Tuesday, June 24, 2008
The Accelerated Stall
Stalling at higher speeds than a normal stall
The accelerated stall usually surprises a pilot because it occurs at a higher airspeed than a normal stall (in which a wing loading of 1 G is maintained). Remember, a wing can be made to stall at any speed—all that has to happen is for the angle of attack to get high enough. As G-loading increases, so does stall speed. If a wing reaches its critical angle of attack when the wing loading is 2 G, twice normal, the stall will occur at a speed that’s proportional to the square root of the wing loading. The square root of 2 is approximately 1.41, so the stalling speed at 2 G will be 1.41 times what it would be under 1 G conditions. Accelerated stalls are often caused by abrupt or excessive control inputs made during steep turns or pull-ups. If you’re in a dive and pull back with enough suddenness and force to load the airplane to a typical design load factor of 3.8 G’s, you’ll enter an accelerated stall if the airspeed drops below 1.95 times the stall speed at 1 G loading (the square root of 3.8 is approximately 1.95).
One of the most dramatic accidents involving an accelerated stall occurred on September 6, 1985, at General Mitchell International Airport in Milwaukee, Wis. Midwest Express Airlines Flight 105, a DC-9, was taking off from runway 19R at about 3:21 p.m. The weather was clear; visibility was 10 miles. During initial climb, at about 450 feet AGL, there was a loud noise and loss of power associated with the right engine. Investigation would later indicate that there had been an uncontained failure of the engine’s ninth- and tenth-stage high-pressure compressor spacer.
The airplane continued to climb to about 700 feet AGL, then rolled to the right until the wings were nearly vertical. While rolling, the airplane entered an accelerated stall, lost control and crashed. Both pilots, both flight attendants and all 27 passengers were killed. The airplane was destroyed in the crash and fire that followed.
The NTSB studied the flight characteristics of the DC-9 and determined that in the event of a right-engine failure, it would be docile, easily controllable and not require any unusual skills or strength from the pilots. Investigators found that about four or five seconds after the uncontained right-engine failure, one or both of its pilots applied incorrect rudder pedal forces followed by aft control column forces that allowed the airplane to stall at a high airspeed—an accelerated stall. The NTSB determined that the probable cause of the accident was the crew’s improper use of flight controls in response to catastrophic failure of the right engine, which led to an accelerated stall and loss of control.
Of course, the aerodynamics of accelerated stalls applies to small airplanes as well as large ones like the DC-9. A Piper Cherokee was involved in an accelerated stall accident after an engine problem surfaced during an instructional flight. The PA28-180 had taken off from runway 28 at New Jersey’s Millville Municipal Airport at 3:28 p.m.; visual meteorological conditions prevailed. An IFR flight plan had been filed to Atlantic City International Airport. At about 600 feet AGL, the private pilot, who was working toward an instrument rating, entered a right turn for the Cedar Lake VOR. During the turn, the engine began to run rough and the tachometer showed that the engine rpm had dropped below 2,000. The instructor told the pilot to return to Millville. At approximately 500 feet, the pilot reduced power, applied flaps and prepared the airplane for a downwind landing on runway 10.
The airplane flew just above the runway, and the pilot noticed that there were workers and a jet aircraft in position for takeoff at the opposite end (runway 28). The pilot and instructor elected to perform a go-around. They applied maximum throttle and started a slight climbing right turn above trees at the runway’s end, then entered a turn to set up for landing on runway 28. During the turn, the airplane entered an accelerated stall; it was too low for a recovery, and the left main landing gear impacted the ground. The airplane then continued across a grass area and struck a group of trees. The pilot and instructor received serious injuries and the airplane was substantially damaged.
In another incident, a Spanish military jet was preparing to be flown around Pueblo Memorial Airport in Pueblo, Colo., so that people on the ground could take photographs for use in a promotional brochure for the upcoming air show season. The Hispano Aviación HA-200 Saeta was flown by a private pilot; visual meteorological conditions prevailed.
The airplane was serviced with 71 gallons of Jet A, and a Prist additive was added to an undetermined amount of fuel that was already on board (total fuel capacity is 152 gallons). At 7:12 a.m., the pilot was cleared to taxi to runway 26L and was issued a transponder code of 0333. At 7:16 a.m., after requesting touch-and-go landings, the pilot was cleared for takeoff and to “make left closed traffic runway 26L.”
Reports from witnesses indicated the airplane made what appeared to be a normal takeoff, lifting off about halfway down the 4,073x75-foot runway, 26L. It then rolled steeply to the left and the nose fell through the horizon. The airplane struck the ground, exploded and burned. The pilot was fatally injured.
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