What happened
On 20 September 2006, a Cessna 750 Citation X, registration G-CDCX, was operating a commercial passenger flight from Newcastle to Luton Airport. During the cruise phase of the flight, the crew observed a master caution light and a low fluid alert for Hydraulic System A. The flight crew identified a decreasing fluid level on the flightdeck display and subsequently determined that the Power Transfer Unit (PTU) had failed.
Following the non-normal checklist, the crew tripped the PTU circuit breaker. After declaring a 'PAN', the pilots recalculated their landing distance requirements and elected to proceed to Luton. The hydraulic failure had specific operational consequences: the left engine thrust reverser was disabled, and the landing gear had to be deployed via the emergency system. Additionally, the crew would have to rely on emergency braking and nosewheel steering systems for the arrival.
While the touchdown at Luton was otherwise unevently, the aircraft encountered difficulties during the deceleration phase. As the speed dropped below 70 knots, the nosewheel steering became ineffective. This caused the aircraft to drift toward the left edge of the runway, eventually coming to a halt with the nosewheel resting in the grass. There were no injuries to the two crew members, and the aircraft sustained no damage.
The investigation
Investigators examined the aircraft and identified two distinct defects within Hydraulic System A that contributed to the fluid loss. The first was a failure of a pressure hose connecting the PTU to the system. The second was a defective O-ring seal located within a union near the hydraulic manifold.
Detailed examination of the O-ring revealed mechanical damage to its outer edge, consistent with being 'pinched' during installation, along with evidence of erosion on the seal face. Regarding the pressure hose, investigators found that the thermal sheath had melted in several places, exposing the thermal layer. This damage was consistent with exposure to temperatures exceeding normal operating ranges. The investigation also looked into the steering shutoff valve, finding that the operating solenoid within the blocking valve was failing intermittently, though no such issues had been reported by the crew prior to the incident.