What happened
On a freight charter flight returning to Winnipeg, Manitoba, a Fairchild SA226-TC Metro II operated by Perimeter Airlines experienced a sudden landing gear retraction during its landing roll. The aircraft, which was carrying no passengers, had completed an instrument approach to runway 36. After touching down approximately 1,200 feet from the threshold, the aircraft traveled about 2,000 feet down the runway before the landing gear began to retract.
As the gear retracted, the aircraft sank onto the runway surface. The impact caused the propellers to strike the pavement, resulting in broken propeller blades and debris penetrating the fuselage. This impact severed critical electrical wiring and hydraulic lines. The crew successfully shut down the engines and evacuated the aircraft without any injuries.
The investigation
The investigation focused on why the landing gear retracted despite the crew having selected the gear down during the approach and the weight-on switches being functional. Investigators examined the electrical circuitry between the cockpit and the hydraulic power pack. While the cockpit selector and weight-on switches were found to be working correctly, the investigation identified significant issues within the left engine nacelle.
Inspectors found that the wiring harnesses for the flap and landing gear systems had been previously repaired but were not protected according to acceptable engineering practices. Specifically, the harnesses exhibited numerous areas of bare, chafed wiring. Furthermore, the investigation looked into the maintenance programs of both the manufacturer and the operator, finding that the inspection procedures for the power pack area were ambiguous and did not explicitly require the removal of certain panels to inspect the wiring harnesses.
Findings
- The landing gear began an uncommanded retraction due to electrical short circuits within the landing gear and flap wiring harnesses.
- The wiring harnesses in the left engine nacelle contained multiple areas of exposed, bare wire caused by abrasion.
- Previous repairs to the wiring harnesses failed to provide adequate protection against chafing.
- Environmental factors, including oily contamination on the wiring and the presence of slush and de-icing fluids on the runway, increased the risk of an electrical fault.
- The manufacturer's Phase Inspection Program and the operator's maintenance manual contained ambiguous instructions that did not effectively ensure the inspection of the power pack's electrical components.
Safety action
- The operator updated the Zone 5 Heavy Inspection sheets to specifically require the removal of the panel below the power pack to inspect the wiring condition.
- Transport Canada requested that the aircraft manufacturer clarify inspection forms to better identify specific panel locations and inspection requirements for the power pack area.