Nobody offloads fuel. Any fuel offloaded (and it is not a regular procedure) is regarded as contaminated and not used for further flights.
Shorthaul aeroplanes frequently carry 'round-trip' fuel- you simply look at fuel required for the return flight, and do some quick sums to ensure that you depart first point with enough to leave at least that plus a buffer for the return flight- it saves bothering to refuel. As the cost is about 3%/hour of excess fuel carried for carrying it, it is a minor cost. Longhaul airlines use sophisticated cost matrixes to assess whether it is worth carrying fuel for say 6 hours if the destination enroute has expensive fuel. So whatever extra you carry, you would burn about 20% just carrying it, so unless the fuel was at least 25% more expensive that departure, not worth it. Often local taxes make it very worthwhile to do, which is why Robber Brown and other thieving politicians may be aching to use it as a means of raising more tax, but can't, because airlines will fuel up all they can abroad.
Other factors must be considered. The fuel will rapidly cool to about -20deg C. If you carry enough fuel for the return, after landing you will discover if the weather is not hot that it causes ice to form on the wing and you then need expensive de-icing, so the skill is deciding how much to carry where supplementing with warm fuel at destination, you will not create an icing problem on the ground. Longhaul flights don't usually have this problem as they uplift so much fuel anyway.
A few years back the fuel system was destroyed at Nairobi and Lagos. Flights had to 'tanker' as much as they could- up to Maximum Take Off Weight (as long as they would not exceed Maximum Landing weight at Nairobi or Lagos), and hang the cost of carrying it. London Heathrow had problems last year following the Buncefield refinery being destroyed, flights had to bring as much fuel as they could back to LHR.
Of extra fuel loaded, you will burn 3%/hour. If you required 6000kgs on top of standard reserves to be landed at destination, you would calculate 4%/hour on top, so for a 4 hour flight you would burn carrying it 4 x 240 = 1000 kgs, so you would load 7000 kgs.
Very common and regular procedure, I think all airlines probably do it when they can (and probably claim to be world leaders!). The nutty eco-Nazis are picking up on it though, and don't like it!
Last edited by Rainboe; 23rd October 2007 at 15:02.