Understanding High-altitude Aerodynamics Is Critical
Reports about the 2009 Air France Flight 447 accident released last summer by the French safety board (BEA) said the three experienced Airbus A330 pilots were unable to recognize they were operating at a too high angle of attack to sustain flight. The reports also said the pilots were unable to see a remedy early enough to recover. But why were these three international pilots confounded by the events of that night?
AIN decided to take a look at some high-altitude basics, with the thought of stimulating a discussion on this topic.
We begin at the “Coffin Corner,” also called the “Q corner,” where AF 447 was operating at the time of the accident. (“Q” is the designation for dynamic pressure). The corner is best described as high-altitude operations where low indicated airspeeds, yield high true airspeeds and Mach numbers at relatively low angles of attack. Surprisingly, high-altitude stalls occur at a significantly lower angle of attack than many once believed, thereby providing a much narrower maneuvering margin. The stall occurs at a lower angle of attack because of the altered dynamics of airflow at higher Mach numbers and compressibility effects.
The recommended maximum altitude on the flight management system provides only 1.3-g stall protection (the g-load is already 1.2 in a level 30-degree bank), which translates to razor-thin margins. The airplane’s climb capability here is a minimum of 300 feet per minute in stable air, although in practical terms it is often less. All of this normally occurs at the upper portion of the maneuvering envelope. Turning maneuvers at high altitudes can increase the angle of attack and result in a significant reduction in stability, as well as a decrease in control effectiveness.
The relationship of stall speed to critical Mach narrows at high altitudes, to a point where any sudden increases in angle of attack or roll rate and disturbances, such as clear-air turbulence, can lead to a stall.
Training in this region in the actual aircraft is certainly dangerous, as well as impractical. Simulator training, however, is sometimes not realistic enough. Although a sophisticated simulator can replicate a high-altitude stall, the data used to run the simulator reflects only the flight-test data made available for approval of the simulator. Beyond that point, it’s guesswork at best, and the software may not actually offer characteristics that accurately duplicate those to be found in the airplane.
So, here’s our discussion question: “How should a flight crew train to best understand high-altitude operations?”
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