Runway length and range?

[Lightening_strikke]

Can anyone explain why shorter runways reduce the range on aircraft.

If the aircraft is still able to take off with full fuel/pax on a short runway it shouldn't affect the range very much, should it?

[Keith.Williams.]

If the aircraft is still able to take off with full fuel/pax on a short runway it shouldn't affect the range very much, should it?

That is true. But if the runway is too short to take off with full fuel and all of the pax, then we need to carry a reduced fuel load (or really annoy some of the pax by leaving them behind). This reduced fuel load will reduce our range.

We could also argue that short runways will prevent us from doing reduced thrust (assumed temperature) take-offs and this will increase the fuel used in getting off the ground. This might reduce range by a small amount.

But the problems really begin when the runway is so short that we are obliged to carry a reduced fuel load.

[Denti]

Keith, nicely put, however reduced thrust take offs do not save fuel, just a lot of maintenance cost.

[Lightening_strikke]

Thanks that explains it, I caught a conversation about this and couldn't work out what the issue was.

[TopBunk]

As an example, MEX-LHR on a B747-400 is not a really long flight - just about 10 hours (the B747-400 has a range of about 14+ hours with max payload).

However, even though the usual departure runway of 05R has 3985 metres available length for takeoff, it is a very limiting runway, with special techniques required to maximise payload available because of the altitude (about 7500ft) and temperature (often about 23deg C) at the time of departure.

So, rather than max structural take of weight of ~397tonnes being available, it is often nearer (from recollection, without the books in front of me), nearer 340 tonnes RTOW, a big penalty.

The same applies at Johannesburg at 5500ft altitude with 4400m runway length @ 28deg C, again with a ~10:20 hr flight time. Also at Cape Town, although it is at sea level but only with a 3200 metre runway and higher temperatures.

[Keith.Williams.]

however reduced thrust take offs do not save fuel, just a lot of maintenance cost.

It's certainly true that saving fuel isn't the reason for using reduced thrust take-offs.

But using reduced thrust will reduce fuel flow during take-off (although admitedly not by very much).

And when airspeed is very low, a large proportion of the energy that is released from the fuel is wasted in producing useless jet wash. Propulsive efficiency is zero until we let the brakes off, and it is never very high at any time during the take-off run. So using anything beyond the absolute minimum fuel flow during the take-off run is wasteful.

But as I said in my original post, this will have only a small impact on range.

[Denti]

Well, if you isolate the take-off you probably use a bit less fuel, however if you take into consideration the whole flight you use a bit more as time to cruise altitude (optimum cruise) is longer. The amount isn't big (around 35 to 55 kg per flight for a B738) and certainly does not offset lower maintenance costs.

And crazy as it sounds, there are really people whos job it is to calculate stuff like that, weird way to make a living.