Luton Minor fatality, and the latest Airprox reports
Aircraft Type: Luton LA4A Minor
Date & Time: 3 February 2019 at 1145
Commander’s Flying Experience: PPL, 317 hours, 150 on type
Last 90 days: unknown
Last 28 days: unknown
The owner/pilot was conducting a Permit to Fly renewal test flight from Waits Farm in Essex, from which he had been operating the aircraft since 2012. The flight followed a period during which it had not flown. Although it is thought that he had previously completed one test flight in December 2018 and a second in January 2019, his logbook did not confirm these.
No one watched the aircraft take off, but a witness working in a nearby garden first saw it climbing away from the airfield heading north-east and “travelling quite slowly”. It then banked sharply to the right and descended at approximately 45° to the ground. This witness did not see it crash, but heard the impact and ran towards the accident site in a harvested arable field close to the extended centreline of the strip from which it had taken off, but before he could reach the scene the aircraft had caught fire.
The engine was partly buried by the force of the impact, but its final orientation was consistent with the aircraft having descended steeply nose-down with the left wing hitting the ground just before the right wing. An intense post-impact fire had destroyed almost all of the wooden structure, leaving only the extreme outboard section of the right wing and aileron unburnt. Charred remains of curved wooden members indicated that the tailplane, rudder and elevators were present, but nothing identifiable as the wooden structure of the fuselage or of the left wing survived the fire. The accident was not survivable.
Due to the absence of the majority of the aircraft, the AAIB’S examination was limited in scope and almost entirely restricted to metallic components, in particular the engine and flying controls. ‘The flying control cable system, although disrupted and with most of its pulleys destroyed by fire, appeared to have been intact before the accident with the possible exception of the turnbuckle barrel on the aileron balance cable,’ says the AAIB report. ‘The process and sequence of separation of the eyebolts from the turnbuckle barrel, the absence of the latter and the absence of a section of locking wire could not be explained. If this turnbuckle barrel was missing prior to flight or failed in flight, it is possible that the aileron circuit would still have functioned whilst held together by the locking wire. However, once the locking wire failed, the pilot would have been left with very limited roll control, although directional control could have been maintained through use of the rudder. Had such a flying control failure occurred, it could support the theory that the pilot tried to return to the airfield. However, with limited roll control and with
a tailwind, maintaining runway alignment would have been challenging and might have necessitated a go-around. The presence of such a flying control disconnect could have either directly caused a loss of control or distracted the pilot from the monitoring of his airspeed which could then have resulted in the aircraft stalling.
‘The condition of the propeller was consistent with an absence of power at impact. No evidence of pre-impact mechanical failure was identified in the engine. The combustion chambers and piston crowns lacked the brown coating generally associated with petroleum fuelled piston engines operating normally. Examination of another Volkswagen derived aero engine had confirmed that such brown colouring was to be expected in the combustion spaces of a correctly functioning engine. Instead, all (this engine’s) piston crowns exhibited a black finish as normally found in engines which have operated for a period with an over-rich mixture. Such over-rich operation can result from a period of running with significant and increasing carburettor ice formation and which will, eventually, cause the engine to stop producing power. The low temperature and recent clearance of ground frost at the airfield at the time of the accident indicates that conditions conducive to carburettor icing near ground level would have been present.’
The AAIB report concludes: ‘It is possible that the pilot experienced a rough running engine due to carburettor icing… It would normally be possible to glide an aircraft into a field if the engine fails, however, this can be particularly challenging when at low speed and with a high nose attitude on a go around. Owners of other Luton Minors reported that the aircraft requires a steep nose down attitude in a glide to maintain a safe airspeed. It is possible that the pilot was not able to react quickly enough to the loss of engine power and the aircraft then stalled. It is unlikely that it would have been possible to recover from such a stall at this low height. A loss of engine power due to carburettor icing followed by a stall would be consistent with the witness observations of the aircraft.’
Originally produced by Luton Aircraft and redesigned for home construction, the LA4A is a development of the one-off 1936 LA3 Minor