Hybrid Air Vehicles Limited has been informed by the Air Accidents Investigation Branch that, after reviewing HAV’s comprehensive internal investigation report, the AAIB has decided to reclassify the November 2017 mooring mast detachment of the prototype Airlander 10-001 as "Non-Reportable". As a result of this finding, the AAIB will not be publishing a report and will close its files on the event. In his letter to HAV, the Inspector of Air Accidents in charge of the investigation wrote:

"On reviewing the comprehensive report provided by HAV to the AAIB, it was concluded that the findings relate only to HAV and there are no wider safety messages that would benefit the general public. That, coupled with the fact that the aircraft was not intending to fly at the time of the incident, led to the decision to downgrade the investigation."

On the morning of Saturday 18th November, the prototype Airlander 10-001 detached from its mooring mast at Cardington, Bedfordshire. The detachment triggered the in-built hull deflation mechanism, which operated until the aircraft came to rest on the airfield’s perimeter fencing. Following the event, HAV launched an internal investigation to establish the chain of events and determine what led to the de-masting.

A key part of the internal investigation was a detailed assessment of the critical aircraft parts and assemblies. This assessment confirmed that there was no evidence of structural or functional failure of the aircraft that could have led to the de-masting. This assessment did, however, confirm that the root cause of the mooring mast detachment was a failure to correctly secure the locking mechanism between the aircraft and its mooring mast, which was not detected. The investigation showed that it was possible to insert the mast locking safety pins in an incorrect position without obvious indications of this being the case, and that this had occurred on completion of Airlander’s test flight on 17th November 2017. This resulted in the aircraft being attached to the mooring mast in an undetected unlocked condition, allowing it to detach from the mooring mast in the light winds experienced on the morning of 18th November 2017.

The internal report recommends a number of corrective and preventative actions to improve the mooring mast locking mechanism and to address all other factors that contributed to the incorrect application of the mast locks, to minimise the possibility of similar events in future. The aircraft design team has begun implementing these actions as it moves from the prototype to the production variant of Airlander.

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