Background
The crew of a scheduled airline flight elected to conduct a practice autoland approach to recertify the aircraft for Category III (CAT III) autoland procedures. Airline policy permitted such approaches when weather was at Category I (CAT I) minimums or better. At the time of the accident, visual conditions were good and weather was not a factor. Wake turbulence was also not a factor.
Approach Procedures
The airline's flight operations manual stated that autoland approaches were permissible to all Instrument Landing System (ILS) facilities without restrictive notes. However, the ILS runway 8 approach chart included a notation: "unusable from 0.8 nm to threshold." Additionally, both FAA advisory material and the airline's manual directed pilots to advise air traffic control (ATC) when conducting an autoland approach to ensure clear areas for localizer signals. The crew did not advise ATC.
Drift and Recovery
The crew reported, and the flight data recorder (FDR) confirmed, that the aircraft began drifting off the centerline at about three-quarters of a mile from the runway threshold. The drift was consistent with the wind direction and the chart notation. The first officer then disconnected the autopilot and hand-flew the aircraft back to the centerline, which prolonged the flare. Airline training material noted that a prolonged flare is a risk factor for tailstrikes.
Landing and Tailstrike
The aircraft did not touch down until approximately 4,000 feet from the landing threshold. Recovering from the drift led to a prolonged flare and a long landing at a lower than nominal speed. FDR information indicated that during the flare, the horizontal stabilizer moved to a position that would command full nose-up trim. During the landing, the tail struck the ground, resulting in substantial damage.
Investigation Findings
An assessment of the Boeing 757 horizontal stabilizer trim control system indicated that the rate of trim change observed during the flare could only be achieved by operating the control yoke stabilizer trim switch or the trim control levers on the center pedestal. Post-accident tests of the trim system revealed no anomalies. During a post-accident training session, the first officer was noted to handle the yoke such that he inadvertently activated the trim switch to the nose-up position. Airline training material noted that use of nose-up trim during the flare is a risk factor for tailstrikes.