Plane & Pilot
Tuesday, June 24, 2008

The Accelerated Stall

Stalling at higher speeds than a normal stall

NTSBThe 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.
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An FAA inspector interviewed the pilot’s HA-200 flight instructor, who reported that the pilot was “above aggressive pilot in command [who] knew the aircraft systems well.” During the fifth hour of instruction, the pilot expressed a desire to do some aerobatics. The instructor asked him to demonstrate a barrel roll at 12,000 feet. The instructor said that during the maneuver, the pilot lost 6,000 feet and rolled out 70 degrees off heading. The instructor then demonstrated a Cuban 8, an aerobatic maneuver. The lesson ended with the instructor advising the pilot not to perform aerobatics in the HA-200. He suggested that the pilot take basic aerobatic instruction in a slower, more forgiving airplane. The instructor submitted a written statement to investigators in which he said the pilot “flew well, flew safe and had a good command authority for his new jet. I was impressed with his command ability and attention to detail.” When the pilot said he planned to fly the airplane in air shows, the instructor informed him that “low-level air show work had huge, unforgiving risks involved” and that performing “aerobatics was totally out of the question.” The pilot said he would seek another instructor.

The instructor described the former Spanish Air Force trainer as “very docile.” He said the HA-200 takeoff profile was “fairly flat,” with retraction of the landing gear occurring between 100 and 110 knots. Gear retraction was slow, with the main gear retracting first, followed by the nosegear.

The instructor said he doubted the airplane could have climbed more than 200 to 300 feet, and airspeed would have been no more than 130 to 140 knots. This speed would allow for shallow turns. If, however, the airplane were to make a steep bank, as witnesses described, a 180-knot airspeed would be required to sustain a 60-degree bank. If the airplane stalled, it would always roll toward the low wing. With its “almost perfect center of gravity,” the airplane would recover from an accelerated stall after the pilot unloaded the wings.

The NTSB determined that the probable cause of this accident was the pilot’s failure to maintain adequate airspeed, resulting in an accelerated stall at an altitude too low for recovery. The pilot’s lack of experience in the aircraft make and model was a factor. The private pilot, who was rated for single-engine and multi-engine airplanes, wasn’t instrument rated and had logged 1,178 hours with 29 in type.

On November 16, 2004, after taking off from Clark Memorial Airport in Williams, Ariz., a Cessna Super Skymaster collided with terrain while scouting for elk around Drake, Ariz. The commercial pilot and two passengers were killed. A witness reported to an NTSB investigator that he had observed the airplane flying low and slow, in and out of the local canyons and valleys over the previous few days and on the day of the accident. He also stated that he had talked to another witness who said the airplane had flown about 100 feet over his camp on the morning of the accident.

The Cessna 337B’s POH includes a stall-speed table for a Skymaster at a 4,300-pound gross weight on a standard day. According to the table, for an airplane configured with landing gear down and flaps at one-third, the stall speed at zero angle of bank is 71 mph (62 knots) CAS. At a 30-degree angle of bank, the stall speed is 76 mph (66 knots) CAS. Data recovered from a GPS receiver found in the wreckage showed the airplane flying in an easterly direction with an average groundspeed decreasing from 90 to 80 knots. The flight reversed course and proceeded west, and the average groundspeed decreased from 80 to 67 knots. Then, the track started to reverse direction and the average groundspeed decreased from 70 to 59 knots. Three seconds later, the track’s average groundspeed was 19 knots. The GPS receiver’s next recorded position showed a groundspeed of zero mph.

The NTSB determined that the probable cause of this accident was the pilot’s failure to maintain airspeed above stall speed while maneuvering at low altitude, which resulted in an accelerated stall and spin. A factor in the accident was the pilot’s performance of maneuvering flight at an altitude insufficient to allow for recovery from an accelerated stall.

Peter Katz is editor and publisher of NTSB Reporter, an independent monthly update on aircraft accident investigations and other NTSB news. To subscribe, write to: NTSB Reporter, Subscription Dept., P.O. Box 831, White Plains, NY 10602-0831.

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