What happened
During the retraction of the landing gear, a flex hose on the left main landing gear downlock actuator ruptured while under pressure. This rupture caused the hydraulic fluid in the left system to deplete, leading to low pressure within that system. Following the crew's decision to shut down the left system, the right hydraulic system began to overheat. This overheating occurred because the power transfer unit (PTU) pressure switch remained stuck in the open position, causing the associated pump to run continuously without fluid in the left system. After the crew deactivated the right engine driven pump, the right system was maintained only by the electric motor driven pump. Because the electric pump's output is lower than the engine driven pump, a pressure warning was triggered as the pump could not meet the simultaneous demands of the system and the PT-U motor.
The investigation
Investigators examined the fractured hose but could not determine the specific cause of the rupture because extensive corrosion on the fracture surfaces prevented a detailed analysis. Notably, the hose showed no external physical damage that would have predisposed it to failure. The investigation also focused on the PTU pressure switch, which was found to be stuck. It was determined that a non-mandatory 2010 service bulletin intended to modify these susceptible switches had not been implemented. The aircraft had been leased to the operator after this bulletin was published, and the subsequent maintenance audit only focused on mandatory directives and airworthiness instructions. Consequently, the operator was unaware of the recommended modification until after the incident.
Findings
- The rupture of the left main landing gear downlock actuator retract flex hose was the primary trigger for the hydraulic depletion.
- The PTU pressure switch likely failed due to contamination or corrosion, a condition that the 2010 service bulletin was designed to prevent.
- The operator's maintenance audit process did not include non-mandatory service bulletins, leaving them unaware of the recommended switch modification.
- The investigation found no evidence that a separate hose failure on the right side of the aircraft, which occurred five days prior, was related to this event, as the hoses had significantly different flight cycle counts.