Amateur-built VANS RV-6 N53MH accident near Lemolo Lake, Oregon

Probable cause

An overvoltage event due to the failure of the automotive voltage regulator, which resulted in electrical anomalies and smoke in the cockpit and a subsequent forced off-airport landing.

NTSB analysis

The student pilot/owner of the experimental, amateur-built airplane was conducting a long cross-country solo flight, and he completed the first two legs uneventfully. He reported that, when the airplane was in cruise flight at 10,500 ft on the final leg of the trip, he began experiencing radio problems, followed shortly by other electrical problems and smoke in the cockpit. Due to concerns about an on-board fire, he opted to conduct a precautionary landing on a road. During the landing roll, the airplane exited the paved surface, nosed over, and then came to rest inverted.

Postaccident examination of the airplane did not reveal any indications of smoke or thermal damage. However, four electrically powered devices, including the voltmeter, were found to be inoperative. Although a visual examination of the alternator, which was a 12-volt automotive-grade unit with an integral voltage regulator, found no evidence of electrical arcing or thermal damage, a functional check revealed that the continuous-regulated voltage output was greater than 18 volts. The alternator and regulator were disassembled, and no internal mechanical anomalies were identified. The electrical anomalies described by the pilot and noted during the tests and examinations, particularly the failed voltmeter, were consistent with an overvoltage event due to the failure of the automotive voltage regulator, which would have resulted in the smoke in the cockpit. Although the engine likely would have continued to run with the noted electrical system anomalies, the pilot had no way of knowing whether the observed smoke and electrical anomalies would result in additional problems or an onboard fire; therefore, his decision to conduct a forced landing was appropriate.

Text from the NTSB. Open the NTSB docket for WPR12LA402 for the full record.