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This question has several parts that are interrelated, and I didn't want to split it up:

Considering that several tons of fuel are consumed during a long haul flight which requires lift to be reduced as the flight progresses, do pilots do this by changing wing geometry, altitude, speed or all of the above?

Is this done automatically or manually?

What is the optimal strategy if you want to minimize fuel costs?

Having chosen the optimal strategy, is the latest phase of the flight where the airplane weighs least necessarily the one that gets the best gas mileage, or is there a minimum at a certain intermediate airplane mass during the flight?

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    $\begingroup$ I'm pretty sure you get the best mileage during the descent at the end of your trip, but that's not really what you're asking, is it? :) $\endgroup$
    – falstro
    Commented Feb 21, 2014 at 18:18
  • $\begingroup$ I also believe this is probably the case, but am not completely sure: Maybe the reduced lift demands an angle of attack that has a poor drag, or the engine has to run at a power level that is suboptimal. However, I presume the relationship between wing configuration, altitude and engine power would be quite difficult to optimize during every point in the flight. $\endgroup$
    – yippy_yay
    Commented Feb 21, 2014 at 18:27
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    $\begingroup$ Descent is most efficient. Lift and angle of attack are the same in the descent as in cruise (provided speed and weight are the same). The only change is less power from the engines. $\endgroup$
    – Khantahr
    Commented Feb 22, 2014 at 15:47
  • $\begingroup$ @Ralgha Lift is less during descent than cruise, pretty much by definition. If lift were the same, you wouldn't be descending! $\endgroup$
    – David Richerby
    Commented Mar 21, 2014 at 4:09
  • $\begingroup$ No, lift is the same. If lift were less you'd feel light in your seat, and you don't, unless it's an unusually sudden pitch down. Stable climbs and descents are caused by excess or insufficient power. Thinking that lift is changing is a common misconception, similar to the "downwind turn" misconception. $\endgroup$
    – Khantahr
    Commented Mar 21, 2014 at 4:21

2 Answers 2

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There are a lot of things that go into determining the most efficient way to operate a jet, and minimizing fuel does not always make it cheaper to operate. Some general "rules" that are used are:

  • There is an optimum speed to fly the airplane, which depends on several factors:
    • Altitude
    • Fuel savings by flying slower -vs- increased maintenance costs because you are taking longer to get there (and putting more time on the airframe/engines).
    • Temperature at cruising altitude
    • Winds at cruising altitude
  • There is also an optimum altitude to fly the airplane:
    • Aircraft weight
    • The higher that you can fly, the better - until you don't have enough power to maintain your optimum speed at the optimum power setting for the engines.
    • Temperature at cruising altitude
    • Winds at cruising altitude
    • Altitudes that are available from ATC
    • The amount of fuel and time that it takes to climb -vs- the amount of time that you will have at the high speed cruise

Anything that makes the airplane heavier makes it unable to climb as high (and lowers the optimum altitude for the current conditions), and makes it burn more fuel. This includes the fuel itself. Assuming a constant speed and constant weather conditions, you will always be more fuel efficient later in the flight.

As fuel gets burned off, the amount of thrust required to maintain the optimum speed goes down and the optimum altitude goes up. Basically, we keep reducing the power to maintain speed until the optimum altitude increases to an altitude that ATC can let us climb to. This is known as a step climb. The speed will be controlled manually by the pilot, or can be controlled automatically by auto throttles.

Today's sophisticated flight planning software takes all of this into account and gives the pilot a very good plan on how to operate the airplane to be the most efficient!

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  • $\begingroup$ Actually, best cruise is flown at a specific lift coefficient, which in turn depends on the engine type, aspect ratio, zero-lift drag and wing efficiency. $\endgroup$
    – Peter Kämpf
    Commented Feb 9, 2015 at 22:01
  • $\begingroup$ @PeterKämpf You left this comment a long time ago, but I just saw it. "Best cruise" doesn't always result in the most economical flight, and for a given design (where all of the things that you mention in your comment are constant), and operating rules, these are the operational considerations which come in to play when trying to minimize total overall cost. $\endgroup$
    – Lnafziger
    Commented May 30, 2016 at 16:34
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To give an example of the weight vs altitude capability for an old airplane, a 747-200 taking off at max weight isn't going to be able to climb higher than FL290 or FL310, and they may stagger to get to the latter. If the flight is in the neighborhood of 12 hours (which generally means a passenger aircraft as opposed to a freighter) FL390 or FL410 become usable. The aircraft is capable of FL450 if unusually light.

Though our old flight plans gave us the points that we could expect to be able to climb, there was a rule of thumb (which I unfortunately have forgotten) using the EPR that we used that quickly told us how many thousand feet above our current altitude we could get to.

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