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Report: Controllability issues on DHC-6 Twin Otter survey plane due to maintenance lapses
18 February 2016
Aircraft schematic showing the EM pod installation (AAIU)

Aircraft schematic showing the EM pod installation (AAIU)

The crew of a DHC-6 Twin Otter survey plane experienced controllability issues after losing the nose cone of an external survey pod near Weston, Ireland. AAIU findings showed ground crew failed to follow standard maintenance procedures the day before.

On August 15, 2015, the aircraft  departed Weston Airport, Ireland, on a geological survey flight. Just after take-off, Weston Tower advised the aircraft that it appeared that the nose cone had fallen from the aircraft. The flight crew checked the aircraft and realised that the nose cone which had been reported falling from the aircraft had come from the right hand Electro Magnetic (EM) pod fitted to the tip of the right wing. The crew levelled the aircraft at 1,500 feet at which time they noted that aircraft control was being adversely affected by a significant amount of yaw to the right; this effect was felt through the flight controls. The Flight Crew transmitted a “PAN” call and advised Weston Tower that they would like to divert to Dublin Airport. The aircraft was transferred to Dublin ATC and was cleared to land on runway 28. On landing the aircraft was met by Airport Fire Service vehicles and was escorted to a parking location.

On 14 August 2015, the day before the event, in accordance with the maintenance instructions prescribed in the STC, the aircraft underwent a 125 hour Supplementary Inspection. This inspection called, inter alia, for the removal of “the EM pod nose and tail cones” and inspection of “the pod internal frames for cracks or other damage”. The personnel who carried out this check advised the Investigation that the Operator’s standard practice calls for the fitting of flagging tape when parts are removed and that the flagging tape should only be removed following re-installation of the removed part(s). On this occasion the personnel involved advised the Investigation that flagging tape was not fitted.
It was reported that during the EM Pod maintenance, while the nose cone was being reinstalled, a fault was detected with its sensor system. Re-installation of the nose cone was halted pending identification of the cause of the fault and consequently only the top two nose cone retaining screws were re-installed. Troubleshooting subsequently traced the origin of the sensor problem to a location inboard of the pod and the fault was rectified. The inspection was then completed but the 14 remaining nose cone retaining screws were not re-installed.

Probable Cause:
1. The right hand side EM pod nose cone was not properly re-installed after the 125 hr Supplementary Inspection.
Contributory Causes:
1. The Operator’s standard practice of attaching flagging tape to highlight when components are removed during maintenance was not followed.
2. None of the personnel who carried out a walkaround inspection noted that the screws were missing from the right hand EM pod nose cone.

More information: