History of the Flight
On 20 September 2006, a Cessna 750 Citation X, registration G-CDCX, was operating a commercial passenger flight from Newcastle to Luton Airport. While the aircraft reached its cruising altitude, the master caution light illuminated along with a low fluid caution for Hydraulic System A. The crew observed the hydraulic fluid level decreasing on the flightdeck display. Shortly afterward, the System A Power Transfer Unit (PTU) failed. The crew completed the non-normal checklist, which included tripping the PTU circuit breaker, and declared a PAN. They recalculated landing distance requirements and decided to continue to Luton.
The loss of Hydraulic System A disabled the left engine thrust reverser and required the landing gear to be deployed using the emergency system. Emergency braking and nosewheel steering systems would be necessary after touchdown. The landing was uneventful, but as the aircraft decelerated through 70 kt, nosewheel steering was required to maintain runway heading. After slowing further, nosewheel steering proved ineffective, and the aircraft drifted left. It came to rest with the nosewheel on the grass at the edge of the runway, while both main wheels remained on the paved surface. The two crew members were uninjured, and the aircraft was undamaged.
Hydraulic System Description
The Cessna 750 has two independent hydraulic systems, each powered by an engine-driven pump. System A, powered by the left engine, provides power for the landing gear actuators, wheel brakes, nosewheel steering, inboard speed brakes, and outboard roll spoilers. System A also includes a PTU that automatically activates when system pressure drops, drawing pressure from System B. An emergency electric pump pressurizes System A if both the left engine pump and PTU fail.
In the event of a complete System A failure, landing gear deployment and emergency braking are provided pneumatically, while nosewheel steering is powered by a hydraulic accumulator. Two valves in the nosewheel steering system—a blocking valve and a steering shutoff valve—must open to pressurize the system. The blocking valve opens when receiving a signal from either main landing gear squat switch; the steering shutoff valve opens upon activation of the nose landing gear squat switch. Failure of either valve to open leaves the nosewheel steering inoperative.
Examination
Initial examination revealed two failures in Hydraulic System A that led to fluid loss. A pressure hose connecting the PTU to the system had failed, and a leak was present between a pipe union and the hydraulic manifold. The union contained a damaged O-ring seal, which showed signs of having been pinched during installation. Erosion of the seal face appeared mechanical, with no chemical erosion detected. It was not possible to determine when the damage occurred.
The failed hydraulic hose was surrounded by a woven thermal sheath and an outer fiber protective layer. The outer sheath had melted in five places, and in one area near the hose end fitting, the thermal layer had blown outward. The damage was consistent with exposure to abnormally high temperatures; no other nearby components showed thermal distress. The aircraft's maintenance organization reported that, prior to this event, Hydraulic System A had suffered an unrelated leak during the descent phase of a previous flight. During that event, the crew had not tripped the PTU circuit breaker before landing, so the PTU ran for an extended period. Operating the PTU with low fluid levels elevated the temperature of the remaining fluid and the PTU itself.
Examination of the nosewheel steering system and landing gear squat switches revealed that the solenoid within the blocker valve was operating intermittently. No abnormalities had been reported during routine testing or by the flight crew before this incident. The valve was removed and, along with the O-ring and failed hose, sent to the AAIB for detailed examination.
Conclusions
The failure of the emergency nosewheel steering was the result of an intermittent failure of the steering blocker valve solenoid. The loss of hydraulic fluid from System A was caused by the failure of the pressure hose connected to the PTU and a damaged O-ring seal in a union associated with the hydraulic manifold. The O-ring failure alone would likely have caused some fluid loss but was considered insufficient to produce the rapid loss witnessed. The thermal damage to the PTU pressure hose indicated exposure to temperatures beyond its normal operating range, which would have affected its ability to withstand loads. It is possible that the heat damage occurred during the incident flight, but it was considered more likely that prolonged PTU operation during the previous fluid leak event caused thermal distress to the hose, which went undetected and probably led to its failure on this flight. No other failures of this nature have been reported, and no safety recommendations were considered necessary by the AAIB.