N20800Cessna 172M · 1975

On April 29, 2013, about 1215 Pacific daylight time, a Cessna 172M, N20800, landed in a vineyard near Lakeport, California, following a loss of engine power. The airplane was registered to Air Ward, Inc., and operated by the pilot under the provisions of 14 Code of Federal Regulations Part 91. The commercial pilot was not injured, and the airplane sustained substantial damage. The local flight departed Lampson Field Airport, Lakeport, California, about 1200. Visual meteorological conditions prevailed, and no flight plan had been filed.

The pilot reported performing an uneventful preflight check, engine run-up, and departure. During the initial climb to 4,000 feet, the engine began to make a rumbling sound, and its speed reduced to about 1,800 rpm. The pilot adjusted the fuel mixture control, and confirmed the fuel selector valve was set to both tanks; however, the airplane would not maintain the climb. He subsequently turned the airplane back towards Lampson Field, and about 3 minutes after the initial anomaly, the engine lost all power.

The pilot set the airplane's airspeed to best glide, and performed a forced landing into a vineyard. The airplane struck a trellis, and sustained substantial damage to both wings and the horizontal stabilizer.

Engine Examination

The engine was removed and examined. The sump contained about 5 quarts of black-colored oil; the oil filter was free of visible particles, and about 1/4 of the oil screen surface was covered in black debris.

External examination revealed that while the engine could be rotated by hand at the propeller hub, rotation required an unusually large amount of force. The valves and pistons for cylinders 1, 3, and 4 moved appropriately, and "thumb compression" was attained for those cylinders after the removal of the lower spark plugs. No movement was observed to any of the components for cylinder two.

The engine was partially disassembled, revealing that piston number two had become seized within the cylinder head. The connecting rod displayed pink discoloration and remained connected to the crankshaft, but had sheared at the small end, obliterating its connection to the piston pin. The engine sump contained pulverized fragments of piston and rod material, connecting rod bearing, and connecting rod bolts. Both the number 1 piston rod and the crankshaft in the area of the number 1 and 2 connecting rods were dry, and displayed discoloration consistent with excessive heat.

Engine Lubrication System

Engine oil for the O-320-E2D series engine is circulated under pressure via a meshed spur-gear pump, housed on the accessory case at the rear of the engine. The pump is mounted in-line with, and driven directly by, the crankshaft via its input coupling.

Oil Pump Examination

The accessory case was removed, revealing the engine oil pump. The pump housing exhibited a crack branching from the upper mounting hole through to the support web at the oil pump input drive, then beyond the web and down to the pump base. Removal of the pump revealed that one tooth from the left-hand impeller gear had fractured and was missing; the right gear retained all of its teeth, but had fractured at the root between two of the teeth, splaying open the gear such that it no longer positively attached to the oil pump input drive shaft. Both gears were of the steel type. The pump housing exhibited mushroom-like deformation at the interface with the input drive.

The oil pump was sent to the NTSB Materials Laboratory for metallurgical examination. Analysis revealed that the fracture surfaces for both gears exhibited dull, tortuous features consistent with overstress failure. No indications of other failure modes, such as fatigue or corrosion were found on the gear fracture surfaces.

The inner surface of the pump housing exhibited smearing and witness marks consistent with gear tooth contact, and the crack from the pump base to the input drive shaft web displayed a witness mark along its entire length consistent with gear tooth contact.

Maintenance Records

According to maintenance logbooks, the engine was overhauled in 2004, at which time it had accumulated a total of 7,061 flight hours, at a tachometer time of 7,866 hours. The most recent annual inspection was performed on April 1, 2013, at a tachometer time of 10,160 hours.