What happened
In May 2012, an EC225 LP Super Puma, registration G-REDW, ditched in the sea approximately 20 nm east of Aberdeen. This followed a mechanical failure in the main gearbox. A second incident occurred on 22 October 2012, when another EC225 LP Super Puma, registration G-CHCN, ditched approximately 32 nm southwest of Sumburlag, Shetland Islands, following similar circumstances.
In both instances, the aircraft experienced a fracture of the bevel gear vertical shaft. This failure triggered warnings for low oil pressure on the Central Warning Panel. Although the crews activated the emergency lubrication system to maintain flight for the required 30 minutes, the emergency lubrication system (EMLUB) captions illuminated erroneously, indicating a failure of the system's intended function.
The investigation
Investigators examined the emergency lubrication system, which uses a mixture of Hydrosafe 620 and bleed-air to cool and lubricate the gearbox. Testing of the pressure switches revealed that a previous modification (MOD 0752520) contained a specification error. This error caused the internal wiring of the switches to be transposed, specifically affecting the connection between Pin 1 and Pin 3.
Further testing of the bleed-air and Hydrosafe 620 supplies showed that under certain operational conditions, the pressure could drop to a level that, while still within the acceptable range for the switches, was low enough to trigger an erroneous low-pressure signal due to the incorrect wiring configuration.
Regarding the Crash Position Indicators (CPI), the investigation found that the crew of G-CHCN had manually selected the 'TRANSMIT' function, which allowed for successful distress signal transmission. However, the CPI on G-REDW did not deploy automatically. While the components were found to be functional, the investigation noted that the water level in the cabin during the ditching was above the level of the water-activated switch, and the crew had not manually activated the device.
Findings
- A wiring error in the pressure switches, resulting from an incorrect manufacturing specification, caused the MGB EMLUB caption to illuminate erroneously.
- The bleed-air pressure could drop to a point that triggered the erroneous low-pressure signal during certain flight phases.
- The bevel gear vertical shaft fracture was the primary mechanical cause of the initial oil pressure loss.
- The automatic deployment of the CPI on G-REDW was not triggered because the water-activated switch was not submerged by the cabin water level during the event.