The final report issued by the UK’s Air Accident Investigation Branch (AAIB) after the 2009 crash of a Bond-operated Eurocopter AS332 L2 Super Puma in the North Sea is highlighting imperfections in the main gearbox’s design and, maybe more important, in monitoring systems and maintenance programs.

Most prominent among the agency’s 17 recommendations (in addition to those made during the course of the probe) are those aimed at avoiding the non-detection of a serious abnormality during maintenance. The accident killed all 16 on board on April 1, 2009, after the helicopter struck the sea 11 nm off Peterhead, Scotland. The main rotor assembly separated from the aircraft in flight.

The crew was not aware of any anomalies with the helicopter until the indication of a main gearbox low-pressure oil warning. Twenty seconds later, the main rotor separated from the helicopter, severing the tailboom. The fuselage descended into the sea.

The investigation found that severe internal damage disrupted the epicyclic ring gear of the main gearbox. Investigators determined this to be the result of a fatigue failure of a second-stage epicyclic planet gear. A magnetic particle was found on the epicyclic module chip detector on March 25, 2009, 36 flight hours before the accident. The operator’s maintenance technicians did take action, but did not recognize the particle as an indication of degradation of the planet gear.

AAIB Recommendations

In its final report on the accident, the AAIB recommended, “Eurocopter should introduce further means of identifying in-service gearbox component degradation, such as debris analysis of the main gearbox oil,” for the AS332 L2 and its successor, the EC225. It pointed out the epicyclic module chip detector’s ability to collect magnetic particles is limited.

Moreover, experience has shown metallic particles generated within the gearbox do not necessarily generate a warning on the health and usage monitoring system (Hums). They may “be captured by the chip detector without closing its electric bridge.” The AAIB also wants the Hums to be improved.

In addition, the investigators recommend that Eurocopter review the Super Puma’s continuing airworthiness program to ensure any defect on a transmission-critical component is understood.

In fact, damage to the outer raceway of epicyclic gears was not a concern for airworthiness. However, this accident and the loss of an SA330J (the predecessor of the AS332) in 1980 have confirmed that an epicyclic gear failure can have catastrophic results. The main gearbox of the SA330J is fundamentally similar in layout to that of the AS332, according to the AAIB. The two gears removed from other gearboxes in 2005 and 2009 due to raceway spalling had been subjected to a laboratory examination. No abnormal failure mode was identified. Had the cracking within the “spalled” planet gear removed in 2005 been identified, mitigating actions could have been developed.

The helicopter’s gearbox was approved in the early 1990s, and in the intervening years certification criteria have become more stringent. The AAIB is recommending that the EASA re-evaluate the continuing airworthiness of the AS332 L2’s and EC225’s main gearbox so that it meets the latest requirements, including those of a 2010 notice of proposed amendment, the investigators said. Their main point is, “Inspection intervals and methods must [...] ensure that failures are detected [before] residual strength conditions are reached.”

Separately, the investigators are concerned that the recorders stopped working before the crash. The likely culprits are the “g switches” used as crash sensors. The AAIB deems them unreliable and recommends the use of more sophisticated sensors. For example, some emergency locator transmitters rely on sensors that heed acceleration profiles.

Oil & Gas UK, the country’s oil producer lobbying association, welcomed the report. It added its Helicopter Safety Steering Group “will conduct its own detailed review of these recommendations.” It will then focus on “monitoring their implementation.”

Also reacting to the publication of the report, Eurocopter claimed to have been proactive since the accident. The manufacturer “initiated the implementation of a number of these modifications before the safety recommendations were issued,” it stated. In addition, Eurocopter is confident that action taken–in information sharing, maintenance and inspection as well as particle detection–is “sufficient.”