CHC Sikorsky S-92A Seat Slide Surprise(s)

Posted by on May 25, 2025 in Accidents & Incidents, Helicopters, Human Factors / Performance, Maintenance / Continuing Airworthiness / CAMOs, Offshore, Oil & Gas / IOGP / Energy, Safety Culture, Safety Management | 0 comments

CHC Sikorsky S-92A Seat Slide Surprise(s): (LN-OQE 2010 and LN-OQG 2013)

On 1 April 2010, at 500 ft on approach to an offshore installation, the Aircraft Commander of Sikorsky S-92A LN-OQE of CHC Helikopter Service found his seat had become detached.  The Accident Investigation Board Norway (AIBN), now the Norwegian Safety Investigation Authority (NSIA), published their safety investigation report, in Norwegian only, on 15 April 2015 (a disappointingly excessive 5 year and one similar incident later).

CHC Helikopter Service Sikorsky S-92A

The Incident Flight

The helicopter was being flown by the Co-Pilot as ‘Pilot Flying‘.  Just before reviewing the Pre-Landing Checklist for landing on the Gullfaks B offshore installation, the Aircraft Commander, the ‘Pilot Monitoring‘, was adjusting his seat to the forward position for landing when it came loose from its mounting rails.

He grabbed the grab handles in the ceiling [believed to be the handhold above the windscreen] and then had to hold on to them to prevent the seat from falling off the rails with the consequent risk that the control sticks (collective and cyclic) could be blocked or pinched.

The Aircraft Commander told the Co-Pilot he could not let go and made it clear the Pilot Flying “was now on his own with regard to the maneuvering”.  The Co-Pilot was able to make an uneventful landing on Gullfaks B without further problems.

A week later CHC issued “Information to Crew (ITC) 80 – 07.05.2010” that stated that the incident involved a potential for the flight controls to be blocked.

The company noted that the incident ended well because the crew remained calm in a difficult situation.

The Co-Pilot informed the AIBN that…

…the weather was as good as it could be in the North Sea, and that the approach was otherwise unproblematic and obstacle-free

After landing offshore the pilots attempted to refit the commander’s seat onto its mounting rails (presumably to certify under an Aircraft Commanders’ Part-145 145.A.30(j) authorisation). However, they discovered that the forward slide stop was missing.  Significantly, they also found the co-pilot’s seat was missing its rear slide stop.

Safety Investigation

LN-OQE had been in for a maintenance check less than a week before.  A Nose Landing Gear
(NLG) Hinge Inspection had been performed, which necessitated removal of the pilots’ seats. 

On these Martin-Baker High Comfort Crew Seats both the locking brackets (slide stop, P/N MBCS13711) and locking pins (quick release pin) must be attached to the mounting rails after the seats have been put in place.

High-Comfort Crashworthy Crew Seat (Credit: Martin-Baker)

After performing the NLG Hinge Inspection, the seats were reinstalled, but:

  1. the locking brackets for both were omitted
  2. on the commander’s seat the front and rear locking pins were omitted
  3. on the co-pilot’s the rear locking pin was missing (though the front pin was fitted)

There is no comment on where the omitted items were.

Two days after the incident, CHC issued Maintenance Alert Notice TI-016, requiring inspection of the pilot seats in all of the company’s S-92 helicopters before the next flight to ensure that locking pins and locking brackets were installed.

No similar defects were found on any of the other helicopters. However, one helicopter was missing a slide stop, but here the locking bolts were in place so that, according to CHC, the seat could not have come off the rails.

In its internal investigation, CHC identified several factors that contributed to the incident:

These conditions were related to the maintenance, more specifically specific elements related to the basis, planning, implementation and documentation.

The AIBN say NLG Hinge Inspection and the seat installations were carried out by two apprentices, and…

…it was found that the communication between the responsible technician and the apprentices had not been sufficient.

The internal CHC investigation resulted in a number of “improvement proposals”, including:

  1. “Tightening up the use of procedures” (discussed further below)
  2. Including the incident as case study in Continuation/Maintenance Human Factors Training (which we would expect as a default)
  3. Including the removal and installation of pilot seats as tasks in MRS 45054 and 46081, with an independent inspection after seat installation.  AIBN do not elaborate on these documents.
  4. “Establishment of a form to register personnel and associated responsibilities for all
    maintenance tasks where more than two aircraft technicians are required”.

Oddly, apart from the introduction of a form, there is nothing further related to supervision mentioned.

Deja Vu All Over Again… LN-OQG December 2013

However, just over three and a half years later, on 21 December 2013, a similar incident occurred with another CHC S-92A, LN-OQG.

