PIPER PA 23-150 — Hayward, CA

What happened

After multiple taxi tests of the twin-engine airplane, the airline transport pilot departed on a local personal flight. About 70 minutes later, the airplane was about 10.5 nautical miles southeast of the airport, and the pilot requested a landing clearance. Witnesses heard one of the airplane's engines "backfiring" as the airplane proceeded northwest towards the airport, and the pilot reported to an air traffic controller that he was having trouble with his left engine and could not maintain altitude. The pilot told the controller that he was going to land in a field next to a railroad track, and the airplane then turned about 45° to the left. A witness about 1/2 mile from the accident site saw the airplane in a wings-level descent for a few seconds before it suddenly rolled into a steep left bank. Security cameras showed the airplane descending with a rapidly increasing left bank angle until it impacted a building, and a postcrash fire erupted.

Postaccident examination of the airplane revealed that the left engine's propeller was feathered, and the left engine-driven fuel pump and carburetor contained trace amounts of water and displayed a white powdery residue, consistent with long term exposure to water contamination. The left wing's inboard and outboard fuel tank caps displayed rust around their circumferences, and the caps did not form a seal when inserted into the fueling portholes, which indicated that both fuel tanks were susceptible to contamination from rain water. Examination of the right engine showed that all four intake camlobes were worn nearly concentric in shape, and the corresponding tappet faces displayed spalling. Worn camlobes can lead to a degradation in engine performance.

The failure of the left wing's fuel cap seals, evidence of water contamination in the fuel system, and the feathered left engine indicate that it is likely the pilot feathered the left engine in response to a power loss resulting from water contamination. Although a witness reported heavy rainfall in the winter that preceded the accident at the airport where the airplane was based, the investigation could not determine when or how water was introduced into the fuel system. The high speed taxi tests conducted by the pilot before the accident flight suggest he may have been aware of a problem with the airplane before departure, but the reason he conducted the taxi tests is unknown.

According to the pilot's logbook, since purchasing the airplane about 10 months before the accident, he had accumulated about 19 hours of flight time in the airplane of which 1.4 hours were in the 90 days preceding the accident. Additionally, the majority of his recent flight experience involved single-engine airplanes, thus the pilot's lack of total and recent experience in the airplane make and model suggest that he may not have been prepared to manage an inflight loss of power in a twin-engine airplane.

Factoring in the reported 11-knot surface wind, radar data indicated that the airplane's airspeed dropped below the airplane's minimum control airspeed with an engine inoperative (Vmc) after it turned into the inoperative engine near the end of the flight and remained there for the remainder of the descent. Therefore, it is likely that the airspeed decayed to the point where the pilot was unable to counteract the asymmetrical thrust produced by the right engine, which led to the rapid left roll seen in the security camera images, further loss of altitude, and impact with the building.

An editorial "what led to it / how to avoid it" analysis for this accident is generated separately and will appear here.