NTSB Factual Report

HISTORY OF FLIGHT

On August 21, 2011, about 1545 Pacific daylight time, a Piper PA-31-310, N1009S, experienced a loss of engine power in the left engine during cruise, and the pilot made a forced landing in open desert terrain near Cabazon, California. Southern California Edison (SoCal Edison) operated the airplane under the provisions of 14 Code of Federal Regulations Part 91 as an aerial survey flight. The commercial pilot and two surveyors were not injured; the airplane sustained substantial damage. Visual meteorological conditions prevailed for the local area flight that departed Banning Airport (BNG), Banning, California, at an undetermined time. No flight plan had been filed.

The pilot was interviewed by the National Transportation Safety Board investigator-in-charge (NTSB IIC). He stated that the purpose of the flight was to survey the power lines between BNG and Palm Springs wind farms. The pilot had flown the airplane to BNG the day before and the morning of the accident; the accident flight in the afternoon was to be the last flight. On the previous two flights, there had been no mechanical discrepancies encountered.

The first part of the flight from BNG to the wind farms was uneventful. The pilot made the turn back to BNG, and was about 20 miles out of BNG when he heard a loud bang come from the left engine. Oil began gushing out of the engine, followed by an engine shutdown. The pilot stated that he then feathered the propeller, and attempted to return to BNG airport. He stated that he was not able to maintain altitude, and decided to make a forced landing in an open field, which damaged both wings, and the undercarriage.

According to the operator, the airplane was hired by Towill, Inc., to perform survey work for SoCal Edison.

The owner of the airplane and the pilot did not return the National Transportation Safety Board (NTSB) Pilot/Operator Report NTSB Form 6120.1/2.

WRECKAGE AND IMPACT INFORMATION

According to the responding Federal Aviation Administration (FAA) Airworthiness inspector, the airplane landed short of the runway on the desert floor. It skidded across brush and rocks for 213 feet. The FAA inspector reported that the engine case had cracked between the numbers five and six cylinders at the top of the engine. Visual inspection of the engine did not reveal any other discrepancies. The spark plugs were removed and were normal and clean in appearance; the oil filter contained grainy debris. The top and bottom of the left engine cowls, as well as the inside of the cowls, were covered in oil. The pilot reported to the FAA inspector that both engines had 10 quarts of oil prior to the flight.

TEST AND RESEARCH

An examination of the left engine was performed at Jet Components in Ovilla, Texas, under the auspices of the National Transportation Safety Board (NTSB). The engine was a Lycoming LIO-540-A2D, serial number L-166861.

The engine remained attached to the engine mounts, and the engine accessories were separated from the engine; all six cylinders remained attached to the engine. Visual examination confirmed that the engine crank case had fractured above the numbers five and six cylinders. The fracture extended into the accessory case. Cylinder numbers one, two, three, and four exhibited normal operating signatures with no damage noted.

Once all of the cylinders were moved, rotational continuity of the crankshaft was established to the aft accessory gears. During the examination, the aft most counterweight was found partially attached to the crankshaft. The trailing edge roller pin, washer, and part of the retaining snap ring were displaced from the counterweight and were subsequently found in the oil sump. The retaining washer was fragmented into multiple pieces. The remaining portion of the retaining snap ring was removed from the counterweight and found to be installed correctly (beveled side facing inward and the sharp side facing outward). The retaining washer was separated from the trailing edge of the aft counter weight. No additional anomalies were noted during the examination. A detailed engine examination report is attached to the public docket for this accident.

According the left engine logbook, on September 24, 2010, the engine had been removed, disassembled, repaired, and reinstalled on the airplane. The last annual inspection was signed off on January 14, 2011.

NTSB Probable Cause

A total loss of left engine power due to the failure of the aft crankshaft counterweight retaining washer for reasons that could not be determined during postaccident examinations.