Understanding Air France 447 by Bill Palmer Page B

that is not necessarily the case for high performance aircraft operating at high altitudes.
    To recover from a stall, the angle of attack must be reduced so that the air can flow smoothly around the wing again. This is properly accomplished by pitching the nose down. However, if there is sufficient engine power available, an application of a large amount of thrust may be sufficient to change the airplane’s direction of movement an thus indirectly reduce the angle of attack.
    On the high speed end, there are two limit speeds to consider. The first is a straight airspeed limit that limits the maximum force on the structure from high airspeeds. The second is a Mach number limit (Mach 1.0 is the speed of sound). The Mach limit references the point that airflow around parts of the wing become supersonic, forms a shock wave, and also disturbs the flow of air around the wing, which results in a large increase in drag. The Mach number that this occurs at is the critical Mach number. The supersonic flow and shock wave formation is also accompanied by a buffet, known as Mach buffet. Most pilots will never experience Mach buffet in airline operations or in simulator training. For the A330, this critical Mach number is beyond the maximum operating speed imposed by other speed limiting factors.
    In older generation aircraft, the onset of this supersonic airflow could also result in Mach tuck , a dangerous loss of control. Mach tuck is a strong pitch down force due to the redistribution of airflow and forces resulting from shockwave formation. In a Mach tuck situation the center of lift is shifted aft toward a swept wing's tips, inducing a pitch down moment. The shock wave may also reduce the effectiveness of the tail reducing its normal pitch-down moment, which may then make a pitch up recovery impossible.  In the early days of commercial and private jet operation, a number of accidents occurred due to this phenomenon.
    In order to allow for high cruise airspeeds, and avoid the effects of Mach buffet, airplanes of the A330's generation employ a “supercritical” airfoil. These air-foils typically have a larger leading edge radius, a flatter upper surface, and a rather distinctive cusp at the trailing edge. These air-foils were developed by NASA starting in 1965 and have improved over time.

    The supercritical airfoil is not “extra critical”, but one with a high critical Mach number (super, meaning high). It allows efficient cruise speeds at relatively high Mach numbers before incurring a large increase in drag due to shock wave formation.
    Modern aircraft with supercritical wing profiles offer numerous advantages, which include:
     
Improved aircraft control characteristics at high speed 21
The position of the aerodynamic center is virtually stable for supercritical profiles, and therefore less susceptible to adverse high Mach effects such as Mach tuck.
They have a higher drag divergence Mach number and greatly reduce shock-induced boundary layer separation.
Their geometry allows for a thicker wing and/or reduced sweep angle, each of which may help reduce the weight of the wing structure.
The increase in drag above a given speed is so great that it is extremely unlikely, or even impossible, to fly faster than the demonstrated dive speeds that ensure the absence of flutter in flight testing (typically set at maximum operating Mach +.07).
    Therefore, the airfoil is better behaved at near Mach speeds than older generation air-foils, as the critical Mach number is higher, and the buffet effects less. As a result, the threat of loss of control due to an over-speed is much less than in older generation aircraft.
    Unfortunately, the characteristics of these new air-foils and the reduced possibility of Mach buffet are not well known to pilots.
    High Mach Stall
    At higher Mach numbers, the stall angle of attack is considerably decreased. Despite the flight school admonition that an airplane always stalls at the same angle of attack, the