Accident Overview
On June 30, 2008, at approximately 1240 Pacific daylight time, a Cessna 172M, registration N61736, was substantially damaged during a forced landing near Oakland, California. The commercial pilot and his passenger sustained minor injuries. The airplane was registered to and operated by Flying Vikings Inc. of Hayward, California, under 14 CFR Part 91. Visual meteorological conditions prevailed, and no flight plan was filed for the local traffic watch flight that originated from Hayward Executive Airport at 1100.
Engine Malfunction and Forced Landing
According to the pilot, during cruise flight, the engine began running rough and experienced a surge in power. The pilot adjusted the mixture to full rich, which temporarily smoothed the roughness. Shortly thereafter, the oil temperature gauge rose, prompting the pilot to divert toward Oakland International Airport. While en route, the engine lost power, and the pilot initiated a forced landing into an open lot. During the landing roll, the airplane struck a large mound of dirt and came to rest upright.
Post-Accident Examination
A Federal Aviation Administration inspector examined the airplane and found structural damage to the right wing and fuselage. Oil was observed on the undercarriage from the engine aft to the empennage.
On July 24, 2008, the airframe and engine were examined by the NTSB investigator-in-charge. The Lycoming O-320-E2D engine (serial number RL-40510-27A) remained attached to the airframe via all four engine mounts. All accessories and cylinders were attached. The four nuts securing the vacuum pump to the mount pad were found loose. Oil was observed on the rear accessory housing below the vacuum pump and between the magnetos. The oil dipstick indicated no oil in the sump. When compressed air was applied to the oil cooler inlet line, bubbling oil was observed from the vacuum pump mounting pad.
The vacuum pump and both magnetos were removed. The bottom portion of the vacuum pump seal was saturated with oil. The crankshaft was rotated by hand and was extremely stiff. Thumb compression was obtained on all four cylinders. The oil suction finger screen contained metallic debris.
Cylinders number one and three were removed. Both had normal combustion deposits, and the pistons exhibited scuffing on the skirts. Internal connecting rods appeared intact. The number three and four connecting rods and the crankshaft area showed thermal discoloration.
The left magneto produced spark on all leads when rotated by hand with impulse coupling engagement. The right magneto produced spark when rotated with an electric hand drill.
The propeller remained attached; one blade was slightly bent aft about 10 degrees, and the other was undamaged.
Maintenance Records and Mechanic Interview
Review of the engine logbook showed the most recent 100-hour/annual inspection was on June 16, 2008, at a tachometer time of 3,036.9 hours and 3,910.6 hours since major overhaul. A squawk sheet indicated that the vacuum pump was replaced after the most recent flight recorded prior to the accident flight on July 27, 2008.
According to the FAA inspector, the airframe and power plant mechanic who installed the vacuum pump reported that he installed the pump and tightened all nuts except the bottom left, which he could not reach. He stated that he used a screwdriver and hammer to tighten that nut.
Probable Cause
No probable cause was stated in the source.