Fuel Imbalance and Quantity Indication Disagreement
A turboprop is in cruise at FL200 with a total fuel load of 3,200 lb. The left tank quantity indicator reads 1,450 lb and the right reads 1,300 lb, a 150 lb imbalance that is growing at about 20 lb per hour. The fuel flow indicators show both engines consuming 620 lb/hr each. The crossfeed valve is closed, and each engine feeds from its own tank. The crew suspects the right quantity indicator is under-reading because the right engine has not shown any anomaly, but the right tank low-quantity caution has not illuminated. The aircraft's fuel imbalance limit is 200 lb, and the flight management system computes total fuel from the quantity indicators. The destination is 90 minutes away, and the alternate is 20 minutes beyond that.
1. Assess whether the imbalance is real and describe how the crew can confirm it without relying on the quantity indicators alone.
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The quantity indicators are the primary evidence, but they can drift. The crew can cross-check by comparing total fuel from the indicators with total fuel computed from the flight management system's fuel flow integration, and by comparing each engine's fuel flow with the expected burn. If the right engine's flow matches the left and the aircraft is not yawing or requiring rudder trim, a real 150 lb imbalance is plausible but not yet critical. The growing rate of 20 lb/hr is small and could be indicator drift rather than actual transfer. The crew should open the crossfeed valve briefly to equalise the tanks and observe whether the indicated imbalance changes, which would confirm a real imbalance. They should also check the fuel temperature and any recent refuelling records, since a quantity indicator fault is more likely than a genuine asymmetric burn when both engines show identical flow.
2. Decide whether to continue to destination or divert, and state the fuel management actions required.
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With 90 minutes to destination and a 200 lb imbalance limit, the crew has time to manage the imbalance before it becomes limiting. The correct action is to open the crossfeed valve and feed both engines from the fuller tank until the imbalance is reduced to below 50 lb, then return to normal feeding. This also tests whether the imbalance is real. If the imbalance continues to grow despite crossfeed, the crew should suspect a transfer or indication fault and treat the fuel total as unreliable, which means diverting to the nearer alternate. The decision should be based on the worst-case fuel estimate, not the optimistic one, because the flight management system total depends on the same indicators. Continuing to destination is acceptable only if the imbalance is stable and the worst-case fuel at destination exceeds the reserve plus 20 minutes.