Since the crash of Air France flight 447 in 2009, flight training departments of all categories of jet operators have taken an introspective look at how to handle high altitude stalls. You may recall that this flight of an Airbus A330 from Rio de Janeiro to Paris plummeted to the ocean after a thunderstorm encounter and loss of airspeed indications as a result of frozen pitot tubes. The pilots, faced with a multitude of warnings and false indications, pitched the airplane up into a high-altitude stall. The stall condition was not corrected and in minutes, the airplane fell 38,000 feet to the ocean in a nose-up attitude.
At my annual recurrent training for the Boeing 737, my airline incorporated high altitude stalls into the script. In the thin air at altitude, jets operate in a very narrow airspeed band between MMO (redline) and the low speed limit—typically a yellow arc that provides a warning zone prior to a stall, which itself is indicated by a variable lower red line. At high altitudes and high weights, this band of airspeed, known as “coffin corner,” can be quite narrow, leaving you a mere 10 knots of usable airspeed range, for example. Load up the wing in a turn and that lower redline will quickly move up toward your airspeed. Making matters worse, at altitude the ordinarily powerful turbines are reduced to comparative slugs. In the simulator, the instructor had us simply pull the power levers to idle at FL380 in level flight. He didn’t give us any instructions on how to handle the situation. That would come later. The 737 dutifully maintained altitude as the autopilot commanded, until the stick shaker warned of an impending stall. At that point, the autopilot kicks off and the drill is to firewall the power and try to recover. I observed as my partner flew the recovery.
You first notice that nothing happens quickly at this altitude. The engines take seemingly forever to spool back up; meanwhile, the airplane feels as if it’s balancing on a basketball as the wing tries to get a bite of air to work with. When the wing stalls, the simulator bucks in protest. All the while, you’re falling at 2,000 feet per minute or more. Having already lost nearly 1,500 feet, the engines have reached max power, but the airplane is so deeply behind the power curve that it makes little difference. The stick shaker was on about 25 percent of the time as my co-pilot gingerly tried to get back to normal flying conditions. Even a slight pull on the yoke resulted in that lower redline (stall) coming up to or through our current airspeed, bringing on the buffet and shaker again. Finally, with a loss of about 3,500 feet, the airplane is controllable, maintaining altitude and slowly regaining speed.
Following some instruction on how best to recover, it was my turn. The instructor recommended following the stall recovery cue (feathers) that appear on the attitude indicator during the maneuver, except to always keep the airplane symbol below the feathers. Once it touches them or goes above, the stick pusher fires and you’re bucking and losing more altitude. Even with a gentle touch, the gyrations can be wild. Granted, lots of this monkey-motion can be attributed to the simulator’s pitch sensitivity, but it’s still a regime that you clearly do not want to explore in the real airplane.
I finally got the airplane recovered with a loss of 2,700 feet, and that was knowing what was going to happen and with instruction on how best to recover. In a surprise situation, such as what occurred to the Air France pilots, it’s easy to see how this could turn into a disaster. Was this training useful? Absolutely. Most eye-opening was how much of a dog a jet can be once it gets behind the power curve. In normal operations, for thousands of hours we never get to see this regime, so it was extremely valuable to do it in the simulator. The ordinarily all-powerful jet is reduced to the performance level of a Piper Apache on one engine on a hot day. Throw in the all-or-nothing aerodynamics of a swept wing and it’s potentially worse. Of course, the first step to avoid all of this is preventing a stall in the first place. Pardon the Monday-morning quarterbacking, but there was one thing the Air France pilots seemingly failed to do. Most pilots who fly one airplane a lot (owners and airline pilots, for example) get to know which power settings provide a certain speed at nearly any given altitude. For example, setting 1,900 rpm keeps a Cessna 172 coming down the glideslope nicely at about 75 knots with some flaps. With each new airplane you check out in, it’s best to note what pitch and power settings work in various conditions. It actually makes flying much easier.
The Air France Airbus zoom-climbed from FL350 to FL380 with a substantial 12-degree nose-up pitch that resulted in a 1.6-G load. Every jet pilot should know that such performance is impossible to attain at that altitude and weight unless it was caused by weather (which may have been what the pilot was thinking). Once the pitot tubes froze up, the pilots’ basic knowledge of attitude and power settings could have avoided the abrupt pitch-up that led to the fatal stall.
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