PALMER, AK, USA
N4911
Pilatus PC-6-350
The pilot stated that while departing an 800-foot off-airport landing area, the airplane did not become airborne and collided with trees at the departure end. The airplane was 1,268 pounds under maximum takeoff weight. The pilot had operated from this location about 500 times. Winds were calm. The propeller had been overhauled 32 hours prior to the accident. The airplane had only been operated from a 3,600 feet long runway since the overhaul. A test run of the engine/propeller combination was performed. The engine did not reach design takeoff static rpm required to produce takeoff power during the first two tests. Subsequent test operation of the engine was normal. Testing and disassembly of the propeller governor failed to reveal any defects.
On September 29, 1997, at 1650 Alaska daylight time, a Pilatus PC-6-350 Porter airplane, N4911, was substantially damaged when it collided with trees at the end of an 800 feet long off airport landing area, near the base of the Knik Glacier, 15 miles southeast of Palmer, Alaska. The commercial certificated pilot and two passengers were not injured. The airplane was operated by Glenn Air, Inc., of Palmer. The flight was conducted under 14 CFR Part 91, as a personal flight. Visual meteorological conditions prevailed in the area, and winds were calm. No flight plan was filed. The pilot indicated during interviews with the NTSB and FAA, that the airplane did not become airborne during the takeoff run, and collided with trees. The pilot aborted the takeoff and placed the mixture control in "idle cutoff" prior to impacting the trees. He reported that he has flown to this strip about 500 times, and has accumulated approximately 250 hours in this airplane. The estimated gross weight at takeoff was 3,582 pounds. Maximum allowable weight is 4,850 pounds. Postaccident investigation revealed that 32 hours had elapsed since the Hartzell HC-B3Z-2 propeller was overhauled. Until the accident flight, the airplane had only been operated from the 3,600 foot long runway at Palmer Municipal Airport. The Lycoming model IGO-540-A1A engine, serial number L-193-49, was mounted on a test stand and operated with the above propeller, and propeller blades model 10151-8, still installed. The engine/propeller combination appeared undamaged from the impact. The first test run resulted in the engine reaching 3,100 rpm at 26 inches of manifold pressure. Engine rpm then decreased with increased throttle application. The design rpm required to achieve takeoff power, according to Type Certificate Data Sheet number P-907, is 3,397 engine rpm. This decrease in engine, and propeller speed, was accompanied by an audible change in noise level. This test was duplicated two times, and then the engine was shut down. A repeat test was attempted, but the anomaly could not be duplicated, and full design speed of 3,397 rpm was attained. Engine and propeller oil was strained, and filters inspected, but no obvious defects were noted. Subsequent flow testing, and a disassembly inspection of the Hartzell Propeller Governor model B-3-1, serial number 332EJ, found no discrepancies.
An underspeed of the propeller system.
Source: NTSB Aviation Accident Database
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