What happened
During an operation, a failure of an O-ring within the number 1 thrust reverser selector valve caused a hydraulic leak. In response to the leak, checklist procedures were followed, which involved deactivating the pumps for the number 1 hydraulic system. This action resulted in the loss of several systems, including the nose wheel steering. Due to the loss of steering, the aircraft required a tug to be towed to the gate following its landing.
To ensure the availability of electrical power and air-conditioning after engine shutdown, the captain decided to start the APU while still in flight. However, a gap between the APU intake door actuator rod cut-souts, combined with the specific location of the leak relative to the APU air intake, allowed hydraulic fluid or vapor to enter the APU system. Once the aircraft was on the ground and the ground-flight logic switch activated, the APU began supplying air to the air-conditioning system, distributing the contamination throughout the cabin. Upon detecting fumes, the captain immediately switched off the APU air supply, but the air-conditioning system had already been contaminated.
During the subsequent emergency evacuation, the R1 door slide failed to deploy.
The investigation
Investigators examined the hydraulic system and the APU air intake mechanism. Maintenance inspections of the R1 door revealed that the door was in a disarmed state, though no mechanical faults were found with the operating mechanism itself. The investigation also looked into the path of the hydraulic vapors through the aircraft's ventilation systems.
Findings
- A damaged O-ring in the left engine thrust reverser selector valve caused the initial hydraulic leak.
- Hydraulic fluid vapor entered the APU air intake through a gap in the APU intake door actuator rod cut-outs.
- The contamination of the cabin air occurred when the APU began supplying the air-conditioning system after the aircraft landed.
- The failure of the R1 door slide to deploy was likely due to the cabin crewmember disarming the door manually during the evacuation, potentially due to reverting to previous training or experience.