Piper PA-28-140 N3922K accident near Richfield, Utah

Probable cause

The pilot’s decision to operate the unairworthy airplane with suspected performance deficiencies in a high density altitude and the subsequent partial loss of engine power due to low compression.

NTSB analysis

Prior to the flight the pilot asked a local airframe and powerplant mechanic to look over the airplane, as it had been sitting a couple of weeks and he suspected that it was not operating properly. The mechanic told the pilot to fly it around the airport traffic pattern and then bring it back so he could do a compression check on the engine; the mechanic was unaware that the airplane was out of its annual inspection. The pilot conducted a preflight inspection of the airplane, started the engine, and prior to takeoff ran the engine up, noting that the magnetos were within tolerances. The pilot said that after taking off and initially climbing without incident, the engine began to lose rpms but was still running smoothly. The pilot stated that he then pushed the mixture in all the way, and soon thereafter the engine rpms were down to about 1,700 and the airplane was losing altitude. The pilot elected to make an emergency landing in a field. During the emergency landing the airplane sustained substantial damage to its left wing after impacting a berm. The airplane subsequently slid between 60 feet to 75 feet before coming to rest in an upright position. A postaccident examination of the engine revealed low compression on the No. 1 and No. 4 opposing cylinders, which most likely resulted in the partial loss of engine power. Because the worst compression readings were on cylinders on opposite sides of the engine, it may have run fairly smoothly but at a reduced power output. Additionally, the pilot’s decision to position the mixture to full rich, coupled with a density altitude of almost 10,000 feet, could have contributed to the partial loss of power. While a review of a Federal Aviation Administration’s Carburetor Icing Probability Chart revealed "Icing - glide power" for the reported temperature and dew point conditions, the investigation could not definitively identify carburetor ice as the reason for the power loss.

Text from the NTSB. Open the NTSB docket for WPR10LA302 for the full record.