AAIB Bulletin: 8/2007

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0 © Crown copyrght 2007 AAIB Bulletin: 8/2007 G-CDCX EW/C2006/09/20 INCIDENT Aircraft Type and Registration: Cessna 750 Citation X, G-CDCX No & Type of Engines: 2 Rolls-Royce AE 007C turbofan engnes Year of Manufacture: 2002 Date & Time (UTC): 20 September 2006 at 1740 hrs Location: Luton Arport, Bedfordshre Type of Flight: Commercial Air Transport (Passenger) Persons on Board: Crew - 2 Passengers - None Injuries: Crew - None Passengers - N/A Nature of Damage: None Commander’s Licence: Ar Transport Plot’s Lcence Commander’s Age: N/K Commander’s Flying Experience: 8,500 hours (of whch 00 were on type) Last 90 days - 20 hours Last 28 days - 0 hours Information Source: Aircraft Accident Report Form submitted and AAIB investigation of failed components Synopsis As the aircraft reached its cruising altitude, the fluid contents of Hydraulic System A were lost. The crew recalculated the dstance requred for a landng wth ths falure, and decded to contnue to ther planned destnaton, Luton Arport. As the arcraft slowed down after touchdown, nosewheel steering became ineffective and the arcraft drfted to the left of the runway. The aircraft came to a halt with its nosewheel in the grass at the edge of the paved surface. The aircraft was undamaged. Two defects were identified in the hydraulic system. One of these, a pressure hose falure, had probably been damaged on a previous flight. History of the flight As the aircraft reached cruising altitude, on a flight from Newcastle to Luton Airport, the master caution light illuminated, together with a ‘LOW FLUID’ cauton for Hydraulic System A. The crew observed the hydraulic fluid level decreasing on the flightdeck display and, shortly afterwards, the A system Power Transfer Unit (PTU) failed. They completed the non-normal checklist for this problem, which included tripping the PTU circuit breaker. After declarng a ‘PAN’, the crew recalculated their landing distance requirements and decided to continue to Luton. The loss of Hydraulic System A dsabled the left engne thrust reverser and requred the landing gear to be deployed using the emergency system. It also meant that emergency braking and nosewheel steering systems would have to be used on landing.  © Crown copyrght 2007 AAIB Bulletin: 8/2007 G-CDCX EW/C2006/09/20 The touchdown was uneventful and, as the arcraft decelerated through 70 kt, nosewheel steerng was required to maintain the runway heading. After it had slowed further, nosewheel steerng proved neffectve and the arcraft began to drft to the left edge of the runway. It came to rest with the nosewheel on the grass to the side of the runway but with both main wheel on the paved surface. The crew were unnjured and the aircraft was undamaged. The aircraft was recovered from the runway and towed to a maintenance hangar for investigation and rectification. Hydraulic system description The Cessna 750 s equpped wth two ndependent hydraulic systems; each is powered by an engine driven pump (EDP). Power for System A is provided by the pump on the left engine and for System B by the pump on the right engine. Both systems provide power for the primary flight controls. System A provides the only power for the landng gear actuators, wheel brakes, nosewheel steering system, the inboard speed brakes and the outboard roll spolers. In order to provde a secondary power supply in the event of an EDP failure, Hydraulic System A is fitted with a PTU, which is essentially a hydraulic pump driven by pressure from System B. The PTU operates automatically when a drop in system pressure is detected and is fitted with a flow limiter. This prevents an excessive drop in pressure in System B in the event of a fluid leak in System A. In addition, an emergency electric pump pressurises System A should both the left engine EDP and the PTU fail. In the event of a complete failure of System A, landing gear deployment and emergency braking is provided by a pneumatic source, and nosewheel steering by a hydraulic accumulator. Two valves are fitted in the nosewheel steering system, which allow the system to become pressurised; a blocking (or sequence) valve and a steering shutoff valve. The blocking valve remains closed until it receives a signal from either of the main landng gear squat swtches, and the steerng shut off valve remains closed until the nose landing gear squat swtch s actvated. Should ether of these two valves fail to open, the nosewheel steering system remains noperatve. Aircraft examination Initial examination of the aircraft revealed that two failures were present in Hydraulic System A which had led to the loss of its hydraulic fluid. A pressure hose connecting the PTU to the system had failed, and a leak was present between a ppe unon and the hydraulc manifold. After the union had been disassembled, an O-rng seal wthn the unon was found to be defectve. Following replacement of the affected parts, tests were carried out on the nosewheel steering system and landng gear squat swtches. These revealed that the operatng solenod wthn the blocker valve was operating intermittently; however, no abnormalities had been reported during routine testing of the system, or by the flight crew, prior to this incident. The valve was removed and, together with the O-ring and the failed pipe, dispatched to the AAIB for detailed examination. Examination of the O-ring revealed signs of mechanical damage to its outer edge, which were indicative of it havng been ‘pnched’ durng nstallaton. There was also evdence of eroson of the edge of the seal face, which appeared to have been produced by a mechanical process; no evidence of any chemical erosion was identified during this examination. It was not possible to determine when the damage to the seal occurred. Footnote  The avalable evdence ndcated that falures of ths valve are