On 16 July 2012, an ATR 42-300 being operated by Blue Islands on a passenger flight from Guernsey to Jersey, Channel Islands with First Officer line training in progress sustained a left main landing gear collapse on touchdown in normal day visibility. The left wingtip and propeller made ground contact but the aircraft remained on the runway and rapidly stopped following which an emergency evacuation of the 40 passengers was initiated via the main (rear left hand) passenger door during which 4 persons received minor injuries.
An Investigation was carried out by the Aircraft Accident Investigation Branch (UK) (AAIB). Data relating to the event was successfully downloaded from the 25 hour Flight Data Recorder (FDR) and the 2 hour Cockpit Voice Recorder (CVR).
It was established that the accident flight was the second of the day and had been without incident during the 11 minutes airborne. A visual approach to runway 27 at Jersey was made with a landing made by the aircraft commander as PF in a slight crosswind (reported as 210°/16 knots) with the left main gear touching first but not especially heavily.
However, shortly after the touchdown, the pilots heard an unusual noise and the commander stated that “the aircraft appeared to settle slightly differently from usual” which had made him suspect that a tyre burst had occurred. The single cabin crew also reported hearing a noise which she too suspected was due to a tyre bursting. The aircraft was stopped using the normal procedure although once the First Officer took over the ailerons as airspeed decreased through 70 knots to allow the commander to operate the steering tiller, he reported that despite applying corrective aileron inputs, he was unable to arrest the left wing down tendency of the aircraft.
Meanwhile, in the passenger cabin, some of the passengers became concerned at the abnormal situation, particularly a smell of burning and began to leave their seats. Although the cabin crew quickly instructed them to return to their seats, as soon as the aircraft stopped, passengers again left their seats and began moving towards the rear of the cabin where they had originally boarded the aircraft through the main (left hand side) passenger door adjacent to which the cabin crew was seated. The latter “realised the aircraft had suffered some kind of accident and that it would be difficult to contact the pilots whilst trying to control the passengers wishing to leave the aircraft” so being aware that the engines had stopped, she decided to initiate an evacuation by opening the main door. Due to the angle of the aircraft and the base-hinge design of the door, it was not possible to fully open it and the exit was additionally awkward to use because the steps normally used are part of the door. Nevertheless, all passengers were able to leave the aircraft through this door.
The Investigation found that the cause of the landing gear collapse was a consequence of structural failure in the side brace upper arm of the left main landing gear - see an illustration of the assembly below. This had allowed the main gear leg to continue to pivot outboard beyond its normal fully-deployed position. This had resulted in an excessive load being transmitted to the actuating cylinder attachment to the fuselage structure which had then also failed, allowing the now-unrestrained main trunnion to continue to extend outwards.
ATR 42 Main Landing Gear showing the locations of the primary and secondary failures (reproduced from the Official Report)
The side brace failure which had initiated the collapse sequence was found to have been attributable to the fracture of both of the lugs at its inner end which held the through bolt which allowed the brace to pivot in the fuselage attachment bracket. A fractured lug recovered from the runway was found to be one of these lugs.
The failed brace was found to have been forged from a billet of Aluminium supplied by a separate smelting company. After re-heating the billet, forging the part and heat treating it to its final temper, the part was then passed to a third company associated with the landing gear manufacturer for finishing which included machining of the bores of the lugs which had failed. It was then forwarded to the landing gear manufacturer and thereafter saw service installed in an unknown number of landing gear assemblies and was subjected to the prescribed action at overhaul. However, it was established during the Investigation that the recorded total life of the component was within its design fatigue life, which is considerably in excess of the normal component overhaul interval.
Metallurgical examination of the brace in order to establish a reason for its failure proved difficult. Eventually, destructive testing was employed and it was then found that the brace contained a small metallurgical feature at the origin of the fracture “which was consistent with titanium rich particles.…..which are introduced as a grain refiner during casting of the billet prior to forging.” The feature was within the defined specification but further analysis of the area around the fracture “revealed an area of static loading before propagating a crack in fatigue, indicating that there may have been a single overload event at some point in the history of the side brace upper arm”.
The summary Conclusion of the Investigation was that the left side main landing gear brace upper arm had suffered a fatigue failure which had rendered it ineffective so that “the unrestrained main trunnion continued to translate outboard, leading to the collapse of the gear”.
It was observed that although the immediate safety significance of a landing gear collapse at or shortly after touchdown on an ATR 42 “is limited”, the Investigation led the aircraft manufacturer to identify a more critical situation should such a failure occur at high speed during a take off roll as “hazardous” in accordance with type certification under CS 25.1309. No Safety Recommendations were made but “the European Aviation Safety Agency (EASA) have been made aware of the findings of this investigation and are reviewing whether there is any safety action required on aircraft components which use (the same) material”.
The Final Report of the Investigation was published on 10 October 2013.