Occurrence
On a day visual flight rules flight from Stony Rapids, Saskatchewan, to La Ronge, Saskatchewan, a Northern Dene Airways Ltd. Metro III (registration C-FIPW, serial number AC524), operating as Norcanair Flight KA1051, carried two crew members and nine passengers. At approximately 1410 Central Standard Time, the crew completed approach and landing checklists and confirmed a gear-down indication. The aircraft landed on Runway 18 in a crosswind, touching down firmly about 1000 feet from the threshold.
Upon touchdown, the left wing dropped and the propeller struck the runway. The aircraft veered left despite full rudder and aileron deflection. The crew applied maximum right braking and shut down both engines. The aircraft departed the runway, traveled about 200 feet through the infield, and the nose and right main landing gear were torn rearward; the left gear collapsed into the wheel well. The aircraft slid on its belly and came to rest approximately 300 feet off the side of the runway. Three passengers suffered minor injuries from the sudden stop when the landing gear finally collapsed; the other passengers and pilots were not injured.
After the occurrence, before shutting down the electrical system, the left main landing gear still indicated down and locked, while the nose and right main gear indicated unsafe. The captain secured the aircraft, and passengers evacuated through the two right overwing exits. Emergency response personnel attended to the passengers.
Weather at La Ronge at the time was: visibility 15 statute miles, clear skies, temperature 16°C, wind 250° magnetic at 17 knots gusting to 24 knots.
Examination Findings
A puff of smoke came from the tires on touchdown. Post-occurrence examination revealed that the outboard tire on the left landing gear had a bald spot worn to the cord on the outboard edge of the tread. The tire had been installed new approximately 63 flight hours prior.
The landing gear design uses a three-piece drag brace attached to the gear struts and wheel wells. Each drag brace hinges at both ends and in the center to fold during retraction. To stay locked down, the center hinge travels upward to an overcenter stop. Positioning cams bolted to the upper drag braces have concave cutouts to receive bellcrank roller assemblies from the wheel well. When fully extended, the bellcrank roller rests against the heel of the cam, indicating down and locked.
Inboard and outboard hydraulic actuators assist extension and retraction. The critical roller-to-cam clearance is adjusted by actuator rod length. The bellcrank roller is held locked by hydraulic pressure in the extended actuator and by spring tension. During normal extension, only the outboard cylinders are pressurized; the inboard bellcrank is held by spring tension alone. Two down-indication micro switches (one at each bellcrank) provide cockpit gear-down indication when activated.
The nose and right landing gear were severely damaged; actuators were pulled away from micro switches, causing unsafe indications. The left gear folded into the wheel well, with both bellcrank rollers driven beyond cam cutouts. The actuators remained extended, contacting the down-indication micro switches and producing a green down-and-locked indication.
When the aircraft was placed on a flatbed truck, the left landing gear was repositioned and a rigging check performed. The Metro III maintenance manual specifies that the measurement from the centre of the bellcrank roller to the edge of the positioning cam must be 0.180 inches, tolerance +0.060, -0.000. The inboard bellcrank roller measured approximately 0.0625 inches, though the cam edge was worn, possibly reducing this measurement, indicating the inboard assembly was out of adjustment.
The outboard bellcrank roller could not be measured because it was undersized: 0.625-inch diameter instead of the designed 0.750-inch diameter. This reduced required rigging allowances. Marks and gouges on the roller showed contact with the lip of the positioning cam.
The installed roller was part number 5453032-1, designed as an uplock roller, not a bellcrank roller. Its smaller diameter reduced clearance relative to the cam heel. Repetitive landing loads were initially absorbed by the roller hitting the cam lip until the lip wore away, eventually allowing the roller to move beyond the cam cutout. Consequently, the drag brace was held down mainly by the inboard bellcrank roller and cam, both out of adjustment and held only by spring tension.
On landing, the crosswind component and firm touchdown produced side loading that overrode the spring tension, snapping the inboard roller beyond the cam cutout and causing the gear collapse.
Maintenance Issues
An Aircraft Maintenance Engineer (AME) installed the incorrect roller, believing it had been redesigned to prevent breakage. The AME did not follow established industry or company practices to check the part number against the manufacturer's parts manual. A rigging check was not completed, which would likely have revealed the undersized roller.
An incorrect roller of smaller diameter and type was installed on the left main landing gear outboard bellcrank assembly, contrary to company and industry practice. The smaller diameter reduced required rigging tolerances and allowed the roller to move beyond the cam cutout, resulting in the collapse. A rigging check was not carried out after replacement; such a check should have revealed that neither the inboard nor outboard bellcrank assembly met minimum rigging requirements.
Post-Occurrence Actions
Northern Dene Airways Ltd. commissioned an independent safety audit of its complete operation. All maintenance staff of the AMO responsible for the company's aircraft met to review procedures. The policy was reinforced that no part should be installed without first referring to the appropriate parts and service manuals to ensure correct part number and system integrity.
This report concludes the Transportation Safety Board's investigation, authorized for release on 16 March 2005.