BmPilot21
You have raised a very good question to which the answer is very clear in the JAR-OPS Fuel Policy.
Most of the replies to your post are close to the answer but no one has hit the nail on the head..so to speak..
Total fuel required =
A. Taxi Fuel
B. Trip Fuel...(Starts at T/OFF) JAR Definition
C. Contingency Fuel
D. Alternate Fuel
E. Final Reserve Fuel
F. Additional fuel for operational reasons.
So you can see that the JAR-OPS regulations clearly make the distinction between TAXI FUEL and TRIP FUEL. Since the CONTINGENCY fuel is based on the Trip fuel you must have this at the commencement of the take off as this is,again by JAR definition,when you start using your trip fuel.
So lets now put it all in the real world. I get to work and collect my Ploggs for the trip. After considering all operational requirements lets say I agree with the fuel figures given in A to E above but decide to take extra fuel in accordance with F.
Everyone is happy and we taxi for departure..Has our flight started? For the purpose of logging flight time it has however by JAR definition we have not started using our trip fuel.
We are now delayed and join the que waiting for departure and I realize that we are in danger of using all our Taxi fuel allowance. The only fuel that I am allowed to eat into is the Fuel I have allowed at F above. That is the additional fuel I took on for operational reasons.
Our company operations manual states that we will use the 5% of trip fuel option in calculating the contingency fuel required however if your ops manual allows you could replan the contingency fuel based on 3% of trip fuel and nominate an enroute alternate which may give you some breathing space.
To sum it all up then you must commence the take off with your contingency fuel in tact...
The company operations manual is your bible and obviously you must conduct your operations in accordance with this document. However if you are asking the question in regard to the JAR - OPS regulations then I feel confident I have given you the right answer. Room for thought