Incident Overview
On August 30, 2010, about 2134 Alaska daylight time, an Aviat A-1B airplane, registration N20HY, sustained substantial damage during a landing on a sandbar approximately 35 miles northwest of Bettles Field, Alaska. The airplane was being operated by the U.S. National Park Service (NPS) as a law enforcement surveillance public use flight under Title 14, Code of Federal Regulations Part 91.
Pilot Reports
During a telephone conversation with the National Transportation Safety Board (NTSB) investigator-in-charge on September 3, a Department of the Interior (DOI) investigator reported that the pilot stated that during cruise flight in a remote area, he saw the engine oil temperature increasing and the engine oil pressure decreasing into the yellow caution area of the pressure gauge. The pilot selected a sandbar for landing. During the landing flare, the left wingtip hit some brush, causing the airplane to yaw to the left. The pilot reported that the right wing and propeller then struck the ground.
In a written report to his employer, the pilot noted seeing smoke in the cockpit and smelling hot/burning oil. He also observed a rise in engine oil temperature and a decrease in oil pressure. Based on these observations, he elected to make a precautionary landing on a sandbar. He additionally reported feeling an uncharacteristic vibration in the stick, rudder, and floorboards.
Landing Sequence
The pilot stated that on short final to the sandbar, he turned the fuel and electrical systems off, and the propeller was windmilling. During landing, he said the left wingtip entered the brush, the airplane veered left, and the right wingtip struck the ground. The airplane nosed down, and the propeller contacted the sandbar.
Post-Accident Examination
At the accident site, the DOI investigator checked the engine oil level; the dipstick indicated about 6 quarts. After recovery on a level ramp, the dipstick indicated 6.5 quarts. After removing the cowlings, a small spot of oil was found on the exhaust pipe below the engine oil dipstick/filler tube.
The engine was removed and, on September 10, under NTSB supervision, was started and run on an engine test stand. No mechanical anomalies or leaks were noted, and temperatures and pressures were within normal limits during the test run.
Propeller Damage
Examination of the propeller showed aft bending, torsional twisting, and numerous leading edge gouges and chordwise scratching, consistent with the propeller contacting the ground several times.
Additional Comments
In their final report to the NTSB, the DOI investigator noted that the distance from the mishap site to Bettles airport was 30.4 miles, which at a cruise speed of 110 mph would take just over 16 minutes. A direct route to Coldfoot was 28 miles, taking approximately 15 minutes. An NPS mentor pilot who flew the route between the accident site and Bettles stated that since the engine was still operating, the pilot could have flown towards Bettles, and if the engine failed, there were numerous areas of willow and alder trees that could have been used for a forced landing. The mentor pilot also noted there are ample, suitable gravel bars closer to Bettles.