S76, vicinity Lagos Nigeria, 2015
S76, vicinity Lagos Nigeria, 2015
On 12 August 2015, a Sikorsky S76C crew on a flight from an offshore platform to Lagos lost control of their aircraft after a sudden uncommanded pitch up, yaw and roll began and 12 seconds later it crashed into water in a suburb of Lagos killing both pilots and four of the 10 passengers. The Investigation concluded that the upset had been caused by a critical separation within the main rotor cyclic control system resulting from undetected wear at a point where there was no secondary mechanical locking system such as a locking pin or a wire lock to maintain system integrity.
On 12 August 2015, a Sikorsky S76C (5N-BGD) being operated by Bristow Helicopters on a non-scheduled passenger flight from the SEDCO Express Platform to Lagos was in day VMC at 1000 feet maintaining 120 knots when it suddenly experienced an uncommanded pitch up, yaw and roll from which recovery was not achieved and 12 seconds later it crashed into a shallow lagoon in a suburb of Lagos. Both pilots and 4 of the 10 passengers on board were killed, the other six passengers were all seriously injured and the helicopter was destroyed.
An Accident Investigation was carried out by the Nigerian Accident Investigation Bureau (AIB). The solid state combined CVDR and an additional CVR installed in the helicopter were recovered from the submerged wreckage on the same day as the accident and together provided access to voice and flight data relevant to the accident, although it was noted that the CVDR did not contain voice data. The main wreckage of the aircraft was subsequently raised from the bottom of the lagoon.
It was noted that the 37 year-old Captain was of US nationality and had accumulated a total of 5,406 hours flying experience of which 1,077 hours were on type. The 26 year-old First Officer was of Nigerian nationality and had a total of 808 hours flying experience of which 570 hours were on type.
It was established that the first flight of the day for the accident helicopter had been the sector from Lagos to the SEDCO Express offshore platform. It had then left there for the return flight to Lagos without refuelling on a VFR flight plan and with the Captain as PF, initially climbing to 3000 feet. First contact with Lagos ATC advised that the flight was approaching 1000 feet and giving at ETA of ten minutes from present. As requested during the initial exchange, the crew subsequently reported “field in sight” and were instructed to report left downwind for runway 18L. Less than a minute later, the helicopter was seen to crash into the shallow water of the Lagos Lagoon.
Recorded flight data indicated that whilst at 1000ft and 120 knots, the helicopter experienced an uncommanded and sudden pitch up, yaw and roll for 12 seconds until it impacted the water. Some of the passengers who survived “confirmed seeing the crew trying to stabilise the aircraft before impact” but it was considered by the Investigation that “the time available was not enough to do much before the aircraft impacted the Lagoon waters”. The Recorded flight data also showed that impact occurred at pitch up angle of about 42°, a roll angle of about 20° and with negligible forward speed.
In the initial investigatory work, it was discovered that the forward main servo input control pushrod assembly, also sometimes referred to as the forward servo clevis rod and bell crank assembly, had failed when the control pushrod tube separated from the control pushrod end with its bearing and jam nut. The jam nut involved was found to have been loose and “not seating against the Control Rod” - see the illustrations below.
It was found that the separation had occurred because of “wear in the rod end fitting shank” and the jam nut end fitting which fits into this rod end shank was found to rotate freely when turned by hand. It was determined that “thread peaks found at the middle and upper end of the end fitting shank were smooth and rounded, consistent with wear”. This meant that once the separation had occurred, pilot inputs to the cyclic control stick did not reach the cyclic control mixing unit and the aircraft had therefore become instantly uncontrollable.
A metallurgical examination of the control rod components by the National Transportation Safety Board (USA) (NTSB) led them to conclude that the separation which occurred “was a pre-impact condition”. A Preliminary Report was then released on 21 September 2015 which detailed these early findings of an airworthiness deficiency and made two Interim Safety Recommendations as follows:
- that the Sikorsky Aircraft Corporation should consider a redesign of the affected control push rod assembly by introducing additional wire locking safety features between the Jam nut and the Pushrod to enhance better security. [2015-009]
- that the Nigerian Civil Aviation Authority (NCAA) should immediately carry out appropriate oversight action on all the Sikorsky S-76 series helicopters flying in Nigeria to ensure the implementation of the Emergency Airworthiness Directives (EAD) and the Alert Service Bulletin (ASB) issued by the FAA and the Sikorsky Aircraft Corporation. [2015-010]
Apart from the findings in respect of the control rod, when the recovered wreckage was examined, it was found on initial inspection that the No. 2 Engine had “damage associated with the uncontained failure of internal components”. However subsequent teardown at the engine manufacturer with AIB investigators in attendance concluded that the damage was attributable to a combination of impact forces and ‘blade shedding’.
NB: Blade-shedding is part of a safety process aimed at preventing turbine discs from bursting in the event of over-speed and if that happens, it facilitates sequential blade separation so as to reduce the centrifugal force sustained by the discs.
It was found that five months prior to the accident, maintenance had been carried out in the vicinity of the main servo area when the main gearbox (MGB) was replaced. It appeared that the forward servo clevis rods and its associated bell crank were removed during the replacement of the MGB but it was not possible to make any link between this maintenance activity and the failure on the evidence available.
The Cause of the Accident was determined as “the separation of the Forward Servo Clevis Rod Assembly from the bearing and Jam nut (Bell Crank Assembly) which is part of the Cyclic Control System responsible for stabilising the attitude of the helicopter which made the aircraft uncontrollable”.
Two Contributory Factors were also identified as follows:
- The absence of a secondary mechanical locking system (lock-pin or wire-lock) in the design of the Forward Servo Input Control Rod assembly (which) contributed to the separation of the Forward Servo Clevis Rod from the bearing and Jam nut.
- The wear that was prevalent at the Forward Servo Clevis Rod end fitting shank, which made the Jam nut rotate freely when force was applied.
Safety Action taken in the early stages of the Investigation prior to the issue of the Preliminary Report and its two Interim Safety Recommendations was noted as follows:
- On 10 September 2015, the Sikorsky Aircraft Corporation issued ASB 76-67-57 applicable to all S-76 helicopters equipped with Control Pushrod Assembly P/N 76400-0034-059 and 76400-00014-071 which required a one-time inspection of the installed Forward, Aft and Lateral Main Servo Input Control Pushrods and Jam nuts and the Tail Servo Input Control Pushrods and Jam nuts for proper installation, condition and security followed by application of slippage mark on all Main and Tail Servo Input prior to the next flight originating from a maintenance facility or otherwise not exceeding 5 flight hours from issue date of the Bulletin.
- On 14 September 2015, the FAA issued EAD 2015-19-51 to mandate this ASB and the Nigerian CAA issued AOL 50 requiring compliance in respect of Nigerian-operated S76s.
On completion of the Investigation, one new Safety Recommendation was made as follows:
- that Sikorsky Aircraft Corporation should redesign the affected control pushrod assembly by introducing wire lock or lock pins as safety features between the Jam nut and the Pushrod to enhance better security and safety.
The Final Report of the Investigation was published on 22 October 2018.