Anthony
With all due repect to our Engineering colleagues Inflight Contingency fuel is a pilot/planning/despatch issue so you probably need to ask this question somewhere else, probably the "Tech Log" section.
However I'll give this as a starter, and emphasise that this is the methodology my operator uses, other States/Companies may work differently:
As I guess you may already know "Trip fuel" is the fuel needed to fly the flight on an ideal day (ideal cruising altitudes, perfect weather forecast, no holding, etc) and then an amount of "Contingency fuel" is added to cater for extra fuel burnt due to unforseen circumstances, e.g. stronger than forecast headwinds, holding, etc, etc.
Once upon a time the Contingency fuel figure was simply an extra X% of Trip fuel, usually 5%, sometimes 3% depending on circumstances.
Nowadays it's more usual for the airline to monitor the actual fuel used on a particular flight on a particular route over several months, look at fuel burnt in excess of trip fuel and then use that data to calculate a "statistical contingency fuel". The 99% contingency fuel figure means 99% of flights have used no more than than that amount of extra fuel ( so it gives 99% coverage)....the 95% contingency figure is arrived at in the same manner.
As for "holding fuel"...for planning purposes, in very simple terms, it's the bottom line, absolute minimum fuel you should land with (anywhere) and it's usually calculated as being the amount of fuel required to hold at the alternate for 30 min.......
Last edited by wiggy; 9th Mar 2009 at 20:42.
Reason: reworded for clarity...?