Air France Flight 447 Analysis
Popular Mechanics recently posted a relatively solid analysis of the 2009 Air France flight 447 accident. It has the rare virtue of being a good read for professional aviators and non-pilots alike.
The article indicates that the pilots — and there were what, three or four of them involved on the flight deck? — were seemingly unaware that the aircraft was aerodynamically stalled. It sounds impossible for a crew with ten thousand hours of flight experience to be so oblivious, but almost the exact same thing happened in the Colgan Air 3407 accident. The aircraft was stalled, the captain didn’t understand what was going on, and he physically held the plane in a deep stall all the way into the ground.
However, in this case, perhaps the problem isn’t that they weren’t aware of the stall warnings, the high pitch attitude or the descending flight path, but rather that they did not believe the airplane could be stalled at all.
That’s on par with believing in the tooth fairy.
The plane has climbed to 2512 feet above its initial altitude, and though it is still ascending at a dangerously high rate, it is flying within its acceptable envelope. But for reasons unknown, Bonin once again increases his back pressure on the stick, raising the nose of the plane and bleeding off speed. Again, the stall alarm begins to sound.
Still, the pilots continue to ignore it, and the reason may be that they believe it is impossible for them to stall the airplane. It’s not an entirely unreasonable idea: The Airbus is a fly-by-wire plane; the control inputs are not fed directly to the control surfaces, but to a computer, which then in turn commands actuators that move the ailerons, rudder, elevator, and flaps. The vast majority of the time, the computer operates within what’s known as normal law, which means that the computer will not enact any control movements that would cause the plane to leave its flight envelope. “You can’t stall the airplane in normal law,” says Godfrey Camilleri, a flight instructor who teaches Airbus 330 systems to US Airways pilots.
Ah, the myth of the un-stallable airplane! Is this what Airbus, airlines, and the FAA are allowing instructors to teach pilots? I certainly hope not. Electronics and fancy design features are no match for the basic laws of physics.
Let’s review. Any airfoil — propeller, main rotor, fan blade, stabilizer, wing — can be stalled. There is no such thing as a stall-proof airplane, just as there are no unsinkable ships (see: RMS Titanic). Anyone who teaches otherwise is a link in an accident chain.
Now, stall resistant? Sure, under specific conditions, there are design elements ranging from canards to stick pushers to computerized flight control systems which may help prevent the airfoil from reaching the critical AOA. But to say that an airplane cannot be stalled is just foolish.
I’m not sure if it’s marketing hyperbole or human pride which causes such claims to be made. Even when an Airbus is flying under normal law, there are atmospheric factors (many of which happen to be found in the type of thunderstorm Air France 447 flew into) which can lead to a stall. Remember: a stall can occur at any airspeed. Mother Nature can dish out things no airliner can handle, even if it’s manufactured by Airbus.
But once the computer lost its airspeed data, it disconnected the autopilot and switched from normal law to “alternate law,” a regime with far fewer restrictions on what a pilot can do. “Once you’re in alternate law, you can stall the airplane,” Camilleri says.
It’s quite possible that Bonin had never flown an airplane in alternate law, or understood its lack of restrictions. According to Camilleri, not one of US Airway’s 17 Airbus 330s has ever been in alternate law. Therefore, Bonin may have assumed that the stall warning was spurious because he didn’t realize that the plane could remove its own restrictions against stalling and, indeed, had done so.
So because they haven’t seen it before, it can’t happen? Even on a clear blue day, computers can fail. Bugs can emerge in the very software they’re counting on to ensure a stall does not occur. The training these pilots receive sounds inadequate, to say the least. Sad to say, this is not a problem limited to US Airways or Air France or pilots flying the Airbus series. Stalls are poorly understood by a the majority of pilots in my experience.
Ironically, the weakest part of the Popular Mechanics piece also happens to be their description of a stall:
Almost as soon as Bonin pulls up into a climb, the plane’s computer reacts. A warning chime alerts the cockpit to the fact that they are leaving their programmed altitude. Then the stall warning sounds. This is a synthesized human voice that repeatedly calls out, “Stall!” in English, followed by a loud and intentionally annoying sound called a “cricket.” A stall is a potentially dangerous situation that can result from flying too slowly. At a critical speed, a wing suddenly becomes much less effective at generating lift, and a plane can plunge precipitously. All pilots are trained to push the controls forward when they’re at risk of a stall so the plane will dive and gain speed.
The author may have a perfectly valid understanding of aerodynamics. Perhaps he just wants to simplify the description for the magazine’s readership. Either way, the description is completely wrong. Stalls have nothing to do with airspeed and they don’t occur from flying too slowly. There is no critical speed at which the wing “becomes less efficient”. Stalls occur exclusively from exceeding the critical angle of attack, period.
Angle of attack and airspeed are not related. You can reach the critical AOA at cruise airspeed. At the opposite end of the spectrum, if you fly at zero G, an airplane will not stall even if the airspeed is zilch.
I don’t understand the reticence to explain AOA, even to a non-flying audience. The concept is stone simple. Everyone knows that any two non-parallel lines will eventually intersect to form an angle. Describing a chord line and the concept of relative wind shouldn’t take more than a paragraph or two. That understanding makes all the difference in the world. At least, it would have to the crew of flight 447.
Now it just so happens that the critical AOA will be reached at a specified speed under a specific center of gravity position IF the load factor is exactly 1g. But this Airbus was flying through a major thunderstorm in the middle of the Intertropical Convergence Zone. The would have been significant turbulence and the load factor on the airplane would have been all over the place. Assuming the load factor will always remain at 1g is simplistic at best.
Had the pilots considered that a stall would result from excessive angle of attack and not from a specific airspeed, they could have compared the high pitch attitude to the decreasing altitude and high vertical speed and figured things out. The problem was exacerbated by the A330′s design, which masked Bonin’s control inputs because there was no force feedback to the matching set of flight controls to let the other pilot know what the aircraft was being commanded to do.
It’s doubt the official accident report will see it this way, but it seems to be that the Air France 447 accident chain started many, many years ago when the Airbus was designed. It continued when the cruise pilots were in primary flight training and learned to associate stalls with airspeed rather than angle-of-attack. Thorough aerobatic training would have disabused them of that notion rather quickly.
It was only after those pieces were in place that a pitot system failure could have resulted in the loss of the airframe when the flight crew had seven miles of altitude with which to reach the conclusion that the airplane was stalled.