this subsequently became the recommended technique when a microburst is encountered with insufficient altitude to recover conventionally. Clearly, the specifics of the circumstances in which this response to a stall warning is applicable has been lost over time.
Obviously, this is hearsay, but you might like to research what happened subsequent to Delta Flight 193.
on July 31, 2011 7:22 PM | Reply
Hi Glenn,
There is a chapter in a book titled, The Limits of Expertise. The pilot on the aircraft stalling pulled up the nose at high altitude because he thought the plane was encountering windshear and so his actions were in line with an Windshear Escape Manoeuvre.
Hi David, I feel one should guard against the bias of deadly set/channelised attention while discussing Air France flight 447 and blaming the pilots of lacking basic piloting skills.
Pranesh
on August 1, 2011 1:46 PM | Reply
For long, Airbus pilots have complained of a 'lack of feel' from the sidestick. Maybe there is nothing wrong with Airbus's flying philosophy, but pilots have for years said loud and clear that FBW undermines their cognitive response and increases psychological stress as they try to stay in the computer's loop. So it's about time Airbus respected their opinion instead of repeating the line that there is nothing wrong with FBW. Airbus aircraft are a designer's aircraft (do they consult pilots?), aircraft that are loved by airline owners (similarity across models, pilots flying one model can fly all others with little training, easy to maintain, value for money). It's never, never a pilot's aircraft. I have always felt First Officers in Airbus aircraft are in a better to manually fly the plane than Captains. At least FOs access the sidestick with their right hands, which I'm sure is more comfortable. But think of the Captains, who fly the plane gripping the sidestick with their left hand. Unless one is a left-hander, I'm sure most Captains would like to grip the control stick with their right hand (or with both hands) in a stressful situation. It's natural. But the very design of Airbus cockpits suggests that pilots were never considered central to flying these planes. Conversely, manual flying was never given a serious thought by the designers, who believed the computer can take care of everything. It's smart design meant to bring in maximum profit to airline owners. Pilots play too insignificant a role in the bigger scheme of things. Maybe investigators should replace 'pilot error' with 'designer error' while apportioning blame for Airbus crashes.
on August 1, 2011 2:23 PM | Reply
Just one point about handedness: a Boeing captain, when flying manually, holds the yoke in his/her left hand, the power levers in the right. It has always been thus, including for Airbuses when they had conventional controls.
on August 1, 2011 7:13 PM | Reply
Thanks for the reply David.
on August 2, 2011 8:02 PM | Reply
Hi David, however much I might try not to blame 447's pilots without evidence, it could be just that. It stalled and they couldn't recover from it. Being at the edge of the flying envelope, there was little room for error, I guess.
on August 6, 2011 2:38 PM | Reply
Following the AF 447 accident, here is my concern about the hidden mini stick.
Sorry for my english, I hope you'll understand my opinion.
When a captain is rushing promptly into the cockpit, feeling something is getting bad and that one of your co-pilot is unclear to explain the situation, the first natural thing he's doing is to check the instruments. As the altitude indicator was functionning, the high rate loss of altitude should have been a serious clue. Showing the Pilot Flying, having the stick in the very aft position with an altitude loss is not the good thing to do to escape an evident stall. Pushing forward the stick and maintaining it, is the way to recover lift.
But a mini stick is located in the external part of the cockpit and unseen from the jump seat. Is the traditionnal wheel stick not preferable than the mini stick ?