Primary finding
Probable cause
The excessive pressure in the brake system resulting from heating of the recently serviced hydraulic fluid in the brake master cylinders, and the resultant locked brakes and inability to maintain directional control of the airplane.
Investigator assessment
Analysis narrative
The 29-inch tundra tire equipped airplane was being landed on a wet, sandy, gravel bar. The pilot stated that the back seat heater had been blowing hot air on the brake master cylinders, which had just been serviced. At touchdown, the airplane began to nose over. The pilot attempted to add power and keep the tail down with airflow, but the airplane continued over onto its back. Skid marks were measured for 154 feet from the touchdown point.
Source record
Factual narrative
On September 13, 1997, at 1300 Alaska daylight time, a tundra tire equipped Piper PA-18-150 airplane, N7054, was substantially damaged when it nosed over during landing on a gravel bar, 10 miles east of Saint Mary's, Alaska. The commercial certificated pilot was uninjured. The airplane was operated by the State of Alaska, Department of Public Safety. The flight was conducted under 14 CFR Part 91 as a public use flight. Visual meteorological conditions prevailed at the time of the accident, and a flight plan was filed with the operator. The airplane was accompanied by another public use airplane. The pilot stated that he was landing on a gravel bar. He stated to the NTSB investigator that the Gar-Aero 29 inch tundra tires contacted soft, wet sand, and the airplane began to nose over. He applied power to attempt to maintain airflow across the tail, but the airplane nosed over onto its back. In his pilot/operator report, the pilot indicated the brakes appeared locked, and there were skid marks in the sand for 154 feet. The pilot also indicated that hot air from the back seat heater hose was blowing on the brake master cylinders, which had just been serviced. -