Preliminary Investigation Report – SAS DHC8-Q400 LN-RDI Accident

Denmark’s Accident Investigation Board reported that the landing gear accident of SAS Dash 8 – Q400 LN-RDI at 27.10.2007 in Copenhagen was caused by a piece of rubber O-Ring trapped in the landing gear, preventing it from extending accurately.
This cause is not related to the two previous accidents which occurred in September 2007. According to the preliminary investigation report, this accident might have happened because of a fault by maintenance personnel and not because of a possible design fault of Bombardier’s Q400.

Summary
During the examination of the retraction/extension actuator assembly from the right main landing gear of the occurrence aircraft, an O-Ring was found blocking the orifice in the restrictor valve.
The blocked orifice within the actuator assembly prevented the normal extension of the right main landing gear.
The O-Ring was similar to that O-Ring for the door solenoid sequence valve (SSV). The only component in the landing gear system that incorporated this O-Ring was the SSV.
An SSV on the right main landing gear system had been replaced on 16 October 2007.
In the past occurrences, O-Rings (situated adjacent to the filter) from the SSV´s are not known to have migrated into the landing gear hydraulic system.
The O-ring found blocking the right main landing gear actuator restrictor valve, was from the SSV that was previously installed on the occurrence aircraft.
The rogue O-Ring could not have traveled from the SSV to its final location in the right main landing gear retraction/extension actuator restrictor valve.
The MSV of the right main landing gear was replaced on 22 October 2007.
According to the maintenance records, the replacement MSV, was initially configured for installation into the nose landing gear hydraulic system. Prior to installation on the occurrence aircraft, the supplied MSV was reconfigured by maintenance personnel. To make it compatible with the installation requirements for the main landing gear, the unions from the replaced MSV were used.
During the replacement of the MSV, the rogue O-Ring could have unknowingly been transferred from one side of the MSV to the other side by maintenance personnel.
Following a possible transfer of the O-Ring, it could travel through the hydraulic lines towards the main landing gear retraction/extension actuator restrictor valve causing the blockage of the valve.

Source:
Preliminary Investigation Report PDF
Technical drawing