Bob I am half way through the performance study for CPL, I am using your text and finding it helpful. There is however one constant issue I cant seem to resolve. In solving fuel requirements you use dividing by 1.15 to exclude the 15% variable reserve from the total fuel. Now this all makes sense until I realised that dividing by 1.15 doesnt actually subtract 15% from a number, it very nearly does but not quite. For example 300us gallons becomes 261 to 1 decimal place if you devide by 1.15 . If you take 15% from 300us gallons it actually becomes 255us gallons. Now obviously this is only a difference of 6 gallons but in the grand scheme of things is this important? I can not explain to you mathematically why you get a correct percentage when you add using your method but I do know when you subtract it is slightly incorrect.

As always, thanks for your help.

Ben Bird.

Sent from my iPad

Giday Ben

The variable reserve is 15% of the flight fuel. So when you have removed the fixed reserve and taxi fuel, you are left with 115% of the flight fuel. Let's use your example. If you subtracted the fixed reserve and the taxi fuel from the total fuel on board at take-off and ended up with 300 US gallons, that 300 gallons would represent 115% of the flight fuel. So the variable reserve is not 15% of 300 - it is 15% of the unknown flight fuel. You can find the flight fuel by dividing 300 by 1.15, that gives us 261 gallons.

Proof: The flight fuel of 261 gallons plus the variable reserve which is 15% of 261 [39 gallons]
and 261 + 39 = 300.

All the best with your study........

Bob