From Flight International
SAS Q400 accident probe points to maintenance error
By David Kaminski-Morrow
Danish investigators are indicating that a maintenance error led to the landing-gear actuator blockage which led a Scandinavian Airlines Bombardier Q400 to conduct a gear-up landing at Copenhagen last month.
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 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 manufacturer 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.
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 transferred…by 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 states that the investigation is continuing and the organisation has yet to reach final conclusions