Safety Matters and Safety Briefs are based on the AAIB Bulletin and UK Airprox Board reports, with additional material from the US National Transportation Safety Board
The Hunter tragedy at Shoreham in 2015 and other incidents
The Shoreham Hunter accident
The Air Accident Investigation Branch’s final report into the crash of a Hawker Hunter T Mk.7 during the Shoreham Air Display on 22 August 2015 was published on 3 March 2017. The following is a necessarily brief synopsis of its 452-page findings, which can be downloaded in full at: gov.uk/aaib-reports
After holding offshore for his display to start and briefly flying inverted, probably to check for loose articles in the cockpit, the pilot rolled upright and wings level, descended to 800ft and made a right turn to line up with the display line to the west of Shoreham’s Rwy 02/20. The Hunter remained right wing low with the angle of bank decreasing as it descended to 100ft and flew along the display line. It then started a gentle climbing right turn, executed a ‘Derry Turn’ to the left, which peaked at 1,800ft, then entered a descending left turn to approximately 185ft agl, approaching the display line at an angle of about 25°.
The Hunter then pitched up into the accident manoeuvre at an indicated airspeed of approximately 310kt and with an engine speed of approximately 7,500rpm. As it approached the vertical, the pilot initiated a roll to the left. In the climb the engine speed first reduced, then increased to about 7,200rpm, then reduced again nearing the apex. The aircraft was almost inverted with its wings level at the apex, at a height of approximately 2,700ft. During the subsequent descent, its ground track was aligned to the west along the A27 Shoreham Bypass. As it descended it accelerated and the nose was raised, but insufficient height was available to recover to level flight before it contacted the westbound carriageway. Eleven people died in vehicles on the road, and thirteeen, included the pilot, were injured.
During 2015 the pilot had flown one display practice and five displays in the Hunter, the most recent fourteen days before the accident. Both his currency and the renewal of his Display Authorisation, completed in a Jet Provost and subsequently RV-8 aircraft, complied with the regulations then in force.
The aircraft’s Rolls-royce Avon engine was subject to a Mandatory Permit Directive (MPD) which imposed a calendar life on the engine type, and provided an option to extend that life using an Alternative Means of Compliance (AMOC). Proposals for an engine life extension using an AMOC inspection programme had to be approved by the regulator. Related tasks were being conducted by the maintenance organisation, but the regulator had not approved either the operator or its maintenance organisation to use an AMOC to this MPD.
On the technical front, AAIB investigators found that defects and excedences of the aircraft’s operational limits had not been reported to the maintenance organisation, and mandatory requirements of its Airworthiness Approval Note had not been met. During prolonged periods of inactivity the aircraft’s Rolls-royce Avon engine had not been preserved in accordance with the approved maintenance schedule. The investigation identified a degraded diaphragm in the engine fuel control system, which could no longer be considered airworthy. However, the engine manufacturer concluded that this would not have affected its normal operation. The AAIB notes that in RAF service there had been several cases involving the Rolls-royce Avon Mk 122, where engine speed had dropped and the subsequent engineering investigation had not established a clear cause.
The Hunter had been issued with a Permit to Fly and its Certificate of Validity was in date, but the technical issues identified in the investigation indicated that the aircraft was no longer in compliance with the requirements of its Permit.
A test pilot flew two data-gathering sorties in a similar Hunter on 19 October 2015 totalling one hour, 25 minutes. Following analysis from these sorties and other information gathered during the course of the investigation, a third, 55-minute sortie was flown on 4 December 2015. Based on the
data gathered during these sorties, the test pilot concluded that:
From the apex height and airspeed achieved in the accident manoeuvre, and for up to at least four seconds after passing the apex, it would have been possible for an appropriately trained pilot to fly a straightforward escape manoeuvre, which would have prevented impact with the ground, by rolling the aircraft through 180° back to erect flight and then pulling out of the dive to regain level flight.
The measured height loss during a representative pull-through from the apex of a loop at the airspeed, all-up mass and density altitude of the accident manoeuvre was between 2,700-2,850ft, and if altimeter reading resolution and instrument errors were considered the range would increase to between 2,600-2,950ft. The height loss appeared to be insensitive to whether one or two notches of flap were selected and to the power setting.
The ‘bent’ loop tests indicated that the apex height and airspeed of the accident manoeuvre was consistent with a maximum performance pull-up from 300 KIAS with significantly less than full thrust and with a 45-90° bend initiated approximately five seconds after pull-up. They also showed that the apex height for a ‘bent’ loop was 300 to 400ft less than for a straight loop with all other parameters constant.
A 90° ‘bent’ loop entered at 350 KIAS with full Hunter T Mk.7 thrust was, for an appropriately trained pilot, a safe and straightforward manoeuvre to fly.
The test pilot also observed that any reduction in thrust was difficult to detect and if the accident pilot had not throttled back intentionally he would probably have been unaware of it. In summary, the AAIB concluded: 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 following contributory factors were also identified:
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.
The pilot had not received formal training to escape from the accident manoeuvre in a Hunter and had not had his competence to do so assessed.
The pilot had not practised the technique for escaping from the accident manoeuvre in a Hunter, and did not know the minimum speed from which an escape manoeuvre could be carried out successfully.
A change of ground track during the manoeuvre positioned the aircraft further east than planned, producing an exit track along the A27 dual carriageway.
The manoeuvre took place above an area occupied by the public over which the organisers of the flying display had no control.
The severity of the outcome was due to the absence of provisions to mitigate the effects of an aircraft crashing in an area outside the control of the organisers of the flying display.
See also the British Air Display Association’s response to the AAIB report in ‘ Pilot Notes’, p.7.
Illustration of the accident manoeuvre, a ‘bent loop’. (This is an approximate depiction of the aircraft’s behaviour/flightpath)
A comparison of audio-determined engine speed during looping manoeuvres flown by the same pilot during different Hunter displays (Accident manoeuvre at top)