Jets A1 refuel in hot WX question

[Ejector]

How come when we go down to Hot areas and order full tanks the gauges (lbs) are always several hundred pounds less that what they are meant to be and when filling ay 10’c ??

[aidey_f]

Howdy.

For full tanks, you normally have high level sensors that close the inlet valves when they change from dry to wet. The difference in the loaded mass of fuel comes about because the density of the fuel decreases with increasing temperature.

[NutLoose]

As said above, the reason your gauges are marked in Lbs or Kg are that a given amount of fuel will weigh the same regardless of expansion or contraction due to temperature.... A gallon on a cold day will take up less area than a gallon on a hot day, but weight wise the gallon will remain the same therefore it is an accurate indication of the amount of fuel you actually have on board regardless of temperature.

[Clarence Oveur]

Eh, NutLoose.

A gallon is a unit of volume. The weight of a gallon will vary with temperature.

A pound is a unit of weight. The volume of a pound will vary with temperature.

But that was what you meant anyway. ;)

[Old Smokey]

Ejector,

The gauges, which compensate for fuel temperature, are telling you the truth.

Full tanks implies a fixed Volume, and if those tanks are full of hot, low density fuel, the total fuel weight / mass will be lower. Conversly, if the tanks are full of cold, high density fuel, the total fuel weight / mass will be higher.

Important to note that all engines, be they Jet, Turbo-Prop, or Piston, require fuel mass, not volume, for their operation, thus Fuel Quantity Indication as Weight or Mass is far more valuable than Volume.

Regards,

Old Smokey

[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?

[Swedish Steve]

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!)