Danish investigators have identified corrosion in the main landing-gear assembly on the Scandinavian Airlines Bombardier Q400 turboprop that suffered a landing-gear collapse at Aalborg.
Corrosion of the internal threads of the starboard main-gear actuator piston, which controls retraction and extension of the undercarriage, led to the piston's rod end becoming detached from the piston.
The Danish accident investigation agency HCL says that the inquiry is still at an early stage, but adds that "the separation contributed to the landing-gear collapse". The main landing-gear on the Q400 retracts rearwards into a cavity behind the engines. The agency says that damage to the actuator piston can cause the undercarriage to deploy too quickly.
The SAS flight SK1209 from Copenhagen had been on approach to Aalborg on 9 September when the crew received a cockpit indication that the starboard main landing-gear was not locked. During the emergency landing that followed, the aircraft's starboard main-gear collapsed a few seconds after touchdown.
Three days after the Aalborg incident a second SAS Q400, destined for Palanga in Lithuania, suffered an apparently-similar main landing-gear collapse after diverting to Vilnius. Findings from that accident have yet to be disclosed, but Canadian regulators have issued an airworthiness directive instructing operators to inspect the landing-gear, and particular sections of the actuator, of Q400 aircraft that have accumulated more than 8,000 cycles or four years' service.
Scandinavian Airlines and Norwegian sister operator Wideroe have withdrawn their entire fleet of Q400s from service pending inspection, while other Q400 operators have been examining their older examples of the type.
See video of the Q400's gear collaspe at Aalborg: flightglobal.com/q400video