What happened
On July 6, 1999, at approximately 16:50 local time, an ATR 42 - 300 operating a flight from Münster/Osnabrück was landing on runway 06 at Dortmund Airport. The aircraft, carrying 27 passengers and three crew members, landed within the touchdown zone under visual meteorological conditions with a light 8-knot wind.
As the aircraft decelerated, the second pilot (operating from the right seat under supervision) applied the brakes. Approximately 350 meters after the touchdown zone, the aircraft began to veer sharply to the left. The excursion continued until the left main landing gear left the paved surface of the runway, approximately 44 meters before the halfway marker. The pilot in command intervened using nosewheel steering to regain directional control, eventually bringing the aircraft back onto the runway centerline about 85 meters past the halfway marker. The aircraft then taxied to taxiway C, where passengers were disembarked. There were no injuries and no damage to the aircraft.
The investigation
The BFU investigation examined track markings, flight recorder data, and conducted technical testing. Analysis of the skid marks revealed that the braking trace from the left main gear was significantly heavier than the right, indicating asymmetric braking. Flight recorder data confirmed the aircraft began rotating left at 95 knots, with the crew applying maximum rudder deflection to the right to compensate.
Technical inspections were performed both on the ground and during a subsequent maintenance test flight. During the ground test, it was discovered that when braking was applied from the right-hand seat, the aircraft continued to pull to the left. A subsequent test flight replicated this behavior. Detailed technical analysis revealed that when the brakes were operated from the right seat, the braking pressure on the right main gear was approximately 200 PSI lower than on the left side.
Findings
- The primary cause of the excursion was an imbalance in braking pressure between the left and right main landing gear.
- The pressure differential occurred specifically when the brakes were operated from the right-hand seat.
- The exact cause of the reduced pressure on the right side could not be definitively determined, though air or debris within the braking system were considered possible factors.
- The pilot in command did not initially report a technical malfunction because, from the left-hand seat, the braking performance appeared normal.