Service Difficulty Report – 510018970
Aircraft: Swearingen SA227-DC Metro 23 (a twin-engine, 19-passenger turboprop often used as a freighter)
Problem: window surround cracked
Possible consequence: depressurisation
During scheduled maintenance, engineers replacing a passenger window on a Metro 23 noticed irregularities on the window doubler. Concerned, they removed a few more rivets to allow further inspection. Investigation of the rivet holes indicated damage so the engineers removed the doubler, which was found to have cracked through in five places.
The doubler is manufactured from magnesium and is a structural component. Magnesium is a highly reactive metal that, although light and strong, is susceptible to corrosion and subsequent cracking.
The worst-case scenario is unlikely, but also very unpleasant: if the doubler and surrounding structure had been allowed to deteriorate further the window could have blown out, depressurising the aircraft.
But a highly likely result of the cracks in the doubler was that, for an unknown time, moisture could work its way further into the structural material layers of the fuselage. This would accelerate corrosion and allow rivets to work loose, continuing to weaken the aircraft’s structure. If this weakening went on for long enough it would increase the risk of depressurisation or other structural failure.
The engineers involved deserve commendation for taking a closer look during maintenance. The operator has now added checking of window doublers to its maintenance schedule.