What happened
On 19 November 2013, a Piper PA-31-350 Navajo Chieftan, registration G-STHA, was conducting a commercial cargo flight from Southend Airport to Oxford Airport. While cruising at 8,000 feet, the crew experienced a sudden, heavy impact accompanied by a leftward yaw and a loss of altitude.
Upon investigating the malfunction, the crew observed that the left engine's top cowling had buckled upwards. The crew promptly shut down the affected engine and feathered the propeller. Despite these actions, the aircraft could not maintain its altitude, and the crew found it difficult to maintain directional control. After declaring a MAYDAY, the aircraft was vectored to Luton Airport, where the crew completed an emergency landing without any injuries to the two crew members.
The investigation
Investigators examined the aircraft and found significant damage to the engine and cowlings. The inspection revealed that the No. 2 piston and cylinder assembly of the left engine had physically separated from the crankcase. This component remained connected to the engine only by the spark plug ignition leads, and the force of the detachment had pushed the cowlings apart.
An inspection of the engine, which had been removed by the operator, indicated that the failure of the No. 2 cylinder assembly began internally. Evidence suggested the engine continued to run for some time after the internal failure began, with signs that the cylinder had been running at an abnormally high temperature. Additionally, the No. 3 cylinder showed signs of distress, likely caused by oil draining from the cylinder walls during periods when the engine was not in use.
Findings
- The internal failure of the No. 2 cylinder assembly caused it to detach from the crankcase.
- The separation of the cylinder assembly caused significant disruption to the engine cowlings, creating enough drag to prevent the aircraft from maintaining altitude.
- The aircraft had experienced periods of storage and low utilization, which was reflected in the condition of the No. 3 cylinder.
- At the time of the report, the specific root cause of the internal failure within the No. 2 cylinder could not be determined.