Approach and Landing
The airplane was conducting a VOR/DME-C approach on an oblique course about 40 degrees to the runway 30 centerline. Wind conditions produced an 8-knot tailwind for landing on runway 30. Moderate to heavy rain had been falling for the past hour, and the runway was wet. The captain elected to land on the 6,600-foot-long runway instead of circling to land with a headwind.
According to the crew, the airplane was flown at the prescribed airspeed (Vref) for its weight with wing flaps fully extended on final approach. They reported touching down just beyond the touchdown zone. After touchdown, the captain extended the wing spoilers immediately and tested the brakes, noting normal brake pedal pressure. During rollout, he reported a lack of deceleration and applied increased brake pressure with no discernible effect.
The airplane's optional thrust reversers had been rendered non-operational by company maintenance personnel and were not functional. The captain stated that he considered performing a go-around but believed insufficient runway remained to ensure a safe takeoff. He did not activate the emergency brakes, which would have bypassed the antiskid system, because he thought there was insufficient time and was preoccupied with maintaining control. He asked the first officer to apply braking simultaneously.
When the airplane was about 2,000 feet from the runway's end, it was still traveling about 100 knots. The airplane rolled off the departure end of runway 30, which was wet, at an estimated speed between 85 and 90 knots.
Post-Overrun Sequence
The airplane traveled 618 feet through a rain-soaked grassy runway safety area before encountering a drainage swale that collapsed the nose gear. As the airplane traversed the soft, wet field, its wheels partially sank into the ground. While decelerating, soil impacted the landing gear wheels and struts where wiring to the antiskid brake system was located.
The crew reported no cockpit annunciator lights indicating a system failure during the rollout; however, after the airplane came to a stop, they observed that the annunciator light for the antiskid system on the No. 2 wheel was illuminated (indicating a failure). The other three annunciator lights were not illuminated.
Landing Performance Data
During the approach, the first officer completed the landing data card using a company-developed quick reference card (QRC). The QRC’s chart contained some data consistent with the Airplane Flight Manual (AFM) but did not include correction factors for tailwind conditions, whereas the AFM charts do include such factors. The landing data indicated a required stopping distance of 3,240 feet on a dry runway in zero wind, with a wet correction factor increasing that to 4,538 feet. The Vref speed was listed as 127 knots for a landing weight of 11,000 pounds; the first officer noted crossing the runway threshold at 125 knots.
Bombardier Lear calculated the wet stopping distance with an 8-knot tailwind as 5,110 feet. The touchdown zone for runway 30 is 1,000 feet from the approach end. The crew estimated their touchdown location at about 1,200 feet from the approach end, yielding a remaining runway of 5,400 feet. On-duty controllers observed the landing and reported that the airplane touched down at a taxiway intersection 1,881 feet from the approach end, leaving about 4,520 feet of runway.
Controllers noted water spraying off the main landing gear just after touchdown.
Post-Accident Testing
Post-accident testing indicated that the brake system, including brake wear, was within limits with no anomalies found. No evidence of tire failure was noted. The antiskid system was removed for functional tests. The control box and left and right control valves tested within specifications. The four wheel speed sensors met electrical resistance specifications. For units 1, 2, and 3, output voltages exceeded the minimum specified voltages for each listed frequency; unit 4 was frozen and could not be rotated, thus could not be tested. Sensors 1 and 2 exceeded the specified 15% maximum-to-minimum voltage variation limit; sensor 3 was within the limit; sensor 4 could not be tested.
Based on all evidence, it is likely that the airplane touched down on the water-contaminated runway beyond the touchdown zone, at a point with about 600 feet less remaining runway than the performance charts indicated required for wet conditions. Since reverted rubber hydroplaning typically follows an encounter with dynamic hydroplaning, the reverted rubber signatures on the No. 2 tire indicate that the airplane encountered dynamic hydroplaning shortly after touchdown. Left main gear wheel speed sensor anomalies allowed the left tires to progress to reverted rubber hydroplaning. This, along with post-accident testing, indicates that the antiskid system was not performing optimally and, in concert with hydroplaning conditions, significantly contributed to the lack of deceleration during braking attempts.