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Several years ago I was a passenger on a near cross-country flight when the pilot came on the PA and announced that we were climbing towards our cruise altitude, on autopilot, and as our fuel load was burning off, and the plane's weight lessened, we would be climbing about another five thousand feet from our cruise altitude.

I wondered why the autopilot, instead, would not make micro-adjustments to the angle of attack to maintain the same altitude, rather than waste fuel making an unnecessary climb.

I realize that maintaining the same exact altitude is probably not realistic, but autopilot, I would think, ought to be able to maintain cruise altitude within about one thousand feet.

This was on a Southwest flight, so it had to be a Boeing 737.

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  • $\begingroup$ Autopilots maintain cruise altitude much more closely than +/-1000ft! Standard vertical separation is 1000ft or 2000ft so planes would crash into each other if they could only maintain altitude to +/-1000ft. If you look at the flight progress information on the seat-back display, you'll see that airliners typically stay within a few feet of the designated altitude, presumably with more deviation during turbulence. $\endgroup$
    – David Richerby
    Commented Jun 30, 2015 at 8:38
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    $\begingroup$ The autopilot does maintain altitude. If they cruise climb it is because it was commanded to climb. It will not drift on its own. $\endgroup$
    – casey
    Commented Jun 30, 2015 at 8:58
  • $\begingroup$ This sounds like a description of a step climb $\endgroup$
    – Phil Miller
    Commented Aug 17, 2016 at 16:58

2 Answers 2

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Climbing further while cruising is saving fuel, not wasting it.

First, every airplane has an optimum angle of attack for most economical cruise. If it loses mass (due to fuel consumed), it needs to adjust either its altitude or its speed to keep that angle of attack. The goal is to keep dynamic pressure, the product of air density and the square of airspeed, constant. Since you want to reach your destination in the shortest time, climbing into less dense air is the better option.

Secondly, gas turbines need less fuel per unit of thrust when flying in colder air. As long as the plane stays in the troposphere, it will fly more economically when it flies in colder air.

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    $\begingroup$ They also need less fuel overall to provide thrust in thinner air, and in fact the fuel flow rate has to be dialed back as the aircraft climbs or the turbines will stall from the rich mixture. So to a point, the higher the altitude, the better the fuel economy, until the air is too thin for the engine's thrust to maintain the desired forward airspeed. $\endgroup$
    – KeithS
    Commented Jun 30, 2015 at 16:38
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    $\begingroup$ @KeithS: Almost - the thrust available from the engines goes down with density, so fuel flow is only dialed back in climb, when they run at maximum power. Note that the rate of climb will also decrease accordingly. $\endgroup$
    – Peter Kämpf
    Commented Jun 30, 2015 at 16:45
  • $\begingroup$ I don't get it - as an aeroplane loses mass, the speed (or rather dynamic pressure) for an optimum angle of attack decreases so it has to slow down (fly at lower IAS) to maintain it. So how it is possible for him to maintain speed while climbing? TAS increase compensates a necessary drop in IAS? $\endgroup$
    – Konrad
    Commented Aug 30, 2021 at 9:42
  • $\begingroup$ @Konrad You can either reduce speed or density. All you need to keep the lift coefficient constant while mass is declining is to reduce dynamic pressure. Climbing reduces density as required. $\endgroup$
    – Peter Kämpf
    Commented Aug 30, 2021 at 17:33
  • $\begingroup$ Now its obvious… thank you! $\endgroup$
    – Konrad
    Commented Aug 30, 2021 at 21:31
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Along with engine economy they may also have been climbing to take advantage of winds aloft at that altitude. Lets take a look at todays winds aloft out of JFK as an example

FD1US1 DATA BASED ON 300600Z
VALID 301200Z FOR USE 0800-1500Z. TEMPS NEG ABV 24000

FT 3000 6000 9000 12000 18000 24000 30000 34000 39000

JFK 2109 2109+10 2614+06 2521+01 2341-10 2348-22 235536 245645 246456

We can see that the winds at 34000 are 240 @ 56 and another 5000 feet up at 3900 are 240 @ 64 a solid 8 knot increase. Over a long distance this can make a big difference.

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