3-May-2017 Source: Allan Blake for HeliHub.com
A Preliminary Report into the fatal accident of the CHC operated H225 helicopter LN-OJF by the Accident Investigation Board Norway (AIBN) was issued on April 28th 2017 (“The Report” is located at https://www.aibn.no/Aviation/Investigations/16-286.
The detailing of the catastrophic events makes very sad reading indeed, particularly the questions as to:
1. Why two similar catastrophic accidents could happen to near identical helicopters only seven years apart?
2. Why Airbus had not undertaken a root cause analysis following the first accident? and
3. The sufficiency of the procedures proposed by Airbus and accepted by EASA that led to EASA lifting its restrictions on the H225 whilst the Norwegian and UK Aviation Authorities declined to do so.
The Report concludes that
“…the accident was a result of failure of the main rotor gearbox (MGB) due to a fatigue fracture in one of the eight second stage planet gears in the epicyclic module.” (para 2.1.1)
The investigation by the AIBN has revealed that there are clear similarities to an AS 332 L2 accident off the coast of Scotland in 2009 (G-REDL) which had a near identical main rotor gearbox to LN-OJF. In HUMS: Safety Alert (February 3rd 2017), I compared these incidents alongside others where accelerated component degradation led to accidents that HUMS and chip alert systems could not give sufficient advance warning of failure. In both these accidents, one of the eight second stage planet gears in the epicyclic module fractured as a result of fatigue. There was one chip warning of possible gear fracture in the case of G-REDL, while there was no advance warning for LN-OJF.
The Report finds that
“In the case of G-REDL, the origin of the crack was in a section of the failed gear which was not recovered. The G-REDL report displayed a stress model prediction of crack growth in the missing section of the planet gear. The crack propagation path in the retrieved second stage planet gear in this accident (LN-OJF) appears to be very similar to the estimated crack growth in the G-REDL. The AIBN will continue the investigation into how and why two similar catastrophic accidents could happen to near identical helicopters only seven years apart. Further assessment of the follow up on the G-REDL safety recommendations and the continuing airworthiness of the gearbox after 2009 is a relevant issue.” (Summary, p.5)
EASA has allowed the H225 to fly again, subject to restrictions that Airbus proposed, but the AIBN is not confident that the real cause of the accident has yet been fully identified, particularly the assertion that a critical component, the planet gear bearings, manufactured by two different manufacturers (SNR and FAG) with different stress features is necessarily of relevance. The replacement of the component which had the same manufacturer in both helicopter accidents was part of the return to service remediation accepted by EASA. The Report states:
“Based on the present available information and as long as the reason for formation of the micro-pit and the underlying driving mechanisms are not yet fully understood, the AIBN has not been able to conclude with regard to a possible connection between the fatigue and the differences in design [between the SNR and FAG components], including both contact pressure and compressive residual stress.” (para 22.214.171.124)
One of the safety recommendation from the G-REDL investigation advises EASA to re-evaluate the continued airworthiness of the MGB to ensure that it satisfies certification requirements and the AIBN
“…regards this as the most important recommendation from G-REDL.” (para 2.5.7)
The AIBN is clearly not satisfied that EASA met this requirement as they
“…based their re-evaluation mainly on removing the ring of magnets from the lower area of the epicyclic module…The re-evaluation done by EASA in conjunction with Airbus Helicopter did not lead to any additional changes in design, operational life limits or inspection process of the planet gears… The accident with LN-OJF contradicts the response to the recommendation given by Airbus Helicopter stating that magnetic plugs and/or chip detectors “are sufficient to ensure flight safety”. In addition, EASA’s response…that “the safety of the fleet relies primarily on the capability of the MGB magnetic plugs to ensure early detection of spalling”…was based on Airbus Helicopters’ in-service experience…For LN-OJF, the fact that the MGB degradation went on undetected by the chip detection system cannot be explained by the ring of magnets or by human factors/maintenance failure, as was the case with G-REDL. A more detailed assessment of the actions in response to the safety recommendations, and how they were documented and closed by EASA and Airbus Helicopters will follow in the final report.” (para 2.5.8)
The Report also comments on the ability of the HUMS system to provide advance warning of this form of failure within the MGB:
“The analysis of the HUMS data for LN-OJF does not show evidence of trends or abnormal vibration behavior for any dynamic parts monitored by the system. Thus, the HUMS was unable to detect the fatigue fracture propagating in the second stage planet gear. The observed failure mode, i.e. crack initiation and propagation with limited spalling, in this accident seems to differ from what was expected or foreseen during the design and certification of this helicopter type. The fracture propagated in a manner which was unlikely to be detected by the maintenance procedures and the monitoring systems fitted to LN-OJF at the time of the accident intended for warning of an imminent failure.” (paras 2.4.6/7)
The Report notes that EASA commissioned Cranfield University to look at alternative technologies to detect damage in the planet gears of the gearbox. This research showed that
“…internal sensors for helicopter main rotor gearboxes are feasible and that they are able to offer improved detection when compared with traditional external vibration measurements” (126.96.36.199)
However, The Report states that Airbus consider no solution presently exists on the market for such degradation detection. According to the Report Airbus are researching possibilities of using accelerometers internally on each crankpin, but the results have so far not been conclusive.
So where does this leave the H225? At present 132 out of a fleet of 264 H225s are flying, mainly the military H225M (73) and non oil and gas (39). There are 35 out of 77 of the AS332L2 fleet operational, again mainly the military version (22) and non oil and gas (8) (source: Airbus Helicopters). I have written elsewhere,
“The cause of the accident may be known, but there is still no proposal for effective monitoring of this critical area: the H225 should not be brought back into service until there is… If there is a return to service Accountable Managers will need to assess carefully whether the proposed monitoring is sufficient.” HUMS: Safety Alert (February 3rd 2017)
The Preliminary Report from the AIBN confirms this approach and casts serious concern as to how EASA came to remove its restrictions on the aircraft.
Copyright and full responsibility for the content of this article remains with Allan Blake, is an independent Aviation Consultant & Journalist. He is the author of “Dynamic Directors: Aligning Board Structure for Business Success” (Macmillan).