Globe GC 1B N3773K accident near Puyallup, Washington

Probable cause

The failure of the exhaust assembly in the No. 3 cylinder for reasons that could not be determined due to the damage incurred to the fractured parts. Also causal to the accident was maintenance personnel’s improper installation of the cylinder.

NTSB analysis

The pilot/owner of the airplane was conducting a local personal flight. The pilot reported that, while the airplane was in the traffic pattern at the destination airport, the engine made a loud "bang" and experienced a loss of power. His attempts to regain engine power were unsuccessful, and the airplane descended and then collided with an embankment located on the airport property before the runway.

Examination of the engine revealed a large mechanically formed hole on the top side of the No. 3 piston, and the cylinder head had sustained significant mechanical damage. The exhaust valve head was separated from its stem and lodged in the exhaust valve seat perpendicular to its normal seated position. The stem was not located. Numerous pieces of the No. 3 cylinder's fractured piston and piston rings and other metal debris were found throughout the engine.

A metallurgical examination revealed that only the No. 3 cylinder's liberated valve head fragment remained, and the nature of the fracture could not be determined due to the subsequent damage sustained from the head battering against the piston crown. The mating valve stem was missing, and there were no remnants of its material in the cylinder. The valve stem seat keepers and the exhaust rocker arm were significantly damaged. Although the damage to the keepers and rocker arm may have occurred after the exhaust valve failed, the wear marks on the interior surfaces of the No. 3 exhaust retainer suggested that the valve seat keepers were not properly aligned, which may have resulted from their being installed improperly at one point or the valve stem being undersized; however, without the missing valve stem to examine, this could not be conclusively determined. There were no indications of foreign material or contamination in or on the No. 3 cylinder assembly components.

A review of the engine's maintenance records and invoices indicated that, about 30 hours before the accident, the Nos. 3 and 5 cylinders underwent repairs due to exhaust anomalies noted during the airplane's most recent annual inspection. The logbook entry for this maintenance action stated, in part, that both the Nos. 3 and 5 cylinders were removed and that the exhaust guides and valves were replaced with new parts.

The next and last maintenance entry in the engine's logbook indicated that, about 19 flight hours following the previous maintenance, the pilot changed the oil and found metal flakes in the oil filter. He sent a sample of the oil to a laboratory for analysis, and the results showed that it contained a high quantity of metal, some of which was identified as metal consistent with valve keys (keepers) material. The oil/material analysis report recommended contacting the engine manufacturer's service representative; however, the pilot did not do so.

Text from the NTSB. Open the NTSB docket for WPR14LA284 for the full record.