What happened
A Boeing 727-200 was performing a scheduled passenger flight into Calgary, Alberta, following a delay in Salt Lake City, Utah, caused by poor weather. During the approach to runway 34, the crew utilized an autopilot-coupled instrument landing system (ILS) approach in low visibility. As the aircraft descended, the autopilot began experiencing pitch oscillations. The captain disconnected the autopilot at approximately 130 feet above ground level after sensing a nose-down pitch. During the subsequent manual transition to landing, the aircraft drifted left of the runway centerline. In an attempt to correct the drift, the captain applied a right bank, which caused the right wing to strike the runway upon touchdown. The impact resulted in damage to the wing tip, flaps, and associated hardware, though no injuries were reported.
The investigation
The investigation examined the aircraft's flight data, the autopilot's sensitivity settings, and the crew's duty period. It was established that the aircraft was flying at a higher-than-normal ground speed due to a combination of light tailwinds, high altitude, and a higher approach speed necessitated by a heavy landing weight. The investigation also looked into the crew's schedule, noting that the pilots had commuted by air to their base, resulting in a duty day of approximately 13 hours and 15 minutes. Furthermore, the technical performance of the Sperry SP-150 autopilot was analyzed, specifically its ability to desensitize during the approach in the absence of a middle marker.
Findings
- The aircraft drifted left of the runway centerline, and the pilot's attempt to correct the bank angle caused the wing strike.
- The autopilot's sensitivity had not sufficiently reduced before the aircraft reached low altitude, leading to unstable pitch oscillations.
- The aircraft's ground speed was higher than the design parameters assumed by the autopilot's desensitization schedule.
- The use of a 30-degree flap setting, while standard practice, was not accounted for in the original autopilot design, contributing to a higher approach speed.
- The crew's extended duty day, driven by weather delays and early morning commuting, may have contributed to fatigue and affected performance during the critical landing phase.