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737 Fuel Anomaly

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Every wondered why the difference in the calculated fuel quantity with the gauges and FMS? Well read below....



"Regarding the FMS quantity, the anomaly you described can be explained by the fact that the fuel upstream of the fuel pumps, downstream of the spar valve is taken into account when calculating the remaining fuel on board, hence always reading a slightly higher value. This is substantiated by closely observing the fuel gauges after the engines have been shut down and the fuel in the lines returns to the tanks, which is seen by a slow increase in fuel tank quantity in the approximately 1 minute after shutdown. Actual arrival fuel read from the gauges should therefore never be done prior to this time, as fuel is still returning to the tanks."


Basically people we need to look at the difference between the fuel gauges which indicate what the fuel quantity is in the tanks and the FMS which calculates fuel on board.


Fuel gauges read the fuel tank quantity only, when engines are shut down, some fuel pumped towards the engine with engines running returnes to the tank, increasing it quantity after shutdown in the first ~60 seconds. This amount is expressed to rounded nearest 10.


FMS calculates the quantity on board, taking into account fuel in tanks and fuel pumped towards but not yet used by the engines, thus always indicates a higher value then fuel quantity gauges. This amount is rounded to nearest 100 as expressed in tonnes.


The two are fundamentally different, with the gauges always under reading actual total fuel on board until engines have been shut down and system returned to its rested state when fuel has returned to the tanks.


Best way to check fuel used is the using the fuel used indicator, expressed to nearest 10. Using fuel on board before departure reduced by fuel used gives most accurate reading to 10.


And as this accuracy is not really ever needed the FMS quantity should be used as it calculates exactly the fuel on board and the fuel used and the fuel not in tanks anymore but not yet used by the engines. It gives accuracy to 100, close enough on an aircraft burning ~37kg per minute, as closer accuracy to the 10 would change every 20 seconds!

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