Endurance is the fuel available to be used for flight enroute from place to place and excludes all other fuel required to be there for other purposes eg holding, final reserve, fixed reserve, variable reserve (VR), alternates, approaches and the like.
Start with total fuel on board, subtract all the fixed amounts such as final reserve, fixed reserve, holding (if required at the landing point) and alternate fuel if an alternate is required for the intended landing point and any other fixed amounts that the company operations manual or local rules require. This leaves an amount of fuel available to be used for flight from A-->B (or A-->A in this case) + any VR the rules might specify. If the VR required is 10% of flight fuel then this remaining fuel must be 110%** of flight fuel. Divide the quantity by 110% to find flight fuel.
Divide flight fuel by fuel consumption to find endurance.
**Don't make the mistake of thinking that flight fuel is 90% of this fuel. Imagine you have 100kg (litres/gallons/gills/whatever) of fuel and a requirement for a 10% VR. If you take 90% of this fuel (90kg for the hard of thinking or alcohol/exam study synergy believer) and call it flight fuel then the VR required will be 9kg, giving you 99kg to use. What about the remaing 1kg? It's there but not accounted for because 90% of a total is not the same as taking an amount and adding an extra 10% to find the total. So, the correct way in this example is to divide the total by 110%, not multiply it by 90%.
Last edited by Tinstaafl; 16th Jul 2009 at 03:53.