It’s looking more and more like the recent AirAsia disaster is essentially a repeat of the 2009 crash of Air France 447. Popular Mechanics has a truly outstanding article (as does Vanity Fair) explaining the events which lead up to the loss of the aircraft. While PM may not explicitly denounce the pilots of AF447 as incompetent, every statement I’ve read by an American pilot on this matter explicitly condemns the pilots, their actions, and their lack of understanding of aerodynamics.
In normal flight, fixed-wing aircraft maintain altitude due to the wings’ lift counteracting the weight of the aircraft. The magnitude of the lift force is a quadratic function of the plane’s airspeed (speed relative to the surrounding air). It is important to note that the thrust from the plane’s engines is not responsible for maintaining altitude directly; rather, the thrust from the engines counteracts the drag force in order to maintain forward airspeed.
During takeoff, pilots use the aircraft’s control surfaces to increase the angle of attack which results in a higher lift coefficient. This increases the aircraft’s rate of ascent. The total engine power required at any time is equal to the plane’s airspeed multiplied by the magnitude of the vector sum of (1) the component of the plane’s weight on an axis parallel to its direction of travel and (2) the drag force (which is always parallel to the direction of travel) experienced by the aircraft. For example, during level flight, (1) is zero, because the plane’s weight is perpendicular to its direction of travel. The engines must only balance the force of drag. During takeoff, however, (1) is nonzero, and may be on the same order of magnitude as the drag force (this I am not sure about, but it is no doubt considerable). But this is not a problem, because the plane’s airspeed is very low, so the total power required is well within the engines’ performance envelope.
However, sustaining a high angle of attack at a high airspeed (such as that associated with cruising at high altitude) would require an enormous power output, far beyond that which the engines on passenger or cargo planes are capable. Additionally, for a given wing profile, there exists a critical angle of attack, above which the coefficient of lift drops precipitously due to flow separation on the upper edge of the wing. If the angle of attack exceeds this critical angle, the aircraft enters a state of aerodynamic stall, and will lose airspeed and altitude rapidly.
One of the ways that aircraft detect stalls is from airspeed. An extremely low airspeed value would imply that the aircraft has entered a stall, and is therefore losing altitude. Recall that during takeoff, pilots gain altitude through large-amplitude inputs to control surfaces. Therefore, it is conceivable that a person with no understanding of the principles I laid out in the preceding paragraphs would interpret a stall warning as simply an indication that the aircraft was rapidly losing altitude, and default to his knowledge of the procedure for gaining altitude following takeoff: command maximum up elevator. Of course, at cruising speed, this would not correct a stall but rather worsen it: it would dramatically increase the aircraft’s drag, thus reducing its airspeed further. To correct a stall at cruising speed, the pitch of the aircraft must be reduced (oriented toward the ground) in order to gain airspeed and thus regain lift force.
Airspeed is determined from the Bernoulli Principle applied to the static pressure measured in pitot tubes. If these tubes are completely blocked (this can occur due to icing), the aircraft may register a “zero” value for airspeed, and sound the stall alarm. Note that in such a scenario, there is nothing wrong with the aircraft whatsoever. This is merely an instrumentation failure. It is relatively easily identified based on other flight parameters, and experienced pilots are usually able to quickly identify the sources of such errors and take corrective action (if any is necessary; in the case of a frozen pitot tube, no such action is necessary).
On AF447, the pitot tubes froze, causing the plane to register zero (or low) airspeed. This caused the stall alarm to sound, which freaked out the junior pilots, who apparently defaulted to their procedure for gaining altitude during takeoff and commanded maximum up elevator and maximum engine thrust (incidentally, because the engines are mounted below the plane’s center of mass, the engine thrust actually increased the angle of attack even further). These inputs at cruising speed put the aircraft into an actual stall, which they did not realize, and the plane fell into the ocean about four minutes later.
228 people died on Air France 447 for absolutely no reason. Three colossally incompetent pilots took a perfectly good airplane and flew it directly into the ocean. If instead they had simply walked out of the cockpit when the stall warning sounded, taken a nap or perhaps had a few drinks, then walked back in when the plane was approaching mainland Europe, everyone would have survived. Based on the AirAsia 8501 reports, it sounds as if something very similar occurred on that flight. It’s not just tragic, it’s frustrating. If a person with a semester of formal aerodynamics training and a shred of insight had been there to watch the sequence of events prior to the crash, he probably could have explained (at least in concept) what had to be done in order to recover from the stall and thus save the passengers and crew from the pilots’ ineptitude.
I know I am armchair-quarterbacking this one, and we do not know for sure exactly what happened on AirAsia 8501. But to prove that I am just as vigorous in my praise as I am in my criticism, consider United Airlines 232, in which the pilots were able to take an aircraft which suffered a complete loss of control and, through an absolutely astonishing show of technical aptitude, resource management, and general coolheadedness, land it and save about two thirds of the passengers (equally noteworthy is a similar incident which occurred at Baghdad Airport). I find the crew’s ability function under unimaginable stress and time constraints incredibly humbling. I have the utmost respect for heroes of war who have received Medals of Honor (or equivalent medals from other nations), but there is something uniquely incredible about these pilots’ ability to function continuously in the face of nearly certain death and not only perform extremely complex technical tasks, but actually solve problems creatively without any procedural or doctrinal guidance.