View Full Version : Planes and fuel
Listening to expert commentary on the appalling Spanish plane crash today I was surprised to discover that planes on short flights take on fuel not just for the outward journey but for the return journey as well so they don't have to refuel before coming back. I suppose it's obvious from the quick turn round by, say, Easyjet at Inverness that they appear not to refuel but is this not
a) dangerous
b) very wasteful of energy.
Since we are told that cars use more fuel if heavily laden, does not the same apply to aeroplanes? For the sake of a slightly longer turnround time, which I'm sure the crew would appreciate on shorthaul flights, and given that flying is apparently so wasteful of fuel and generally bad for the planet, would it not be both safer and more economical if they only took on enough fuel for the outward journey? Or am I missing something?
Surely if they have to divert having enough fuel to get somewhere else might be useful too!
They would need to carry more than the minimum but surely not enough for two journeys and they could never guarantee to have spare for a diversion if, for instance, they were near the end of the return flight.
There's probably a number of factors here.
Maybe the cost of fuel in invreness, where it would be tankered in, is more expensive than in London where the major airports have pipelines direct to them?
Many planes are flying virtually 24 hours a day. Maybe the extra time needed for multiple refuelings would cut into flight time?
Whatever the reasons, I'm sure they're financial!
One thing that you have to consider is the carbon footprint of an aerolane just sitting around at an airport and the taxiing around for the purpose of refuelling. I believe that this contributes significantly to their carbon emissions but I've no idea how the alternative would compare.
George Brims
22-Aug-08, 21:00
There is a minimum amount of fuel mandated for each type of plane, for each journey. This includes considerations of how far it is to alternate landing sites in an emergency, plus a certain amount of idling time (circling around the destination waiting for a runway to be cleared after some incident). If you add all that up for the return leg, it means the extra bit that is for the outbound leg is probably not a big fraction of the whole. It's quite possible also that loading the fuel for both legs is the only way to do things if there aren't sufficient facilities at the other end. Commercial considerations like filling up on fuel that is cheaper in one place than another might also come into it.
The fuel plan goes something like this.
Trip Fuel
Alternate Fuel
Taxi APU and start up.
Holding fuel (30 mins)
Contingency (5% of trip fuel)
Plus extra at the captains discretion, ie weather, traffic, airport etc.
Tanking fuel, ie carrying fuel for the next trip is very common practice and usually based on price. On short hops the trip fuel can be quite a small percentage of the total carried. It isn't a dangerous practice at all, not having enough is though.
True, the fuel burn is greater the heavier the aircraft is, however the bean counters have a say in this. The crews would always appreciate an extra bit more time on the turnaround, but the turn around times got cut short with the advent of the low cost carriers.
One interesting point about tanking, is that it can go wrong on longer trips. Flying at high altitude cold soaks the fuel to very low temperatures -40 is quite common. When the aircraft lands in warm humid air, ice quickly forms on the wings, even in Egypt. The solution is not to carry too much, and/or get hot fuel into the tanks immediately on landing. Its difficult to get a de-icing rig in Sharm El Sheikh! and you cant go with ice on the wings.
George Brims
22-Aug-08, 21:35
I was reading a "message from the chairman" in an American Airlines flight magazine, and he mentioned they had discovered several ways to save on the amount of fuel carried. One was to use tractors to push out planes instead of them using the plane's engines (not sure where that was happening as most airports I've been through always use tractors). But their biggest surprise was how much they could save by making sure the pilots stuck to the mandated amount. It turned out that the "pilot's discretion" part was getting to be very high, to the point that some pilots would just tell the fuel crew to "fill 'er up". So they were expending a lot of extra fuel just carrying a lot of extra fuel around!
I was reading a "message from the chairman" in an American Airlines flight magazine, and he mentioned they had discovered several ways to save on the amount of fuel carried. One was to use tractors to push out planes instead of them using the plane's engines (not sure where that was happening as most airports I've been through always use tractors). But their biggest surprise was how much they could save by making sure the pilots stuck to the mandated amount. It turned out that the "pilot's discretion" part was getting to be very high, to the point that some pilots would just tell the fuel crew to "fill 'er up". So they were expending a lot of extra fuel just carrying a lot of extra fuel around!
You are correct, aircraft are pushed back from the stand, jets may not use reverse thrust on the ramp areas. I think what some companies have looked at, is to tow the aircraft to the holding point for the runway. Theoretically it could save fuel, but not that much, as engines and other systems need to warm up to operating temperature. Normally this happens during taxiing. Much has been made of the extra fuel pilots put on board, what the company execs don't explain is that any delay at destination means either an imediate divert, OR, burn reserve fuel to hold and have little or no fuel for an alternate airport. True, there are some guys who put on the extra with little thought as to why. If its mid week, and the weather is blue skies everywhere, then there is little reason to put on too much extra.
I thought it probably wasn't as simple as it seemed on the surface but thank you all for those interesting answers. I had assumed a plane would need to carry more than the minimum required for one trip but had not thought of all the other complications, particularly temperature. The commentators' comments about all the pilots' routines were also not something I knew much about before, like there is a point when the plane has to take off no matter what but up to then the pilot can decide. Terrifying responsibility.
.......The commentators' comments about all the pilots' routines were also not something I knew much about before, like there is a point when the plane has to take off no matter what but up to then the pilot can decide. Terrifying responsibility.
The final point of go/no go is called V1. Its a speed calculated for a given runway where the aircraft can be safely brought to a stop. Beyond that point a stop usually ends in disaster. The format usually goes along these lines: Up to 80kts stop for any malfunction, between 80kts and V1 stop only for ANY fire, engine severe damage or failure or control malfunction. V1 or above, go regardless. BTW a stop from V1 on a big jet on a wee runway is violent.
The recent Madrid disaster is very sad, and many, not least the relatives who lost their loved ones, need some explanation as to what went wrong. Up to now, all we have seen on the telly is just speculation. I hope the Spanish authorities will have enough information to put together an initial report soon.
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