Incident Overview
Perimeter Airlines Flight 625, a Fairchild SA226-TC Metro II (serial number TC 278), was returning to Winnipeg, Manitoba, after a freight charter flight from Regina, Saskatchewan. The aircraft was operated by a crew of two pilots and carried no passengers. The crew executed a vectored instrument landing system (ILS) approach to runway 36, with the first officer at the controls. The aircraft touched down approximately 1,200 feet from the runway threshold and rolled about 2,000 feet before the landing gear retracted. The aircraft came to rest on its fuselage on the runway near the intersection of runway 07/25, about 3,300 feet from the touchdown point. The crew shut down the engines and evacuated the aircraft. No injuries occurred.
Crew Information
The captain had approximately 4,500 total flying hours, including 70 hours in the previous 30 days. The first officer had about 750 flight hours, with 70 hours in the previous 30 days. Both pilots were appropriately certified and qualified for the flight.
Flight and Landing Details
Both pilots reported that the landing gear was selected down during the approach and that the gear selector remained in the down position. The hydraulic system pressure was indicated at 2,000 psi after gear extension. Standard company procedure called for gear extension at glide slope intercept; normal hydraulic pressure ranges from 1,750 to 2,000 psi. The landing gear indicator showed all three gear legs as down. The aircraft broke out of cloud at about 200 feet above ground level, slightly right of the runway centerline, then maneuvered into position during descent. Witnesses described the landing as normal. After touchdown, the aircraft rolled about 2,000 feet and slowed to an estimated 40 knots before disappearing from view. The gear was reported extended during landing and the initial roll. Shortly thereafter, all three gear legs retracted, and the aircraft settled onto the runway. The propellers struck the ground, curled back, and broke; pieces penetrated the fuselage, severing a large electrical wiring bundle and hydraulic lines.
Environmental Conditions
The incident occurred at 1251 central standard time. Weather at Winnipeg at 1300 CST included wind 360° true at 14 knots, visibility one-half statute mile in snow, vertical visibility 200 feet, and temperature and dew point both 0°C. A surface condition report at 1200 CST indicated the center 100 feet of runway 36 had snow and slush one-eighth inch deep, while the remainder had two inches of snow and slush; de-icing chemical had been applied. The crew reported slush on the runway at Regina at departure.
Aircraft Weight and Balance
The maximum allowable gross weight was 12,500 pounds. The takeoff weight from Regina was 11,145 pounds, and the landing weight at Winnipeg was 10,295 pounds. The allowable center of gravity limits at that landing weight were 256.3 to 277 inches aft of datum; the actual center of gravity was 257.5 inches aft of datum.
Landing Gear System Examination
The landing gear is hydraulically actuated, using two engine-driven pumps supplying fluid to a hydraulic power pack. The power pack contains electrically operated valves directing fluid to the gear lines. The gear selector was found in the down position after the incident. When the aircraft was lifted, the left main gear was up-and-locked, while the right main and nose gear were partially extended. The gear extended easily to the down and geometric over-center position after doors and uplocks were released. Fuselage damage precluded power application to electrical or hydraulic systems, but inspections were conducted.
Electrical System Testing
The gear selector and flap selector switches route 28-volt electricity to the power pack. The cockpit circuitry to a terminal strip in the left engine nacelle was tested with no discrepancies found, though intermittent faults could not be ruled out. The aircraft has weight-on (squat) switches at each gear leg, designed to prevent an "up" signal when the aircraft is on the ground. These switches and the gear selector switches were tested and found functional. The landing gear oleo extension exceeded manufacturer recommendations but remained within range for proper weight-on switch operation. Examination of electrical wiring in the left engine nacelle revealed that the flap and gear wiring harnesses had been replaced after the aircraft entered service. The wiring displayed numerous small areas of bare insulation where nylon had been abraded. The flap harness was partly enclosed by soft insulating tubing; the gear harness was wrapped with spiral wrap. Neither harness was attached to the aircraft structure between the terminal strip and the power pack. Oil contamination was found on the harnesses in the power pack area.
Maintenance Records
Operator records indicated the aircraft was maintained per airworthiness regulations. No specific entry described repair of the wiring harnesses from the terminal strip to the power pack. However, an entry dated August 9, 1994, noted: "landing gear control circuit breaker keeps popping; repaired wiring for landing gear hydraulic control valve as per S/B 226 29 008, installed terminal block and relocated diodes for flap and landing gear wiring at power pack." Bare wiring can cause short circuits through contact with other conductive material or contaminants. Airworthiness regulations do not mandate specific wiring protection methods, but acceptable practices include tying or clamping wiring to structure or enclosing it in tubing or conduits.
Hydraulic Power Pack Inspection
The hydraulic power pack was removed and examined at the TSB Engineering Branch (LP 166/96). No damage or operational anomalies were found. There is no time-before-overhaul limit on the power pack or related wiring; they are maintained "on condition." The power pack was manufactured in 1983, but no installation record was available.
Sequence of Gear Retraction
For the power pack to retract the gear after landing, the electrical "down" signal must be removed and an "up" signal applied. The bare wiring is electrically downstream from the gear selector and the weight-on switches. The weight-on switches are designed to prevent inadvertent "up" signals while on the ground.
Inspection Program
The aircraft was on the Fairchild Phase Inspection Program, dividing inspection into 10 zones over an 800-hour cycle. The operator's Transport Canada-approved Maintenance Control Manual required phase inspections every 150 hours over 1,200-hour cycles, with the objective of inspecting the power pack and associated electrical harnesses. Zone 5 (landing gear and wheel wells) inspection required removal of 11 access panels but not the hydraulic power pack transmitter access panel. The power pack area was to be inspected through panel 551 in the forward wheel well, with no specific mention of checking electrical harnesses or the terminal strip. Complete inspection of electrical components required removal of the transmitter access panel. Zone 9 (engines) heavy inspection required panel removal but did not illustrate those panels; inspection items were accessible only through the transmitter access panel below the power pack area.
Conclusions from Investigation
The aircraft's weight and center of gravity were within limits. Electrical and hydraulic systems functioned normally during flight until landing. Crew reports and witness observations indicated the gear was selected down and extended before landing. The distance traveled and speed reduction suggest the aircraft's weight was substantially on the gear before retraction. The serviceability of the weight-on switches and oleo extension make it unlikely that the cockpit gear selector sent an "up" signal after touchdown. Part of the flap wiring harness was enclosed by soft tubing, and the gear harness was wrapped with spiral wrap, but these measures were in place.