Dutch investigators have determined that a Flybe Bombardier Q400 crew received a false indication that the undercarriage was down and locked before the right-hand main landing-gear collapsed on touchdown.
The inquiry into the accident at Amsterdam Schiphol airport found that the Q400 had undergone maintenance during which the right-hand landing-gear stabiliser brace assembly – along with its associated proximity sensors – was replaced.
This brace is a folding structure which keeps the yoke and shock-strut in position when the landing-gear is extended or retracted.
Ground tests were performed, with the undercarriage lowered and raised several times while the Q400 was jacked, and the aircraft was released for service.
"The maintenance crew reported nothing unusual was found during and after the maintenance activities," says the Dutch Safety Board. There were no maintenance deferred items on the aircraft.
But analysis of the undercarriage found that the Q400 had a deformation in the right-hand landing-gear yoke, which had existed before the landing.
The inquiry was unable to determine when this deformation had occurred, adding that it would have been caused "at any time" when the stabiliser brace was not installed, from production to its last maintenance session.
There was no need to check for bending, twisting or misalignment of the yoke during the replacement of the stabiliser brace.
On the approach to Schiphol's runway 22, on 23 February last year, the crew lowered the undercarriage and three green lights on the instrument panel indicated the landing-gear was down and locked.
But the inquiry says deformation of the yoke meant the landing-gear was "outside the certification state". The deformation affected the stabiliser brace's final over-centre position as the landing-gear was deployed, while not affecting the over-centre position of the brace's lock link – which, in turn, drives the landing-gear down-lock sensors.
This resulted in the main landing-gear being down, but not locked, while indicators in the cockpit falsely showed green lights to the crew in the cockpit.
Investigators state that the only potential sign of a problem might have been an amber light indication caused by the aft landing-gear door remaining open during the approach, a sign of a sequencing fault detected in the proximity sensor electronic unit.
"The crew stated they did not notice an amber caution light," says the inquiry. "The fact they did not notice the light was probably the result of the crew being focused on the landing and looking outside, after they had confirmed that they had three greens for the landing-gear."
While the right-hand gear on the aircraft (G-JECP) collapsed almost immediately after touchdown, causing the wing-tip and engine nacelle to strike the runway, none of the 63 occupants was injured.
Source: Cirium Dashboard