Eight of Scandinavian Airlines' Q400s were returned to service after undergoing modification to correct the corrosion problem linked to the incidents last month.
Danish investigators identified corrosion of threads in the undercarriage actuator as the source of the problem, and SAS Group subsequently discovered similar corrosion on virtually all of its grounded turboprops.
SAS Group grounded all 27 Q400s from the Scandinavian Airlines and Wideroe fleets following the 9 and 12 September landing accidents, in which the right-hand main gear on two separate aircraft collapsed on touchdown.
SAS Group has opted to change the main landing-gear actuator pistons and rod-ends on its entire fleet. Scandinavian Airlines vice-president for technical operations Geir Steiro stresses that the carrier has used only new parts during the refit.
Actuator piston installed in the main landing gear
"We have not replaced these components with repaired or reworked parts," he says.
Steiro adds that the maintenance procedure has also involved applying an anti-humidity and corrosion-prevention sealant, known as Mastinox, to the rod-end threads.
Anti-humidity and corrosion-prevention sealant appears as yellow coating
The three-week grounding has prompted SAS Group to pursue the Q400's manufacturer, Bombardier, for compensation. Horizon Air, another major Q400 operator with 33 in service, declines to say if it will request compensation for the service disruptions that resulted from inspections.
Credit rating agency Standard & Poor's last month estimated the financial impact from the grounding at SKr10-15 million ($1.5-2.2 million) a day.
SAS Group says the aircraft being returned to service have undergone "extensive" inspections of both the main landing-gear and the nose-gear and been subjected to a number of test flights.
Bombardier will not comment on any discussions about compensation, saying only tht its priority "has been to work with operators to facilitate the return of our Q400 aircraft to service". It adds that in many cases it has "worked around the clock with Goodrich, the landing-gear manufacturer, to make this happen".
Bombardier said that the SAS Q400s were the only aircraft that remained grounded. "It is very encouraging that nearly 90% of Bombardier's 165 Q400 aircraft [in the global fleet] will soon be back in service."