What happened
On September 28, 2010, a Bombardier CL600-2B19, operating as SkyWest Airlines flight 3074, landed at General Mitchell Airport (MKE) in Milwaukee, Wisconsin, with its left main landing gear (MLG) retracted. The flight had originated from Eppley Airfield (OMA) in Omaha, Nebraska.
During the approach to runway 07R, the crew selected the landing gear to the down position and received a "GEAR DISAGREE" message on the Engine Indicating Crew Alert System (EICAS), accompanied by an aural alert. The crew performed a go-around and attempted to troubleshoot the issue using the Quick Reference Handbook (QRH). During the troubleshooting process, the crew attempted to manually extend the gear by pulling the manual release T-handle, but the left MLG failed to extend and lock.
Due to critical fuel levels, the crew decided to proceed with the landing. The captain took control of the aircraft and used right aileron input to keep the left wing elevated for as long as possible during the landing roll. The aircraft eventually came to rest on the right MLG, the nose wheel, and the left wing. There were no injuries among the 39 passengers and 2 crew members.
The investigation
Investigators examined the aircraft N498CA and its landing gear system. Post-incident inspections revealed that the left MLG remained in the up and locked position within the wheel well. Maintenance personnel attempted to extend the gear using both the selector lever and the manual release handle, and the gear eventually extended only after an additional pull of the manual release handle.
Technical examinations of the hydraulic system revealed the following:
- The pressure filter contained significant metallic particles and discolored, black hydraulic fluid.
- The left MLG sidestay actuator contained small pieces of aluminum (approximately 1mm by 3mm) and other debris in its restrictor manifold.
- Fluid within the sidestay actuator cylinder failed particle count and water content tests.
- The runaround and bypass valve exhibited binding and sticking during operational tests. Furthermore, the force required to operate the bypass valve was measured at 60 pounds, exceeding the specified 40-pound limit.
While the left MLG uplock pin and latch showed wear, the measurements were within the permitted maintenance limits. Ground testing of the aircraft after the incident was unable to replicate the failure, as the gear extended normally during subsequent cycles.
Findings
- The left main landing gear failed to extend through either the normal or manual extension systems.
- While no single component or level of contamination was identified as the sole cause, a combination of factors—including hydraulic system pressure, contamination, bypass valve operation, and component wear—likely contributed to the malfunction.
- The interaction of these factors, potentially influenced by environmental conditions like temperature and humidity, resulted in the gear remaining in the uplock position.