What happened
On 27 June 2014, an Agusta Bell 206B Jet Ranger III, registration G-OMDR, was conducting a private flight from Bicester towards Yorkshire. While navigating near Gamston Airport, the aircraft encountered deteriorating weather conditions associated with a weather front. During the flight, the helicopter climbed into cloud, resulting in a temporary loss of control. The pilot experienced symptoms consistent with a vortex ring state, including rapid descent and uncommand to yaw. During the subsequent recovery maneuvers, the aircraft underwent extreme movements that caused significant internal damage.
Following the event, the pilot performed a precautionary landing at Gamston Airport. An inspection was conducted, but because the pilot did not use a checklist and focused primarily on the drag pin, the structural damage was not identified. The helicopter subsequently completed two more flight sectors to Breighton and then to Scarborough.
The investigation
The AAIB investigation established that the aircraft's main rotor drive system had sustained extensive damage. This included damage to the gear teeth of both the forward and aft main driveshaft couplings, as well as deformation of the freewheeling unit attachment flange. The isolation mount top plate was gouged through its entire thickness, and the internal elastomeric damper had disbonded. Additionally, the transmission drag pin had made such heavy contact with the stop mount plate that the rivets failed in shear, causing the stop mount plate to detach entirely.
The investigation also examined the subsequent flights. On 30 June 2014, the pilot sought advice from a maintenance manager. While reviewing photographs sent by the pilot, the maintenance manager noted a gouge in the isolation mount but did not consult the Aircraft Maintenance Manual (AMM) for allowable damage limits. The manager also failed to notice that the stop mount plate was missing from the images. Based on the pilot's report that the aircraft felt normal, the manager verbally approved two further flight sectors before the aircraft was finally withdrawn from service at Biggin Hill.
Findings
- The flight was planned through an area of deteriorating weather in an aircraft only certified for VFR operations.
- The aircraft entered cloud, leading to a temporary loss of control and extreme cyclic control inputs.
- Significant damage to the main rotor drive system occurred during the loss of control but remained undetected during the initial post-flight inspection.
- The pilot's post-event inspection was incomplete as it did not follow a checklist and omitted the isolation mount.
- The maintenance manager's assessment of the damage was influenced by confirmation bias and a failure to refer to the AMM for specific damage limits or to notice the missing stop mount plate.