Danish investigators have indicated that a maintenance error led to the landing-gear actuator blockage that caused a Scandinavian Airlines Bombardier Q400 to make a gear-up landing at Copenhagen in October.
It follows their discovery that an O-ring from a newly replaced door valve caused the blockage of a restrictor valve in the actuator assembly. The blockage prevented the right-hand landing gear from deploying.
The subsequent gear-up landing was the third suffered by a Scandinavian Airlines Q400 since early September and the carrier opted to remove the entire fleet permanently from service.
But while the underlying reasons for the first two accidents, which resulted from actuator corrosion, have yet to be determined, Danish investigation agency HCL is suggesting that a maintenance error contributed to the third.
The finding lends support to claims from Bombardier that there is no inherent problem with the aircraft.
HCL says that in-depth analysis of the Q400's hydraulic system shows that the O-ring "could not have travelled" from the solenoid valve to the actuator because certain components, such as the mechanical sequence valve, would prevent passage.
But it points out that the right main landing gear's solenoid valve was replaced on 16 October and the mechanical sequence valve was replaced on 22 October - just five days before the Copenhagen accident.
The 9 September incident at Aalborg airport was caused by corrosion
Scandinavian Airlines replaced a number of landing-gear components on its entire fleet after the first two gear-up events, at Aalborg and Vilnius, on 9 and 12 September.
HCL says that, during replacement of the mechanical sequence valve, the rogue O-ring could have "unknowingly been transferredby maintenance personnel" from one side of the valve to the other.
If this was the case, it says, the O-ring would have been able to travel through the hydraulic lines towards the landing-gear actuator. HCL says that the investigation is continuing and the organisation has yet to reach final conclusions.
But Bombardier says the Danish evidence "clearly supports" its view that the Q400 is "safe and reliable". The manufacturer has been forced to defend the Q400 in the wake of the three closely spaced Scandinavian Airlines accidents and the carrier's decision to axe the type.
"We have unwavering confidence in the Q400 aircraft and we stand by our product," says Bombardier Regional Aircraft president Steven Ridolfi.
"We are concerned about the impact negative comments might have had on Bombardier and the Q400 turboprop's reputation and we will do all that is necessary to protect our brand and, by association, the reputation of our Q400 aircraft customers."