S-61N Damaged During Take Off When Swashplate Seized Due to Corrosion

S-61N Damaged During Take Off When Swashplate Seized Due to Corrosion (G-ATBJ)

British International Sikorsky S-61N G-ATBJ was preparing to fly from Marchwood Military Port in Hampshire to a maintenance base on 1 February 2018 when a series of factors resulted in a dramatic dockside drama.

British International Sikorskt S-61N G-ATBJ Pitching Down Upon Ttakeoff at Marchwood, Hants, 1 February 2018 (Credit: Port CCTV via AAIB)

British International Sikorsky S-61N G-ATBJ Pitching Down Upon Takeoff at Marchwood, Hants, 1 February 2018 (Credit: Port CCTV via AAIB)

The Accident Flight

In their safety investigation report, published 3o January 2020, the UK Air Accidents Investigation Branch (AAIB) explain that:

G-ATBJ had previously been operating in the Falkland Islands for four years until its last flight on 31 December 2017. It was then prepared for return by sea to the UK. This included having its main and tail rotor blades removed; no covers were used to protect the rotor head and transmission. On 8 January 2018, the helicopter was moved onto a roll-on/roll-off sealift ship…G-ATBJ was transported below decks during the voyage…to Marchwood…where it was unloaded on 29 January 2018. The following day, the helicopter was prepared for flight which included having its main and tail rotor blades fitted; a ground run was then performed by flight crew [the same Aircraft Commander who was to make the ferry flight but a different Co-Pilot].

It appears these were done in haste due to concerns about a limited number of external batteries and no external power cart being available.  On the CVR recording there was comment yo…

…just check that the blades were moving in “the right sort of way”. During the check, the range of cyclic pitch movement recorded was between -12% aft and 18% forward as opposed to the full range of movement required by the procedure of approximately -44% aft to 26% forward.

On the next day…

…the co-pilot performed the external checks while the commander commenced the internal checks. Two of the operator’s engineers were also in attendance and remained outside the helicopter throughout. The crew discussions indicated the need to progress quickly [also due to to battery concerns].

They initially encountered difficulties with the engine start.  Subsequently:

The engine 2 start was successful but, because the subsequent checks required the rotors, and hence the hydraulic pumps and electrical generators, to remain disengaged, the commander commented that they needed to be “as quick as we can” to minimise the use of the battery to pressurise the hydraulics from the DC motor-generator (motorising).

During the after-start checks, a flying controls servo system check was completed, but not to the full extent of control movement.  The pilots believed that a full and free check to the extremities of the controls’ movement was not possible as the helicopter’s electrically-driven hydraulic pumps would have disengaged under a high demand as they were being powered by the battery. However, the helicopter manufacturer has advised that, if the controls are moved slowly, full movement could be obtained without the pumps disengaging.

During these checks the Pilot Flying (PF) stated “…and they [main rotor blades] are moving in the right sense…I’m not doing full and free, we haven’t got time.” Both pilots highlighted that they did not intend to avoid full and free checks, but they felt that they were not achievable in the circumstances having previous experience on the S-61 after failed attempts using batteries.

The helicopter’s rotors were then engaged and the crew then started engine 1.  Pre-flight checks were completed and ATC clearance to depart was received.

The commander released the parking brake and unlocked the tailwheel. The co-pilot then advanced the engine speed select levers to achieve 104%, monitoring the triple tachometer as he did so. While the co-pilot monitored the engine instruments, the commander started to raise the collective until the helicopter was light on its wheels. Initially the helicopter started to move forward, so the commander arrested this movement with the cyclic and trimmed out the aft cyclic input; he then continued to raise the collective.

As the helicopter lifted it started to move forward again and the co-pilot caught the movement in his peripheral vision. As the commander continued to raise the collective, the helicopter pitched nose-down and started to climb. As it started to pitch, the co-pilot observed a large amount of aft cyclic being applied by the commander.

The helicopter did not respond to the aft cyclic input, so the commander promptly lowered the collective to land the helicopter. The crew felt a “thump through the seats” as the helicopter struck the ground with its nose. The mainwheels made ground contact causing the tail to pivot downwards onto the tailwheel.

The time between the last wheel leaving the ground and the initial impact was less than three seconds. Despite the co-pilot being slightly dazed he commenced the emergency shutdown checklist and called to the commander to apply the rotor brake. Both pilots then evacuated the helicopter and, once outside, went to check that the engineers were unhurt. There were no injuries.

The aircraft came to rest 10m from the centre of the marked helipad.

