What happened
On 9 February 1998, an SD3-60 Variant 100, registration G-BLGB, was performing a scheduled public transport flight to Stornoway Airport, Scotland. The flight was the final sector of the day for the crew. While approaching Runway 36 in instrument meteorological conditions, the crew transitioned to a visual approach after sighting the runway.
Upon sighting the runway, the commander noted the Precision Approach Path Indicators (PAPI) were displaying 'four whites', indicating the aircraft was above the correct glideslope. Although the commander briefly considered a go-around, he decided to continue the approach, reasoning that the length of the Strendoway runway would allow for a touchdown further down the strip. During the final moments of the approach, the aircraft's airspeed decreased as the commander configured the aircraft for landing flaps without a corresponding increase in engine power.
The aircraft touched down with a high rate of descent. The impact caused the left main landing gear to collapse and the left tyre to burst. The aircraft bounced, the fuselage scraped the runway, and the aircraft eventually slewed to the left, coming to rest at the edge of the runway. There were no fatalities and no injuries among the 26 passengers or the 4 crew members.
The investigation
Investigators examined the aircraft's mechanical condition, the flight data recorder (FDR), and the cockpit voice recorder (CVR). The engineering examination revealed that the left main landing gear's swinging arm had suffered a longitudinal split due to overload. This allowed the axle to rotate upward, causing the tyre to strike the diagonal brace and subsequently burst. The right landing gear also sustained damage, with a pin in the shock absorber shearing, though the gear itself did not collapse.
Analysis of the CVR and FDR indicated that while the crew worked professionally, the aircraft's descent rate at touchdown was between 1,000 and 1,400 feet per minute. This exceeded the manufacturer's estimate that a descent rate greater than 960 feet per minute could lead to undercarriage failure. The investigation also noted that the commander's decision to rely on runway perspective rather than the PAPI indications contributed to the unstable approach.
Findings
- The primary cause of the accident was the high rate of descent at touchdown, which exceeded the structural limits of the landing gear.
- The commander'1s decision to continue the approach despite being above the glideslope and the lack of sufficient power to maintain airspeed contributed to the unstable landing.
- The reduction in airspeed, caused by increasing propeller pitch without increasing engine torque, made the aircraft more difficult to control during the final approach phase.