The following is taken from Aircraft Accident Report AAR 1/2017. The full report including an animation of the flight path can be found on the Air Accident Investigations Branch website.
At 1322 BST on 22 August 2015, Hawker Hunter TMk7 G-BXFI crashed on to the A27, Shoreham Bypass, while performing at the Shoreham Airshow, fatally injuring eleven road users and bystanders. A further 13 people, including the pilot, sustained other injuries.
The investigation identified the following causal factors in the accident: the aircraft did not achieve sufficient height at the apex of the accident manoeuvre to complete it before impacting the ground because the combination of low entry speed and low engine thrust in the upward half of the manoeuvre was insufficient. An escape manoeuvre was not carried out, despite the aircraft not achieving the required minimum apex height.
The pilot either did not perceive that an escape manoeuvre was necessary, or did not realise that one was possible at the speed achieved at the apex of the manoeuvre. He had not received formal training to escape from the accident manoeuvre in a Hunter, he had not had his competence to do so assessed, he had not practised the technique for escaping from the accident manoeuvre and did not know the minimum speed from which an escape manoeuvre could be carried out successfully.
Information included in a previous AAIB report indicated that there had been several cases involving the Avon Mk122 engine type fitted to this aircraft where an un-commanded reduction in engine speed had occurred and subsequent engineering investigation had not established a clear cause. This investigation was unable to determine whether a reduction in engine speed recorded during the accident manoeuvre was commanded by the pilot. A test flight on a similar Hunter indicated that any un-commanded reduction in thrust would be difficult to detect.
The Hunter was flying with a valid Permit to Fly but technical issues identified in the investigation indicated that the aircraft was no longer in compliance with the requirements of the Permit. It was also found that defects and excedences of aircraft operational limits had not been reported to the maintenance organisation and mandatory requirements of its Airworthiness Approval Note had not been met. Also, during prolonged periods of inactivity the engine had not been preserved in accordance with the approved maintenance schedule.
As a result of this investigation all Hunters on the UK civil register are grounded.