What happened
The flight was performing a night landing on runway 01 under instrument meteorological conditions (IMC). The weather featured drizzle, a wet runway, and a right crosswind from 110 degrees at 14 knots, gusting to 18 knots. Before beginning the approach, the crew established an approach speed (Vapp) of 128 knots and a landing reference speed (Vref) of 122 knots.
As the aircraft followed the ILS approach, DFDR data indicated it was on autopilot while passing the middle marker at 200 feet AGL at 130 knots. Approximately three seconds after the first officer reported "runway in sight twelve o'clock," the captain disconnected the autopilot at a radio altitude of 132 feet and a heading of approximately 18 degrees. Within 11 seconds of this disconnection, deviations in the glideslope and localizer increased. Shortly before touchdown, the aircraft experienced a series of lateral rolls.
The airplane crossed the runway threshold at an altitude of 35 feet and 130 knots. It touched down 2,802 feet from the approach end of the 5,495-foot runway (which featured an 844-foot displaced threshold) at 125 knots on a heading of 10 degrees. The aircraft overran the pavement and struck a ditch located 175 feet beyond the departure end of the runway.
Calculations showed a total ground roll of 2,693 feet after touchdown. This included 1,016 feet before braking was initiated and 1,677 feet after braking began. According to Saab, the required distance from the start of braking to a complete stop on a wet runway is 1,989 feet.
The investigation
An examination of the aircraft revealed no mechanical anomalies that would have prevented it from operating according to its design prior to the excursion and impact with the ditch. Additionally, an FAA flight check of the ILS for runway 01 determined all components were operating within prescribed tolerances.
The investigation also reviewed the airport's runway safety area (RSA). In 1992, a proposal to extend the north end of runway 01 by 194 feet was approved, despite initial FAA disapproval because the extension reduced the RSA to less than the recommended 1,000 feet for a Part 139 airport. Following this extension, a drainage ditch was installed in the north RSA, perpendicular to the runway and approximately 175 feet north of the departure end. While FAA inspection reports from 1998 and 1999 noted the inadequate RSA, the letters of correction sent to the City of Killeen did not mention the issue.