Safety Mat­ters

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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

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