When did the military/Air Force start measuring fuel in pounds instead of gallons?

Watching a training film for the B-47 from 1951, they mention (around 12:30) that the fuel is now measured in pounds instead of gallons on the instruments. Was that a new concept with jet aircraft? Did previous propeller aircraft instruments always measure gallons instead?

• I don't think it's just military that measures fuel in pounds - I believe everyone does it. (Except for those who measure in kilograms, of course.) – FreeMan Aug 17 '18 at 19:39

1 Answer

Good question! It makes sense for the conversion to have been done in 1951, as the film you saw states, because that is the year when JP-4 fuel was introduced. JP-4 has a lower freezing point (-60C) than earlier JP-x fuels, which were all derivatives of JP-1, first introduced in 1944. Lower freezing point means the aircraft can fly higher, and the changes in volume would have led to the change in measurement basis.

The reason is that the weight of fuel doesn't change with temperature or pressure, but the volume does. Rule of thumb is about one percent change in volume for every ten degrees centigrade up or down.

A little experiment to prove this would be to weigh a glass of water, cool it in the freezer for a few hours, and weigh it again.

If fuel quantity indicators on jet aircraft were based on volume they would indicate different levels depending on the aircraft's altitude and outside ambient temperature. That would not be very useful.

It does also make weight and balance calculations easier, otherwise the loadmaster would have to compensate for temperature when calculating center-of-gravity.

• While the information in this answer is useful, I believe the OP is asking about when as in what year / decade, and the question is tagged with "aircraft history" – kevin Aug 24 '18 at 15:54
• As @kevin says, while this is great information it answers the question of “why” not “when” – Notts90 supports Monica Aug 24 '18 at 16:57
• Just as a matter of information, weight & balance software does typically compensate for temperature when calculating the c.g. I ran a quick test putting 344,000 lb of fuel into an empty 747-400. The results at 6.43 lb/gal, 6.7, and 7.1 were a zfw c.g of 14.3% mac, 15.5%, and 16.9% respectively. The c.g. of the filled portion of a tank changes with volume, and since the fuel volume is temperature dependent. a tank c.g. is temp dependent, and hence the aircraft c.g. is temp dependent. The 6.43 and 7.1 values are the minimum and maximum fuel densities allowed for the 747-400. – Terry Aug 24 '18 at 19:31