Cessna 172S N562AD accident near Oceanside, California

Probable cause

A defective alternator wire splice that resulted in an intermittent electrical system malfunction and erroneous fuel gauge readings, and, a loss of engine power for an undetermined reason.

NTSB analysis

The pilot was on an instrument clearance in cruise flight at 6,000 feet mean sea level above a cloud layer. He said that while he did not see any fuel leaking, the fuel gauges were moving steadily toward zero at a higher rate than normal. He moved the fuel selector to the right tank position, yet the fuel gauges still kept moving rapidly toward zero. The pilot then attempted to make a radio call to declare an emergency; however, there was a complete electrical failure. He decided to make an emergency landing at a nearby airport, and as he started descending, the engine lost power. The pilot said that since the airplane was above clouds, he could not see the airport until the airplane was directly above it. As he approached runway 24, he realized he was too high and fast, so he overflew the runway and reversed course at the west end of the airport to attempt a landing on runway 06. He was too high and fast to land on runway 06 and did not have enough altitude or airspeed to make another attempt on runway 24. The airplane impacted the ground and obstacles about 0.5 miles east of the airport. During post-accident examination, fuel was found in both wing tanks and in the firewall-mounted fuel strainer. No evidence of significant fuel leakage was noted. No evidence was found of any mechanical malfunctions or failures that would have prevented the engine from producing power. The reason for the reported loss of engine power could not be determined. A defective splice repair to the alternator output electrical wire was observed, and when the wire was removed, the splice came apart and revealed evidence of electrical arcing. The alternator was bench tested and found to be functional. The defective splice likely resulted in the anomalous fuel gauge readings noted by the pilot. The fact that air traffic control reported the airplane’s transponder was broadcasting the codes for radio failure and then for emergency as the airplane descended indicates the airplane had not lost all electrical power. The pilot’s inability to communicate by radio during the descent likely resulted from his improper positioning of avionics bus 1 switch to the OFF position where it was found, which would have cut off power to communications radio 1.

Text from the NTSB. Open the NTSB docket for WPR09LA098 for the full record.