What happened
On October 16, 1999, a Bell 407 helicopter was conducting a commercial passenger flight, traveling from Paderborn-Lippstadt to a landing site at a university sports field in Hannover. During the approach to the Hannover control zone, the crew noticed the engine oil pressure steadily rising, eventually exceeding the maximum limit of 130 PSI and stabilizing at 150 PSI. While other instruments remained normal, the crew elected to continue the flight while monitoring all parameters.
During the final approach, approximately one minute from landing, the gearbox oil pressure warning light illuminated, and the instruments indicated a total loss of pressure. The crew continued the landing sequence. Upon landing, the engine oil pressure dropped below the minimum threshold. Subsequent inspection revealed that the engine oil tank had ruptured.
The investigation
The BFU investigation involved technical experts and the aircraft manufacturer. The investigation looked into the maintenance history of the aircraft, noting that it had recently undergone technical work at a maintenance facility in Riesa. Investigators also examined a previous incident where the pilot had noted a small oil leak during a post-flight check, which had been incorrectly attributed to an overfilled oil tank.
Technical analysis focused on the separation of the lubrication systems for the main gearbox and the engine. While both systems were found to be free of leaks, the investigation centered on the dual seal separating the two systems.
Findings
- The investigation determined that the dual seal on the freewheel side was cracked, allowing gearbox oil to leak into the engine oil system.
- This seal had been installed by the manufacturer and had not been replaced during the component's 741-hour service life.
- The faulty installation of the seal by the manufacturer caused the pressure imbalance.
- The crew continued the flight because the flight manual lacked specific "Abnormal or Emergency" procedures for high engine oil pressure.
- The previous oil leak noted by the pilot was not caused by overfilling, a misconception supported by inadequate documentation in the maintenance logs.
- The rupture of the engine oil tank was caused by the excessive internal pressure resulting from the accumulation of gearbox oil.