What happened
On 12 April 2007, a Piper PA-28-180, registration SE-GAV, was conducting a local flight over the Stockholm archipelago with a pilot and three passengers. While positioned over Frihamnen and preparing to land at Bromma Airport, the engine began to run roughly and lost power at an altitude of approximately 1200 ft.
As the engine malfunction progressed, the pilot identified an open field on the mainland to avoid a forced landing on the water. During the descent, the engine ceased operation entirely. The pilot executed a gliding turn to reach the field, but despite applying full flaps and brakes, the aircraft maintained high speed. Upon reaching the end of the field, the aircraft struck trees and an outcropping rock, causing the right wing to detach. All four occupants sustained only minor injuries.
The investigation
The Swedish Accident Investigation Board (SHK) conducted a technical examination of the engine and the wreckage. Investigators found that the propeller could not be rotated, indicating a mechanical seizure within the engine block.
Detailed inspection of the engine revealed that the connecting rod for cylinder no. 2 had failed. Furthermore, the investigation identified that a portion of an aluminum plug from piston-pin no. 4 had broken away. This fragment had been circulating within the engine, causing progressive damage. The investigation also uncovered a manufacturing defect in the crankshaft, where the oil supply channel was improperly drilled, and noted that the oil pressure regulator spring was too weak, leading to low oil pressure.
Findings
- The primary cause of the accident was the inadequate design of the piston-pins and their plugs in this engine type.
- Aluminum fragments from the broken piston-pin plug clogged the oil filter, allowing contaminated oil to enter the lubrication system.
- The failure of the connecting rod no. 2 was exacerbated by a manufacturing defect in the crankshaft's oil supply channel.
- Low oil pressure contributed to the overheating and eventual seizure of the connecting rod bearings.