Misassembled Anti-Torque Pedals Cause EC135 Accident

Misassembled Anti-Torque Pedals Cause EC135 Accident

The US National Transportation Safety Board (NTSB) has recently determined that misassembled anti-torque pedals caused an accident to Airbus Helicopters (formerly Eurocopter) EC135P1 N911KB, during a post-maintenance check flight by Metro Aviation at Shreveport, Louisiana on 9 November 2013.

The Accident

The pilot and two mechanics sustained minor injuries and the helicopter was substantially damaged after the Loss of Control – Inflight (LOC-I) while transiting to a nearby field for a hover test.  In their report the NTSB state:

The pilot attempted to regain control of the helicopter using the antitorque pedals, but they were ineffective. The pilot reduced engine power and performed an autorotation to the field. The helicopter landed hard and rolled on its right side.

Metro Aviation EC135 N911KB Accident Site (Credit: via WFAB)

Metro Aviation EC135 N911KB Accident Site (Credit: via WFAB)

The Investigation

On examination of the wreckage the NTSB:

…found that the antitorque pedals had separated from the antitorque levers. The attachment hardware was not located in the wreckage or the surrounding area.

N911KB Anti-Torque Levers Missing the Pedals (Credit NTSB)

N911KB Anti-Torque Levers Missing the Pedals (Credit NTSB)

Neither the antitorque pedals nor the lever attachment holes displayed elongation, which is consistent with the hardware bolts not being in place at the time of impact.

Metro Aviation EC135P1 N911KB (Credit: via Shreveport Times)

Metro Aviation EC135P1 N911KB (Credit: via Shreveport Times)

The NTSB report (emphasis added) that:

A review of the helicopter’s log book found an entry dated October 31, 2013, that a mechanic performed the action “disassemble, inspect, and reassemble tail rotor pedals”. According to a statement provided by the company’s director of maintenance, after the accident a search of the area maintenance area was conducted. Near the area where the helicopter was previously repaired, a small parts bag was found tied to the tail rotor control cable that had been replaced. Inside of the bag were bolts similar to the bolts used to secure the anti-torque pedals.

Circumstances

The aircraft had been undergoing an 800 hour check and a scheduled engine change.  Closer examination of the NTSB public docket reveals, in a statement by the facility’s then Director of Maintenance (DoM), that they were under pressure from the operator of the aircraft:

The scope of work had originally been slated for a four to six week work schedule, but demand by the using customer dictated that the scope of work be shortened to the minimum time needed to complete only the inspections and any repair as needed to ensure airworthiness of the aircraft. They (the using customer) had requested that this work be completed in 5 days or sooner if possible. I explained this was not possible due to personnel availability, and depth of inspection requirement.

Also, personnel shortages:

…created a situation where repair station personnel were bouncing back and forth between aircraft undergoing other maintenance activities [and] helping those personnel who were working on this aircraft. This was not ideal and made job continuity difficult.

The DoM goes on:

I did observe one of the…technicians with the pilot pedal shaft assemblies in his hands shortly after removal performing the required inspection. I quizzed him to ensure he understood the inspection requirement on the pilot pedal shaft and support. He gave me enough information to make me believe he did In fact understand the inspection requirements.

The NTSB do not unfortunately appear to have interviewed the technician, reducing the learning opportunity available.

NTSB Conclusion

The NTSB determines the probable cause to be:

The mechanic’s improper installation of the antitorque pedals, which resulted in an in-flight loss of helicopter control.

Observations

This is a further accident that highlights the need for effective independent inspections during maintenance and is again an example of a failure to learn from earlier accidents.  In this case, while not conclusive, it appears from records released by the NTSB that contain single signatures, that no independent inspections were occurring:

n911kb-4

Although it appears the DoM actually had chance to query the technician’s understanding of the task while it was in progress, knowledge and experience do not vaccinate against the possibility of a slip or lapse during a task.

We have previously looked at several accident which featured relevant maintenance human performance issues:

The concept of a failure to learn has been raised in relation to BP after the 2005 Texas City refinery explosion, prior to the 2010 Macondo / Deepwater Horizon disaster.  It is also the subject of a book of the same title by Australian National University Emeritus Professor Andrew Hopkins and our own article on a Dutch refinery explosion (Shell Moerdijk Explosion: “Failure to Learn”).

In the 2008 book Resilience Engineering Perspectives, Volume 1: Remaining Sensitive to the Possibility of Failure, John Wetherall writes:

…one hallmark of a resilient organisation is that it is prepared not only for its own failures those of which it can learn from others – the more resilient it is, the ‘bigger’ are the lessons it has learnt from others.

This is also not the first accident were bagged components were found after the accident or serious incident.  In the case of British Airways Boeing 777 G-YMME in 2004, fasteners and an uninstalled panel were found tie-wrapped to structure.  Nor is it the first case were missing components were found on racks in a base maintenance facility, as demonstrated by missing access panels in another BA serious incident, involving B757 G-CPER in 2003.

We have commented in the past on managing the risks of maintenance check flights and a fatal helicopter check flight accident (also on Louisiana):

Aerossurance is pleased to have sponsored a recent Royal Aeronautical Society (RAeS) seminar, Maintenance Human Factors: The Next Generation, at Cranfield University. Presentations from that seminar are available here. Additionally we have also written several more general articles related to maintenance safety:

Also see:


Aerossurance is pleased to sponsor the 2017 European Society of Air Safety Investigators (ESASI) 8th Regional Seminar in Ljubljana, Slovenia on 19 and 20 April 2017.  Registration is just €100 per delegate. To register for the seminar please follow this link.  ESASI is the European chapter of the International Society of Air Safety Investigators (ISASI).

Aerossurance also is delighted to be sponsoring an RAeS HFG:E conference at Cranfield University on 9 May 2017, on the topic of Staying Alert: Managing Fatigue in Maintenance.  This event will feature presentations and interactive workshop sessions.

Aerossurance has extensive air safety, airworthiness and accident analysis experience.  For practical aviation advice you can trust, contact us at: enquiries@aerossurance.com