April 1, 1998
A few days before Christmas of last year, an Airborne Express Douglas DC-8-63 freighter crashed in mountainous terrain near Narrows, Virginia, killing all aboard. The accident occurred during a functional flight test flown at night in IMC conditions. The test hop was scheduled to include a series of systems and performance checks, including approach-to-stall recoveries, required as a result of major modification and inspection work that had just been completed on the aircraft. Following departure from GSO (Greensboro, North Carolina), the crew requested and received clearance for a block altitude assignment of 13,000 to 15,000 feet in which to perform the checks. Half an hour after takeoff, the aircraft was observed on radar to descend below the floor of the altitude block at low airspeed. The crew declared an emergency while descending at a high rate through 8,000 feet, after which there was no further radio contact. The aircraft struck terrain at an elevation of 3,400 feet msl.
The National Transportation Safety Board determined that the probable causes of the accident included "inappropriate control inputs" applied by the flying pilot during a stall recovery, and "the failure of the nonflying pilot-in-command to recognize, address, and correct those inputs." Contributing to the accident was an inoperative stick-shaker stall warning system, as well as Airborne's DC-8 simulator, which the agency said lacked sufficient fidelity to accurately reproduce the airplane's stall characteristics.
Although the formal investigation is over, the flight's tragic ending nevertheless prompts questions about the nature of sweptwing jet aircraft.
Do, for instance, such aircraft stall differently than light piston-powered aircraft with considerably less sweep? Are the kinds of stall recoveries that we learn early in primary flight training relevant to large jet aircraft with highly sweptwing designs?
A wing is a wing, of course. Each has a particular angle of attack that results in the greatest lifting efficiency, or maximum coefficient of lift. Straight or swept, the amount of lift produced decreases when this optimum angle of attack is exceeded. If increased enough, airflow around the wing is disrupted to the point that the wing stalls.
Why sweep a wing in the first place? One natural (and correct) assumption is that it lets the airplane go faster. It does so in part by delaying the onset of the airfoil's critical Mach, the point where localized airflow over portions of the wing reaches the speed of sound before the aircraft itself does. Exceeding critical Mach can lead to undesirable aerodynamic effects in subsonic aircraft. The most notable, perhaps, is shock-wave-induced airflow separation from the wing, with a resultant loss of lift. Sweeping the wing allows the relative airflow over its surface to occur in a more spanwise direction. Compared with a straight wing, the airflow encounters less pronounced camber along this path and so doesn't accelerate as much. This allows the aircraft itself to reach higher speeds before the wing's critical Mach is reached.
To optimize a sweptwing jet aircraft's efficiency in the high-altitude, high-speed regime where it spends the most time, aerodynamicists employ other sleight of hand, such as boundary layer strips, winglets, variable camber, and wing twist. But there are limits to everything. Jets come up against theirs in the coffin corner, the morbid but well-deserved name given to the edges of their high-altitude operating envelope.
There are two distinctly different "buffet boundaries" that mark the extremes of the coffin corner, and pilots must be aware of both. High-speed buffet refers to the aerodynamic buffeting that occurs as a jet aircraft approaches its critical Mach. The resulting shock wave interferes with the wing's lifting ability by causing some airflow to separate from the wing. The design of typical sweptwing aircraft is such that this flow separation develops first at the wing root, then progresses gradually outward toward the wing tips. The aircraft's center of lift moves aft as the outer, more rearward portions of the wing assume more total lift. This in turn produces an increasing nose-down moment. If allowed to progress unchecked, Mach tuck may eventually occur. Although Mach tuck develops gradually, if it is allowed to progress significantly, the center of lift can move so far rearward that there is no longer enough elevator authority available to counteract it, and the aircraft could enter a steep, sometimes unrecoverable dive.
Low-speed buffet marks the coffin corner's other "wall." It begins when the wing's angle of attack approaches its stalled condition. This is the same kind of pre-stall buffeting that precedes a low-altitude stall in most light aircraft. At the high density altitudes associated with flight-level flying, however, low-speed buffeting will occur at higher indicated airspeeds than would be true at low altitudes.
The margin between high- and low-speed buffet in the coffin corner may be just a few knots, and it can even disappear altogether. Pilots are well-advised to choose a lower flight level if the coffin corner's high- and low-speed buffet boundaries converge too closely. Otherwise, a small increase in bank angle or an encounter with turbulence may be all it takes to induce an actual stall.
For instance, the 1.3-G Buffet Boundary chart for the Boeing 737-500 aircraft shows that at FL370 and a weight of 115,000 pounds, both high- and low-speed buffet will occur at 233 KIAS while in a 1.3-G maneuver. This obviously leaves an uncomfortably small margin for safe operation. At FL350, however, the envelope widens considerably. Low-speed buffet occurs at 220 KIAS, while 259 KIAS marks the high-speed buffet threshold.
Pilots undergoing stall training in jet aircraft are taught to recover at the first sign of an impending stall. Normally, this is indicated by pre-stall buffeting or by the aircraft's stick shaker, which activates at around 107 percent of the actual stall speed. At such slow speeds, very high sink rates can develop if aircraft pitch is decreased below the horizon, as is normal recovery procedure in most piston-powered, straight-wing light aircraft. Therefore, at low altitudes where plenty of engine thrust is available, proper recovery in many sweptwing jets involves applying full available power on all engines, rolling wings level, and holding a slightly positive pitch attitude. The amount of pitch required varies by aircraft but should be sufficient to maintain altitude or begin a slight climb.
At high altitudes, where there may be little excess engine thrust available to effect a recovery using power alone, the procedure is somewhat different. Here it may be necessary for the crew to lower the nose below the horizon in order to accelerate away from the impending stall. Some aircraft require several thousand feet or more to recover from a fully developed stall entered at high altitude. Pilots must resist the temptation to raise the nose prematurely during recovery, since a secondary stall is likely with an increased G load experienced near the stall speed.
Certain sweptwing jet aircraft are capable of entering so-called deep or super stalls, from which recovery may not be possible. Deep stalls are the result of design characteristics that cause these aircraft to pitch up markedly after a full stall occurs. The disturbed airflow streaming from the stalled wing then blankets the tail, causing a loss of tailplane effectiveness. Once this happens, there is insufficient pitch control available for the pilot to lower the nose to recover. To preclude such a situation, aircraft susceptible to deep stalls are equipped with stick pushers that automatically do as their name implies, before the aircraft reaches a fully stalled condition.
Unlike many light GA aircraft, which commonly touch down close to or even at stall speed, sweptwing jet aircraft are flown onto the runway at a target speed commonly calculated at 1.3 times the stall speed for the particular landing configuration. Proper landing technique calls for maintaining most of this speed all the way to touchdown (a small amount is lost in the flare). Attempting to hold the aircraft off the runway so as to touch down close to stall speed can use up hundreds or even thousands of feet of additional runway. Also, the drag rise associated with increased pitch for sweptwing aircraft is very rapid at slow speeds. Bleeding off too much speed during landing can result in a sudden and dramatic sink rate. At this point, the aircraft is well behind the power curve. Even large amounts of thrust may not be enough to arrest the resulting "sinker" in time to prevent a hard landing, or worse. Avoiding "low and slow" or idle power approaches thus becomes especially important in sweptwing jet aircraft.
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