NTSB Factual Report

On August 30, 2010, about 2134 Alaska daylight time, an Aviat A-1B airplane, N20HY, sustained substantial damage during a landing on a sandbar, about 35 miles northwest of Bettles Field, Alaska. The airplane was being operated by the U.S. National Park Service as a law enforcement surveillance public use flight, under Title 14, Code of Federal Regulations Part 91.

During a telephone conversation with the National Transportation Safety Board (NTSB) investigator-in-charge (IIC) on September 3, an investigator for the Department of the Interior (DOI) said the pilot related to him that during cruise flight in a remote area, he saw that the engine oil temperature was increasing, and the engine oil pressure was decreasing into the yellow caution area of the pressure gauge. The pilot reported that he selected a sandbar upon which to land. The pilot related that during the landing flare the left wingtip hit some brush causing the airplane to yaw to the left. He reported that the right wing and propeller struck the ground.

In a written report to his employer, the pilot reported that during the flight he saw smoke in the cockpit, and smelled hot/burning oil. He also said he saw a rise in engine oil temperature, and a decrease in oil pressure. Based on those observations he elected to make a precautionary landing on a sandbar. He noted that he also felt an uncharacteristic vibration in the stick, rudder, and floorboards.

The pilot reported that on short final to the sandbar he turned the fuel and electrical systems off, and indicated the propeller was windmilling. During the landing he said the left wingtip got into the brush, and the airplane veered to the left, with the right wingtip striking the ground. The airplane nosed down, and the propeller struck the sandbar.

At the accident site, the DOI investigator said he checked the airplane engine oil level, and the dipstick indicated about 6 quarts. After recovery, sitting on a level ramp, he said the dipstick indicated 6.5 quarts. After removing the cowlings, he said he found a small spot of oil on the exhaust pipe below the engine oil dipstick/filler tube.

The engine was removed from the airplane, and on September 10, under the supervision of the NTSB IIC, the engine 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.

An 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.

In their final report provided to the NTSB, the DOI investigator reported that "from the mishap site to the Bettles airport was actually 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 and would have taken approximately 15 minutes. An NPS [National Park Service] 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. He also stated there are ample, suitable gravel bars closer to Bettles."

NTSB Probable Cause

The pilot's selection of an unsuitable off-airport landing site for a precautionary landing.