The front of the helicopter had struck the ground during the accident sequence. The front equipment bay was crushed resulting in the detachment of the bay door.

Nose Bay Damage British International Sikorsky S-61N G-ATBJ (Credit: AAIB)

Nose Bay Damage British International Sikorsky S-61N G-ATBJ (Credit: AAIB)

Scuff marks on the concrete surface, 9.5 m from the centre of the helipad, indicated the location that the helicopter initially struck the ground. The tailwheel strut had been driven through the upper stops, with buckling of the skin around frame 493 at the rear of the fuselage. gatbj bi S61n marchwood impact Both pilots stated that everything appeared normal until the moment when weight came off the wheels. The commander also stated that he believes the cyclic forces were unusually light when moved fore and aft during the takeoff but the side-to-side movement forces felt normal.

AAIB Safety Investigation

During the investigation, G-ATBJ’s historic [FDR] data was reviewed to determine the use of the full range of cyclic movement. In the 78 hours of recorded data there was only one check where the controls were fully exercised. The operator was asked to review the full and free checks from previous recordings in its flight data monitoring programme. A sample of these recordings indicates that the full and free check was not always conducted to the full extent of control movement available. The operator noted that observations in simulator checks and during flights also indicated that the check was not always completed to the full range.

During examination of the aircraft:

…it was identified that the spherical bearing within the stationary swashplate assembly was seized. gatbj bi S61n marchwood swashplate The swashplate was able to translate up and down the guide tube but could not tilt. There was evidence that the guide tube had deflected during operation as grease on the mast had come into contact with the inner surface of the guide tube. gatbj bi S61n marchwood grease However, there was no evidence of metal-to-metal contact between the mast and guide tube. Once it had been removed, examination of the swashplate assembly confirmed that the spherical bearing could not articulate in any direction. gatbj bi S61n marchwood swashplate 2 Examination of the sockets identified that both the upper and lower sockets had become skewed such that there was a gap between the lower socket and its retaining flange of up to 0.150 in. The upper socket had also skewed with a variation of up to 0.023 in. The sockets were re-seated thus allowing the spherical bearing to be released. The grease between the sockets and the spherical bearings was in very poor condition, with crystalline deposits within it. gatbj bi S61n marchwood bearing Elemental analysis of the deposits found them to contain aluminium, chlorine, sodium, copper, tin and oxygen. This indicates that the aluminium spherical bearing had corroded, and deposits had been retained within the grease. A section of the grease was cleaned from the bearing to reveal areas of corrosion pitting to a depth of 0.018 in. gatbj bi S61n marchwood bearing pit A build-up of plaque-like material was adhered to the bearing. Analysis of this material found it to be bronze material that had released from the socket, combined with the grease and adhered to the bearing surface. Once the spherical bearing was cleaned, a ring showing signs of severe corrosion was identified around the location where the lower socket had been. Closer inspection identified that a number of deep corrosion pits were located along the intersection between spherical bearing and where the top of the lower socket bearing face made contact. gatbj bi S61n marchwood bearing 2 The swashplate…was last overhauled in January 2014. It was fitted to the helicopter in June 2016, 44 months before the accident, during which it had accumulated 2,493.5 operating hours. In-service assessment of the vertical play is required every 500 hours and was most recently completed 33.5 hours prior to the event. During the assessment, the vertical play was measured as 0.008 in, 0.001 in outside of limits and so the shim was adjusted to increase spherical bearing clamping. There were no reports of any issues with the swashplate subsequent to the 500-hr check.

AAIB note that:

The level and type of maintenance to be completed on the S-61N is defined by the helicopter manufacturer and is detailed in the S-61N Equalized Inspection and Maintenance Program, SA 4047-13 (EIMP).  The EIMP defines several inspection types, the most frequent being a Pre-flight Inspection. There are also Safety Inspections, Progressive Period Inspections, Special Frequency Inspections, Unscheduled Maintenance Check and Major Inspections. As G-ATBJ had not been operated for over 30 days while it was in transit, a Safety Inspection should have been carried out on the aircraft prior to flight.

The operator’s AMP [Aircraft Maintenance Programme] (S-61N AMP MP/01016/1381) subsumes the intent of the Safety Inspections defined in the EIMP into its ‘Daily’ maintenance requirements. These Daily inspections contain mandatory items that must be carried out within a period not exceeding 10 flying hours or 24 elapsed hours prior to flight. For the swashplate assembly, the EIMP required an Inspection and Check to be carried out as part of the Safety Inspection, a remark was also made to ‘Check for binding in the ball-ring socket per Maintenance Manual’.

