Hydraulic Case-Drain Leak and Inoperative Crossfeed Valve
On a 777-300ER at the gate, the left hydraulic reservoir indicates 68% and the EICAS shows an advisory. A drip from the left engine-driven pump case drain line is measured at 12 drops per minute. The left circuit powers the left aileron and elevator; the center and right circuits are normal. The operator's MEL permits one hydraulic system to be 20% below nominal if the leak is classified as 'slow' and no further loss occurs in 15 minutes, but the AMM defines a slow leak as less than 10 drops per minute. The flight is 7 hours with 300 passengers. Separately, the left main fuel tank is 2.5 t lower than the right, and the center tank contains 12 t; the crossfeed valve is inoperative under a deferred MEL item. The dispatcher wants to release the aircraft. The maintenance controller must assess.
1. Classify the hydraulic leak and determine whether the reservoir quantity meets the MEL. Show the arithmetic.
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The measured leak is 12 drops per minute. The AMM defines a slow leak as less than 10 drops per minute, so this leak is above the slow-leak threshold; it is an active leak. The reservoir is at 68%, which is 32% below nominal. The MEL permits only 20% below nominal. Both conditions fail. For context, 12 drops/min over 7 hours is 12 x 60 x 7 = 5,040 drops; at about 0.05 mL per drop that is roughly 250 mL, but that calculation is not a release argument because the leak rate can increase with temperature and pressure, and the reservoir is already below the allowed quantity. The correct decision is no dispatch until the leak is repaired, the system is serviced, and a pressure check shows a stable quantity.
2. How does the inoperative crossfeed valve affect the fuel balance and the dispatch decision?
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With the crossfeed valve inoperative, the fuel manifold cannot be reconfigured to feed both engines from one side. The left main tank is already 2.5 t lower than the right, and the center tank holds 12 t. While the center tank is feeding, it can supply both engines and mask the imbalance, but once the center tank is empty each engine will draw from its own side. The left engine will then reduce the left main further, and the aircraft may exceed the lateral imbalance limit before the flight ends. The MEL for the crossfeed valve may allow dispatch only if the fuel balance can be maintained within limits for the planned route. With a 2.5 t pre-existing split and no crossfeed, I would not release without a fuel plan that keeps the imbalance within limits, or without repairing the valve. Combined with the hydraulic leak, the aircraft is not dispatchable.
3. If the hydraulic leak is repaired but the crossfeed valve remains deferred, what would you require before release?
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After the leak repair, I would pressure-test the left hydraulic system, service the reservoir to the correct level, and check for external leakage and stable quantity. For the crossfeed deferral, I would review the MEL conditions and the fuel imbalance limit, calculate the fuel load and expected burn, and balance the aircraft on the ground before departure. The flight plan should keep the center tank feeding as long as practical and include a procedure to monitor main-tank quantities. If the imbalance cannot be kept within limits without crossfeed, the valve must be repaired before dispatch. I would also record the deferral, the fuel calculation, and the crew briefing. The hydraulic repair alone does not clear the fuel-system limitation.