Like LN-OQE, both the front locking bracket and locking pin in the captain’s seat were missing, which came loose when he was adjusting it forward. 

The captain of LN-OQG also had to hold on to the grab handles in the ceiling to prevent the seat from falling off the rails and blocking the control.

This time the crew managed to partially stabilize the seat so that it rested on the two rear attachment points. However, the seat tilted 15 degrees to the right, and the captain had to use considerable force and concentration to prevent it from falling back down.

The first officer landed the helicopter without the captain being able to assist during the landing if necessary.

The AIBN reveal that CHC were unable to determine when the locking pins had been removed from the pilot seats on LN-OQG, but did conclude that omission in maintenance was likely.

The internal CHC report determined that…

….the measures adopted after the incident with LN-OQE had been implemented with the exception that MRS 45054 and 46081 [that] had not been revised.

It is not clear from the [CHC] report why these had not been revised…

Crucially this meant that introducing a requirement to conduct an independent inspection had not been actioned.

The concept of a failure to learn has been raised in relation to BP after the 2005 Texas City refinery explosion for example, 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.  It also can apply to a failure to learn from accidents that occured to other organisations (as we discussed in the case of a US helicopter operator).

Following the incident with LN-OQG in 2013, further recommendations were made.  These again included introducing an independent inspection.  An item was also added to the pilots’ checklist to check that the locking pins are in place.

AIBN also say that “the report also indicated that technical management and personnel had been informed of the incident” and rather vaguely they were “encouraged to follow up on possible root causes”.  No specific follow up was reported by AIBN.

AIBN Comments

The incident in 2010 illustrates, among other things, the importance of maintenance personnel having appropriate procedures in place to be able to perform their work tasks correctly.

This seems to contradict the observation of “tightening up the use of procedures” in the internal investigation, though readers will have noted that two of the four changes proposed were procedural changes, suggesting at least some ‘tightening up the procedures’ themselves.

In particular, modifications or the use of non-standard equipment can have ripple effects that affect the maintenance base [sic]. It can sometimes be a challenge to capture all the areas affected. In this case, it turned out that the use of special pilot seats required changes to the maintenance base that remained undetected.

It appears the ‘High Comfort’ seats, which became available in 2006, were the ‘non-standard equipment’ referred too.  Its not clear if CHC therefore had a mixed fleet with different modification standards and how different the seats might be.  As mentioned above, disappointingly:

After the 2013 incident, it was discovered that not all of the measures proposed in 2010 had been implemented.

This suggests a weakness in tracking and executing actions, a feature at about that time discussed in another accident report.

Furthermore, those measures that had been implemented did not have sufficient effect to prevent a recurrence of the first incident.  Quality management involves organisations conducting systematic assessments of all their activities, introducing corrective measures where required and verifying their effectiveness.  

Our Observations: Pilot Seat Installations

On fixed wing aircraft, seats sliding back on rotation, due to installation errors or undetected wear, can easily lead to a stall due an unintended rearward movement of the control column.  They have even resulted in runway excursions where the seat has slid back while accelerating leaving the pilots feet unable to reach the rudder pedals. 

Cirrus SR22 N161DL (credit via NTSB)

We have previously discussed a Royal Canadian Air Force (RCAF) Loss of Control – Inflight (LOC-I) accident that occurred to Search and Rescue (SAR) Leonardo CH149 Cormorant (AW101) CH149903 at Gander, Newfoundland on 10 March 2022.

During the final clockwise hover turn sequence, the pilot flying’s seat unexpectedly descended to its lowest position.  The aircraft…accelerated counter clockwise left yaw about the aircraft mast [and] the attitude of the aircraft became unstable resulting in an increasing right bank attitude. The aircraft rotated through approximately 400° and as the right rolling moment intensified, the right outboard wheel, the horizontal stabilizer assembly located on the right side of the tail section, and main rotor blades impacted the runway.

RCASR SAR CH149 Leonardo Cormorant (AW101) CH149903 at Gander 10 March 2022 (Credit: RCAF Flight Safety)

That investigation determined that…

…a combination of factors, including seat non conformances and the horizontal position that was maladjusted, allowed the seat to be in a false lock condition.

As we observed in April 2022 when we examined the RCAF accident:

This accident is a good reminder to pay attention to the maintenance, adjustment and operation of pilot’s seats.

Our Observations: Simultaneous Incorrect Maintenance

In this case the NLG maintenance required both pilot seats be removed at once.  This made the reinstallation vulnerable to errors performed consistently on two simultaneous tasks.  While LN-OQE featured multiple omissions on both seats but fortunately only one became dislodged at once.

The phenomena of simultaneous incorrect maintenance gained attention after a serious incident to British Midland Boeing 737-400, G-OBMM on 23 February 1995. 

