SA-227 AC Metro III
Maintenance Airframe; Maintenance Powerplant
Confusion; Time Pressure
Aircraft Equipment Problem Critical; Ground Event / Encounter Ground Strike - Aircraft
Aircraft Aircraft Damaged; General Maintenance Action
Procedure; Aircraft
Was brought out to tow aircraft to hangar for hangar installation. Hooked up tow bar after shutdown; hooked tug to towbar; got okay to tow; wing walkers thumbs up; towing was successful until aft right Main Landing Gear (MLG) tires hit a small one inch lip at asphalt to concrete change point; right main gear folded at this point and right side of aircraft hit the ground asphalt/concrete.It was discovered that the cause of the problem was shearing of the main gear cam bolts. My only recommendations are a fleet campaign and or; if the problem proves to be affiliated with specific design of the aircraft possibly a Service Bulletin (SB) or Airworthiness Directive (AD) that makes the change of the bolts at regular intervals mandatory. Towing after ferry flight.
Reporter stated his air taxi has already started a fleet campaign to check the four bolts and self-locking nuts that attach two crescent moon shaped cams on the forward drag brace of each main gear strut. Two hydraulic actuators one for (up) and one for the (down) movement of each Main Landing Gear (MLG) have a bearing that ride in the crescent section and rotate the MLG. The Main Landing Gears do not use a traditional overcenter locking brace.Reporter stated the Fairchild (Swearingen) Merlin had just arrived after a ferry flight for a landing gear door system problem. The prop; nose cone and planetary gears were damaged after the MLG collapsed. Three of the four cam attach bolts at one of the two cams for the right gear had completely sheared and the fourth was half sheared. He suspects that was an isolated incident; but is also aware the cam bolts are not normally removed and inspected for condition during Maintenance checks. He believes the Main Landing Gears had been changed recently. Aircraft should be jacked up to replace the bolts. He recommends at least a torque check of the cam bolts and self-locking nuts which may indicate if some have already sheared.
I was walking in; the right wing of the aircraft; as it was being pushed back into the hangar; the right Main Landing Gear (MLG) collapsed and the right wing hit the ground in front of me.[Recommendations]: Nothing comes to mind since any defects were unknown at time of occurrence [during] aircraft push back into hangar.
Two Aircraft Maintenance Technicians (AMTs) report a right Main Landing Gear (MLG) on a Fairchild (Swearingen) SA-227DC Merlin had collapsed as the aircraft was towed into the hangar across an asphalt to concrete transition line. Three of the four bolts attaching one of the two crescent shaped cams on the right MLG were found sheared.