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.
That is still the best way to recover from an aerodynamic stall.
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.
This was “the procedure” that was initially adopted because someone decided that the method to recover from an approach to stall would be exactly the same as the recovery from an aerodynamic stall – and the industry desired to reduce the “cost” of training, and the use of simulators was enticingly held out as the method desired to reduce those costs. Essentially, a conclusion of convenience.
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).
You may have some trouble in accepting what I’m going to say here, but, I assure you, it’s accurate. This was another of the “reading into what is written.” The idea of minimizing the loss of altitude during a stall recovery, was intended to focus the pilot on not indiscriminately losing untold thousands of feet, simply to “ensure” that there was sufficient airflow over the wings before asking those wings to support the weight of the airplane, plus the “g-force” encountered with the rotation back to “level flight.” Dumb? Sure! But, that is where it came from. AND then (!) someone determined that if a “minimum” altitude loss was “good,” then it had to be that “zero” altitude loss would be “better.” No one out there wanted to acknowledge that THEY allowed their pilots to perform to a lesser standard than absolute “perfection.”
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.
Colgan, Buffalo, NY, Feb 12, 2009
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!