In reviewing the AMP, it was identified that this check was not annotated…and therefore was not carried out as part of the Daily inspections. Section 80 65-12-7 of the S-61N Maintenance Manual, SA 4045, refers to swashplate maintenance. Paragraph 3 D, titled ‘Check for Binding in Ball-Ring Socket’ defines the check referred to in the EIMP. This check requires a binding check to be carried out by motorising the servos systems and, by exercising the collective and cyclic with an observer on the service platform, assessing that the motion and travel of the swashplate is smooth and continuous. This check was not completed during the pre-flight preparations on 29 or 30 January 2018.

Very oddly, no mention is made of any Vibration Health Monitoring (VHM) / HUMS data being analysed.

AAIB Analysis

The uncommanded pitch nose-down on lift off…was most likely as a result of a restriction of the swashplate which had seized. The seizure had not been detected during the reinstatement of helicopter in preparation for flight or in the pre-flight control checks. It is possible that there was some corrosion present within the spherical bearing sockets prior to the shipment of the helicopter back to the UK. However, with regular operation, there was no opportunity for any corrosion to dwell and allow the ball-ring to seize.

During the shipping of the helicopter from the Falkland Islands, where the rotor head was unprotected, water ingress between the sockets is likely to have occurred… Any water that was captured in the socket area would have welled above the lower socket. The dissimilar materials of the spherical bearing and the socket would promote galvanic corrosion if exposed to salt water. This would promote rapid corrosion propagation and adherence between the socket and bearing surfaces.

It is likely that [the Sikorsky defined Safety Inspection], had it been carried out, would have identified the seized swashplate before the accident flight. Upon application of cyclic control inputs during the pre-flight checks the swashplate will have initially tilted. With the socket and bearing locked together the force will have lifted the lower socket out of position, in doing so it will have wedged the spherical bearing in position, preventing any further movement. Any cyclic load applied subsequently would have resulted in guide tube deflection with an associated small change in blade pitch.

Despite the seizure, the investigation determined that full fore/aft travel of the cyclic control could still be achieved which indicates that this is not a reliable indication that the swashplate is free to move. During pre-flight checks by maintenance engineers and the flight crew, the flight control servo system checks were not completed to the full extremes of travel. With a seized swashplate, the rotor blades changed pitch due to flexing of the guide tube and the blade movement was incorrectly identified as a positive confirmation of control authority. There was no confirmation by external observation of the main rotor and swashplate operation during the limited range pre-flight checks.

The perceived limitations of the hydraulic system when pressuring the hydraulics from the battery powered DC motor (motorising) compounded by the restrictions of using an external battery for starting were identified as contributory factors because control movements were not made to the full extremes of the cyclic envelop during the pre-flight checks.

Safety Actions

On 20 June 2018, the operator issued Flying Staff Instruction (FSI) 2018-35 to remind all crews to conduct the flight controls servo system check, which includes a full and free check, as required by the Operations Manual Part B, Section 02, Appendix 2. The FSI contained the detailed check as an Appendix.

After FSI 2018-35 was issued, the operator carried out a review of compliance on a sample of flights. This review identified that the control extremes were mostly but not always being reached; the majority of deviations being associated with a lack of full travel of aft cyclic. The operator considered that this may have been associated with seating positions and physical body shape, and that it would review this possibility in more detail.

The operator has advised that it will continue to monitor that its pilots perform the check, to the extremities, through routine simulator checks and, through its flight data monitoring programme, during operational flying.

On 22 July 2019, the helicopter manufacturer issued a Safety Advisory to highlight to operators the necessity of performing the prescribed Safety Inspections after long-term storage of the aircraft, specifically the inspection/check of the swashplate. The helicopter operator has incorporated the assessment of the ball ring socket for freedom of movement in the Daily inspections. In addition, it has made the decision that, in the future, helicopters that have been transported by sea will then be road transported from their port of entry to the maintenance facility. The operator has also undertaken to investigate increased environmental protection for its helicopters during sea voyages.

Other Safety Resources

Aerossurance has previously written:

We have discussed human factors and error management more generally here:

A public inquiry chaired by Anthony Hidden QC investigated 1988 Clapham Junction rail accident. In the report of the investigation, known as the Hidden report, he commented:

There is almost no human action or decision that cannot be made to look flawed and less sensible in the misleading light of hindsight.  It is essential that the critic should keep himself constantly aware of that fact.

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, which fills a gap in maintenance that in operations is filled by FDM and LOSA.

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