What happened
On 25 September 2020, a Sikorsky S-9COA helicopter, registration LN-ONQ, was descending from 7,000 ft to 1,000 ft while returning from the West Elara oil platform to Stavanger Airport Sola. During the descent, the crew received an MGB OIL PRESS caution, indicating that the main gearbox oil pressure had fallen below 45 psi. Shortly after, a warning for high temperature in the left engine input module appeared.
The flight crew immediately initiated emergency checklists, reducing the airspeed and setting the left engine to idle. As the oil pressure continued to decline, the MGB BYPASS caution illuminated, signaling that the gearbox oil cooler had automatically disconnected. The crew also instructed the three passengers to prepare for a potential ditching at sea. The helicopter continued its descent to a low altitude of 200 ft before successfully landing at Sola. The aircraft was met by airport fire and rescue services upon arrival.
The investigation
The Norwegian Safety Investigation Authority (NSIA) conducted a technical investigation into the cause of the pressure drop and overheating. The investigation involved analyzing flight data recorders, maintenance logs, and a physical examination of the gearbox components conducted by the NTSB in the United States.
Upon disassembling the left input module, investigators discovered a metal stop-washer (type MS20002-6) lodged within the scavenge return oil system. The washer was positioned on its edge, which significantly restricted the flow of oil through the passageway. This obstruction led to the observed drop in pressure and the subsequent rise in oil temperature, which reached a peak of 214°C.
Findings
- The primary cause of the incident was a metal washer obstructing the oil flow in the main gearbox scavenge return passageway.
- The washer likely entered the gearbox during maintenance activities, specifically through openings exposed when the generator is disconnected from the accessory module.
- The washer had been inside the gearbox for at least 487 flight hours, as the last time the relevant module opening was accessible was during a maintenance event prior to that duration.
- The crew's decision-making was found to be appropriate, as they followed emergency procedures and prioritized a landing at a controlled aerodrome rather than attempting a landing in the terrain of Jæran.
Safety action
Following the investigation, the NSIA issued a safety recommendation to the Norwegian Civil Aviation Authority. The recommendation advises the authority to monitor and follow up on operator procedures regarding Foreign Object Debris (FOD), particularly concerning maintenance routines for safety-critical gearbox components.