The fatal April 29, 2016 accident near Norwegian coastal island Turøy that killed all 13 aboard an Airbus Helicopters EC225 LP after its main rotors separated from the helicopter in flight was the result of “a fatigue fracture in a second stage planet gear in the epicyclic module of the main rotor gearbox," according to a final report issued this morning by the Accident Investigation Board of Norway (AIBN). The board further found "cracks initiated from a micro-pit at the surface and developed subsurface to a catastrophic failure without being detected. There are no connections between the crew handling and the accident. The AIBN has also excluded material unconformity and mechanical failure, as well as maintenance actions by the helicopter operator.” The helicopter was operated by CHC for Statoil.
Specifically, the AIBN found, “The fatigue fracture initiated from a surface micro-pit in the upper outer race of the bearing, propagating subsurface while producing a limited quantity of particles from spalling, before turning towards the gear teeth and fracturing the rim of the gear without being detected. The investigation has shown that the combination of material properties, surface treatment, design, operational loading environment, and debris gave rise to a failure mode which was not previously anticipated or assessed.”
The AIBN said the crew had no warning of the failure. “The helicopter had just descended from 3,000 feet and had been established in cruise at 140 knots at 2,000 feet for about one minute. The flight was normal and the crew received no warnings before the main rotor separated from the helicopter. The helicopter impacted a small island near Turøy, northwest of Bergen. Wreckage parts were spread over a large area of about 180,000 sq m both at land and in the sea. The main rotor fell about 550 meters north of the crash site. The impact forces destroyed the helicopter before most of the wreckage continued into the sea. Fuel from the helicopter ignited and caused a fire onshore.”
During the course of its investigation, the AIBN found that “only a few second stage planet gears in Airbus Helicopters EC 225 LP and AS 332 L2 ever reached their intended operational time limit before being rejected during overhaul inspections or non-scheduled MGB removals.”
As a result of the accident, the AIBN made a dozen safety recommendations, among them:
That EASA develop regulations for engine and helicopter operational reliability systems, which could be applied to helicopters that carry out offshore and similar operations to improve safety outcomes.
That EASA review and improve the existing provisions and procedures applicable to critical parts on helicopters in order to ensure design assumptions are correct throughout its service life.
That EASA commission research into crack development in high-loaded case-hardened bearings in aircraft applications.
That EASA assess the need to amend the regulatory requirements with regard to procedures or instructions for continued airworthiness for critical parts on helicopters to maintain the design integrity after being subjected to any unusual event. (The accident helicopter’s main gearbox (MGB) had fallen off a truck while in transit and was inspected and returned to service.)
That EASA revise the certification specifications for large rotorcraft (CS-29) to introduce requirements for MGB chip detection system performance given that it does not currently specify chip detection system functionality and performance.
That EASA develop MGB certification specifications for large rotorcraft to introduce a design requirement that no failure of internal MGB components should lead to a catastrophic failure.
The AIBN also recommended that “Airbus Helicopters revise the type design to improve the robustness, reliability, and safety of the main gearbox in AS 332 L2 and EC 225 LP.”
In a prepared statement issued today, Gilles Bruniaux, Airbus Helicopters director of safety, acknowledged the AIBN’s findings, but also noted, “Before 2016, the available degree of scientific and technical knowledge meant it was neither foreseeable nor foreseen that a crack in a plant gear could propagate in a sub-layer, and as a result generate very low levels of detectable particles. With knowledge gained from this investigation, Airbus Helicopters has introduced a series of safety measures on the H225. Some of the technology that has been developed is ground-breaking for the helicopter industry. Airbus Helicopters will continue to pursue innovations and improve safety standards through a proactive approach that sees us challenge internally everything we do. Work on a number of potential improvements to the H225 are in progress, and I remain optimistic that this concerted and complex work will yield new advances.”
Specifically, since the accident, Airbus has changed out the types of planet gears on the accident helicopter and reduced their life limits, improved capture of particles in the oil with the introduction of a new full-flow magnetic plug that increases the capture rate from 12 percent to 50 percent, and strengthened inspection criteria and equipment allowing a more detailed analysis of these particles, including very small ones, through the reduction in the size of the defined surface for the epicyclic module removal to three square millimeters.