Bombardier Inc CL-600-2B1 N857AS incident near Atlanta, Georgia

Probable cause

The failure of the left main landing gear to extend due to high levels of particulate contamination in the hydraulic fluid.

NTSB analysis

While configuring the airplane for landing, the left main landing gear light indicated that it was not down and locked. The flight crew accomplished the landing gear abnormal procedures, including recycling the landing gear once. The left main gear continued to indicate unsafe, and the captain landed the airplane. After touching down on the right main and the nose gear, the captain used right aileron to keep the left wing up as long as possible. The left wing eventually dropped to the runway and the airplane came to a stop at the runway’s left edge. During recovery of the airplane, the left main gear was partially extended, and no physical obstructions were observed that would prevent the gear from extending. Loosening a hydraulic line to the left main gear actuator allowed the gear to fall into the extended and locked position. The airplane was examined and the landing gear was cycled numerous times using the normal and alternate extension methods. The examination failed to duplicate the incident conditions; the gear extended and retracted normally without hesitation or binding. All major components of the main landing gear system, including the left main landing gear, left gear door, hydraulic pumps, valves and actuators were examined at the manufacturers’ facilities. Although some minor anomalies were noted, no evidence was found to explain the failure of the gear to extend. Analysis of fluid from the numbers 2 and 3 hydraulic systems revealed that the fluid exceeded the maximum particle contamination limits. The system 3 sample also contained metallic particles. Hydraulic system 3 powers gear extension and retraction. After the incident, the operator enhanced its maintenance procedures for testing hydraulic fluid to include contamination. The operator also added verbiage to its pilot procedures to further explain that, in certain cases, it is appropriate to make several attempts at normal gear extension and provided more explanation and guidance regarding the use of the back-up (freefall) system.

Text from the NTSB. Open the NTSB docket for ERA09IA338 for the full record.