British Midland 737-400 G-OBMM (Credit Aero Icarus CC BY-NC-SA 2.0)

The aircraft was climbing to cruise altitude when there was an indicated loss of first oil quantity and then oil pressure on both engines.  The crew diverted to Luton Airport, landing safely. The aircraft had been subject to Borescope Inspections on both engines during the night prior to the incident flight by the same licenced engineer.  The High Pressure (HP) rotor drive covers, one on each CFM56 engine, had not been refitted.  

Subsequently UK CAA published Airworthiness Notice 72 on 16 March 1998 on Safety Critical Maintenance Tasks.  This recommended staggering tasks so…

…the accomplishment of similar critical tasks on two or more systems are segregated. 

Consideration should be given to introducing procedures that will ensure that such tasks are separated by at least one flight cycle. Where it is not practical to introduce staggered maintenance, inspections and functional checks should be performed independently to ensure system serviceability.

Where [this] is not practical…the use of separate work teams together with the accomplishment of appropriate functional checks…should ensure a similar level of system integrity.

AN72 later became CAP562 Leaflet 11-21 and then Leaflet B-150.

Our Observations: Independent Inspections in Norway

Since the LN-OQE investigation was published, AIBN reported on a serious incident involving Airbus Helicopters EC130B4 LN-ORR operated by Helicopter Utleie that occurred on 12 August 2014.  We summarised that AIBN report in February 2016:  EC130B4 Accident: Incorrect TRDS Bearing Installation.  

In their EC130B4 report AIBN note that EASA completed a ‘Standardisation Inspection’ of the Norwegian CAA in June 2014, making a finding that:

The CAA-NO does not ensure that the requirement for independent inspections as specified in M.A.402(a) is applied in maintenance organisations.  Substantiation: Both Part-145.A.65(b)(3) ‘capture errors on critical systems’ and M.A.402(a) ‘flight safety sensitive maintenance tasks’, require methods to control maintenance errors that could endanger the safe operation of an aircraft if not performed properly.  Independent inspections are required to capture these ‘safety related’ errors according to M.A.402(a) and these should be carried out by at least two persons (the second person not necessarily CS, but at least appropriately qualified to perform the task), UNLESS otherwise specified by Part 145 or agreed by the competent authority.

AIBN comment:

Based on the EASA inspection, the Norwegian CAA has dedicated more focus on what the organisations do to prevent maintenance errors when performing audits. The AIBN considers this change a good contribution to flight safety.

Re-inspection of ones own work has “traditionally been a common practice in this part of Norwegian aviation” say AIBN.

EASA has previously noted in NPA 2012-04 (issued in June 2012, and implemented via Commission Regulation (EU)  2015/1536, see: Critical Maintenance Tasks: EASA Part-M & -145 Change): 

A re-inspection as an error capturing method should only be used in unforeseen circumstances when only one person is available to carry out the task and perform the independent inspection. The circumstances cannot be considered unforeseen if the organisation has not programmed a suitable “independent qualified person” onto that particular line station or shift.

Helicopter Safety Study 3 (HSS3) was issued in March 2010 just days before the first seat incident

The report, produced by research organisation SINTEF, was funded by Civil Aviation Authority Norway and 8 oil and gas companies.  It perversely calls such re-inspection as “not acceptable from a safety standpoint” (page 145) but grades the value of a stricter regime (‘M06’) as just ‘low’ (page 162). 

Even when conducted, independent inspections are not a ‘silver bullet’ as they are vulnerable to confirmation bias, especially where they are verifying the work of trusted colleagues.

Footnote: CHC AS332L LN-OMF 3 May 2025

On 3 May 2025 CHC AS332L LN-OMF cut its tail off during a ground run at Stavanger.  Published images indicate a main rotor blade had been fitted inverted.

CHC AS332L LN-OMF 3 May 2025

Safety Resources

The European Safety Promotion Network Rotorcraft (ESPN-R) has a helicopter safety discussion group on LinkedIn.  You may also find these Aerossurance articles of interest:

You might find these safety / human factors resources of interest:

FSF Maintenance Observation Programme (MOP)

Aerossurance worked with the Flight Safety Foundation (FSF) to create a Maintenance Observation Program (MOP) requirement for their contractible BARSOHO offshore helicopter Safety Performance Requirements to help learning about routine maintenance and then to initiate safety improvements:

mop

Aerossurance can provide practice guidance and specialist support to successfully implement a MOP.

Aerossurance has extensive air safety, flight operations, SAR, airworthiness, human factors, helidecks, aviation regulation and safety analysis experience.  For practical aviation advice you can trust, contact us at: enquiries@aerossurance.com

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