Introduction
On 2 February 1999, the commander of a Sikorsky S76A helicopter (registration G-BIBG) became aware of a crackling sound on his No 1 VHF radio during an approach to Den Helder, Netherlands. The approach controller and tower controller reported poor radio transmissions. After switching to the No 2 VHF radio, normal transmissions resumed. During taxiing, the co-pilot commented on restricted yaw pedals, but the commander attributed it to tail wind effects. After shutdown, a ground crew member reported the right-hand tail rotor gearbox fairing missing. Inspection found the No 1 VHF aerial lead damaged near the tail rotor gearbox. The operator's staff initially suspected a bird strike. On 10 February, the operator's Engineering Department issued an inspection alert worldwide, calling for examination of tail rotor gearbox fairing attachments at the next Daily Inspection. No defective fairings were found at that time.
Incident Flight
On 22 March 1999, another Sikorsky S76A helicopter of the same operator, registration G-BITR, suffered a similar failure. The helicopter was on a charter flight for a North Sea oil/gas company. After an uneventful flight to a platform in the Netherlands sector, the co-pilot left the cockpit to supervise refuelling with rotors running. While walking past the right side, he noticed the right-hand tail rotor gearbox fairing missing and immediately signaled the commander to shut down.
Operator Safety Action
The following day, the operator re-issued the inspection alert with an enhanced inspection technique, copied to other UK operators and the manufacturer. Within 48 hours, six of the operator's S76 helicopters were found with tail rotor gearbox fairings that had suffered bonding failures between the fairing and the pylon. These helicopters were grounded until repairs were effected. One other UK operator found two of its S76 helicopters with bonding failures in the same region. All bonding failures were on the right-hand tail rotor gearbox fairing; none were detected on the left-hand fairing, which is on the same side of the pylon as the tail rotor. The operator produced a repair scheme implemented on its helicopters.
Safety Recommendation
On 21 April 1999, a Safety Recommendation was drafted to the helicopter manufacturer and the FAA (Primary Certificating Authority), with a copy to the CAA. Recommendation No 99-19 urged the manufacturer and FAA to urgently promulgate an inspection requirement for all Sikorsky S76 operators to inspect the bonding attachment of tail rotor gearbox fairings for evidence of bonding failure, and to require repair of any defective fairings before further flight. The concern was that fairing detachment could potentially lead to contact damage to adjacent tail rotor blades.
Manufacturer's Response
On 12 May 1999, the helicopter manufacturer issued Alert Service Bulletin No 76-55-16, introducing a one-time inspection for disbonding of attachment angles on the tail rotor box forward fairings at the vertical pylon skin, plus a recurrent 1,500 hour inspection. The bulletin also specified a repair scheme for any disbonding found. The FAA was proposing to issue an Airworthiness Directive requiring the inspections specified in the Alert Service Bulletin.