Originally Posted by
novicef
Would I be right in explaining it like this? If a fuel tank takes X gals of fuel at a certain temperature. When the temperature of the fuel is warmer the volume of that parcel of fuel due to expansion is greater. Therefore if the tank is full there are less gals in it which in turn would lead to a lower mass being indicated on the gauges?
NO.
A gallon is a unit of volume. The fuel tank is a fixed volume. The capacity of the fuel tank is X galls(or litres). That is a fixed value.
When the fuel gets hotter it gets less dense so the weight of each gallon goes down. There are the same number of gallons( or litres), but less Lbs (or kgs)
i.e.
An aircrafts wings hold 10000 litres of fuel. This is fixed.
In the Winter this fuel has a S.G. of 0.81. You have 8100 kgs of fuel on board.
In the Med the sg is 0.79. You have 7900 kgs of fuel on board.
In both cases the wings are full, the sg had changed, so the weight has changed.
For info BA uses 0.800 for sg on all shorthaul aircraft all the time. On fuel loads under 20tons its not worth getting your calculator out (you can multiply by 8 in your head, right!)