To quote an example; how much additional fuel is burnt to carry the fuel needed for a 20 mins hold at FL100 by a B777 operating a 3,500nm sector into LHR? And how much by a B737 on a 850nm sector into LHR? Does anyone know the answer?
Carriage of extra fuel is costed at 3%/hr of extra fuel carried
at start of flight. So your example- 2000 hrs extra x 3% x 8 hours=500 kgs fuel burnt carrying 2000 kgs extra initially. so you've used 1/4 of it just carrying it.
However- you want that fuel at the end of the flight, so of that fuel, you are burning 4%/hr during the flight to end up with that much
at the end of the flight. Not sure of 777 figures- (say 7000 kgs/hr? Guess- 747 is about 10,000kgs/hr). So for 7000/3 (ie 20 minutes) kgs remaining end of flight, 7000/3 (=2,333) x 4% x 8hrs= 750kgs, so you would need to load 2333+750=3100 kgs of fuel to end up with 2333 kgs for your 20 minutes holding. If you don't believe that, try working the other way reducing 3100 by 3%/hr for 8 hours.
Pretty solid figures established by experience. This is why that '20 minutes' at the end of the flight is not popular with accountants. Statistically- once in a while you can expect occasional minor fuel emergencies. However the record of aviation is just about right- how many airliners have dropped out of the sky without fuel due to current fuel rules?
A lot of the 20 minutes fuel can be made by unused contingency fuel plus anticipated savings from being able to exclude your diversion (ie commit to destination) assuming certain criteria are followed.
*411A- not going to get suckered in on that fishing lure you floated! I think 20 minutes holding
on every flight into the UK is bizarrely extravagant on a valuable resource. It is not flight planned, and rarely ever used. It is as safe to not carry it
as long as your decision making process is brought forward and you take a divert decision earlier. That is the key point- thousands of flights can carry less fuel, but the point must be accepted that occasionally one or two flights will divert
because they did not carry the extra fuel. Then you do a sum- is it cheaper to carry 20 minutes extra fuel for all flights, or accept that with less fuel, there are extra diversions (actually incredibly small in number). Remember, we're talking about probably a broad average of 750kgs per every 777 flight fuel burnt just to carry enough fuel to maybe delay or avoid the once in a blue moon diversion? The accountants tell us....don't carry unnecessary fuel- it's cheaper to accept a small number of diversions. That's good enough for me. Safety is not affected. The extra fuel just means you can hold longer- you can still have a crisis if you do not take action, you just have longer to play with before the crisis. It's a cost/benefit thing.
In my experience, extra fuel
still often does you no good. Through having round trip fuel last month, I was able to hold for nearly 2 hours waiting for fog to clear, at 2000kgs/hr. It still didn't work- the fog didn't, although I thought there was a good chance. I would have been better off diverting that much quicker. So what good did that extra 4000kgs achieve? Nothing.