What happened
Two separate incidents involving EC2TR225 LP Super Puma helicopters resulted in ditchings in 2012. The first occurred on 10 May 2012, when G-REDW experienced a loss of Main Gear Box (MGB) oil pressure approximately 20 nm east of Aberdeen. The second took place on 22 October 2012, when G-CHCN ditched roughly 32 nm southwest of the Shetland Islands.
In both instances, the crews activated the emergency lubrication system following warnings of declining oil pressure. However, in both accidents, the MGB EMLUB caption illuminated on the Central Warning Panel approximately 30 seconds after activation, indicating a system failure.
The investigation
Investigators examined the mechanical integrity of the MGBs and the functionality of the emergency lubrication and Crash Position Indicator (CPI) systems. Analysis of the MGBs revealed that a fracture in the bevel gear lubrication systems and the CPI vertical shaft had led to the associated warnings. Strip examinations showed no significant heat damage, suggesting the emergency lubrication had successfully cooled the gearbox.
Regarding the CPI, the investigation found that on G-REDW, the beacon failed to deploy automatically. While the crew on G-CHCN had manually selected the 'TRANSMIT' function, the investigation noted that this manual selection had rendered the automatic deployment feature redundant due to the system's design.
Findings
- A specification error in a 2010 modification (MOD 0752520) caused the internal wiring of the pressure switches to be transposed. This error meant that the MGB EMLUB caption would always illuminate after the system was activated, necessitating an immediate landing.
- The pressure switches were operating within their specified thresholds, but the wiring error triggered erroneous low-pressure signals.
- The crew of G-CHCN was unaware that manually selecting the 'TRANSMIT' function on the CPI would prevent automatic deployment via the water-activated switch.
- The primary cause of the MGB EMLUB caption illumination was a wiring transposition error in the pressure switches caused by an incorrect manufacturer specification.
Safety action
- The AAIB issued Safety Recommendation 2012-034, requesting EASA require Eurocopter to review the MGB emergency lubrication system design to ensure accurate status indications.
- Safety Recommendation 2013-006 was issued, advising manufacturers of aircraft with Type 15-503 CPI systems to review Flight Manuals to ensure no features are listed that could inhibit automatic deployment.