Altitude and fuel burn

[KLOS]

Apologies if this qualifies for the dumbest question award:sad: but here goes.......

When flying the Atlantic on the B777, I have heard the Captain say that we shall be increasing our flight level when we have burnt off some fuel.

What is the connection between going from an initial cruise height to a higher one and fuel weight? I see the obvious connection between weight and getting airborne but don't get the connection from say level 350-390.

Please feel free to award the dunce cap-:(

Thanks

[DaveReidUK]

There are no dumb questions, only dumb answers. :O

Put simply, a wing produces less lift at higher flight levels, because the air is less dense.

So, in your example, your 777 that's heavy with fuel has enough lift to maintain FL350, but not to climb higher.

Once it has burnt off fuel, and is lighter, it can maintain a higher FL.

[Georgeablelovehowindia]

Put simply, the engines work most efficiently at a certain number of high revs per minute. If, in your example, you remained at FL350, as the mass of the aircraft burned off, to maintain your cleared Mach number, you would have to reduce power into a relatively inefficient regime. In order to get the engines back into their most efficient revs per minute, you request climb clearance into thinner air.

There are other factors to consider, of course. There's no sense climbing into a greatly increased headwind component which would cancel out the benefit. That's where the job still calls for a bit of the cerebrals in making the decision.
:)

[DaveReidUK]

Put simply, the engines work most efficiently at a certain number of high revs per minute. If, in your example, you remained at FL350, as the mass of the aircraft burned off, to maintain your cleared Mach number, you would have to reduce power into a relatively inefficient regime. In order to get the engines back into their most efficient revs per minute, you request climb clearance into thinner air.

Good answer, though not to the question the OP asked.

Still, now he knows both why you are able to climb higher as you burn off fuel and also why you would want to. :O

[BBK]

Hi KLOS

On a B744 (GE) engines the fuel burn is approximately 10 tonnes per hour in the cruise and the optimum flight level, ignoring wind effects, increases at about 700 feet per hour. On a typical day, say westbound, you might start at say FL330 as you enter the oceanic area west of Ireland and exit the ocean over Newfoundland at FL350...if ATC will give the level!

Hope that helps. :)

regards

BBK

[Lightning Mate]

Put simply, a wing produces less lift at higher flight levels, because the air
is less dense.

After 26 years flying, that's a new one on me. :eek:

[KLOS]

Dave/George/BBK,

Now even I get it :D Many thanks for the info and for taking the time to reply :ok:

I am travellng to TPA in early January so shall be watchig the flight map with interest. It is great to have you pros at hand to explain.

Compliments of the season to you all :)

[BOAC]

Put simply, a wing produces less lift at higher flight levels, because the air
is less dense. After 48 years flying, that's a new one on me. http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/eek.gif I too am busy re-writing my theory books.:)

[Lightning Mate]

Better not say any more mate - I got banned last time.

[DaveReidUK]

I too am busy re-writing my theory books

Fair enough - I'd estimate that there are several thousand out there, so it should keep you occupied for the rest of your lifetime.

Out of interest, what do you plan to use to replace ρ with in the lift equation ?

[Georgeablelovehowindia]

Hmm ... well I've got as far as blowing the dust off my copy of 'Handling the Big Jets' by D.P.Davies (Second Edition) which explains all this with graphs and everything.

But ... oh look! Here come the Patrouille de France, up from Salon for their morning practice, so I'm off outside with a mug of steaming coffee to watch.

:)

[BOAC]

Out of interest, what do you plan to use to replace ρ with in the lift equation - I always found V or Cl worked pretty well? (PS not 'replace' of course but I know what you meant.

If you had qualified your statement with something like "at the same alpha, speed and Mach" and perhaps mentioned buffet margin we might have been happier. Otherwise aircraft would be dropping out of the sky - wait a minute, here comes one now......................

[DaveReidUK]

If you had qualified your statement with something like "at the same alpha, speed and Mach" and perhaps mentioned buffet margin we might have been happier

Fine, then (pithy comments aside) the only thing we appear to disagree about was how much (over-)simplification was appropriate to the OP's question.

We're in the context of Spotters Corner, after all. When in Rome ... :O

[Peter47]

Out of interest, could someone quantify the reduction in specific fuel consumption, all things (such as weight) being equal, by flying at a higher altitude? For example would it take 5% less fuel to fly 1km, or nm at 35,000 feet rather than 30,000 feet?

Presumably flight time increases at higher altitudes as mach number stays constant but the speed of sound decreases?

[Mark 1]

In simple terms the fuel consumption is proportional to the drag and drag is proportional to air density and speed squared.

Air density reduces roughly 3% for every 1000' increase in altitude so increasing altitude, in principle, allows you to go faster for the same fuel burn or use less fuel for the same speed.

This only holds true though to a limited degree as the drag increases disproportionately at low airspeeds due to the high angle of attack and at high airspeeds due to compressibility effects as you get to higher Mach numbers.

For small piston engined aircraft, the maximum range varies little with altitude, but the speed used to achieve max range increases with altitude. For atechnical explanation of that have a look at this paper (http://www.db.erau.edu/research/cruise/piston.frame.html).

As you get higher the usable range between low indicated airspeed and high Mach number gets smaller for jet aircraft.