History of Flight
On August 21, 2011, about 1545 Pacific daylight time, a Piper PA-31-310, registration N1009S, suffered a loss of engine power in the left engine during cruise flight. The pilot executed a forced landing in open desert terrain near Cabazon, California. The aircraft was operated by Southern California Edison (SoCal Edison) under 14 CFR 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 was filed.
The pilot informed the National Transportation Safety Board investigator-in-charge (NTSB IIC) that the flight's purpose was to survey power lines between BNG and the Palm Springs wind farms. The pilot had flown the aircraft to BNG the previous day and on the morning of the accident; the afternoon flight was intended to be the last. No mechanical discrepancies were noted on the earlier flights. The first segment from BNG to the wind farms was uneventful. While turning back toward BNG, approximately 20 miles out, the pilot heard a loud bang from the left engine. Oil began gushing from the engine, followed by an engine shutdown. The pilot feathered the propeller and attempted to return to BNG but could not maintain altitude, prompting a forced landing in an open field, which damaged both wings and the undercarriage.
According to the operator, the aircraft was hired by Towill, Inc., to perform survey work for SoCal Edison. The owner and pilot did not submit the NTSB Pilot/Operator Report Form 6120.1/2.
Wreckage and Impact Information
The responding Federal Aviation Administration (FAA) Airworthiness inspector reported that the airplane landed short of the runway on the desert floor, skidding across brush and rocks for 213 feet. The engine case had cracked between the number five and six cylinders at the top of the engine. Visual inspection revealed no other discrepancies. Spark plugs were normal and clean; the oil filter contained grainy debris. The top and bottom of the left engine cowls, as well as their interiors, were covered in oil. The pilot reported that both engines had 10 quarts of oil prior to the flight.
Test and Research
An examination of the left engine was conducted at Jet Components in Ovilla, Texas, under NTSB auspices. The engine was a Lycoming LIO-540-A2D, serial number L-166861. The engine remained attached to its mounts, with accessories separated; all six cylinders remained attached. Visual examination confirmed that the crankcase had fractured above the number five and six cylinders, with the fracture extending into the accessory case. Cylinders one through four exhibited normal operating signatures with no damage noted.
After moving all cylinders, rotational continuity of the crankshaft was established to the aft accessory gears. The aftmost counterweight was found partially attached to the crankshaft; its trailing edge roller pin, washer, and part of the retaining snap ring were displaced and found in the oil sump. The retaining washer was fragmented. The remaining snap ring portion was correctly installed (beveled side inward, sharp side outward). The retaining washer had separated from the trailing edge of the aft counterweight. No additional anomalies were noted. A detailed engine examination report is in the public docket.