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Some people said airplanes would fly higher in the late period of each flight, because the fuel is consumed, and the airplane is lighter. Higher altitude and less dense air is enough to support the airplane, not to mention, less drag in high altitude can save the fuel further.

This statement sounds reasonable. But would this be practical in the real world?

Because you can always lower your pitch angle, and get less drag and save fuel. In this way, you don't need climb to consume additional fuel. And you don't mess with other traffic.

What would a typical 2-hour domestic flight's altitude change be like? What about a typical 10-hour international flight?

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    $\begingroup$ Related: What is the flight trajectory of a commercial airplane in regards to altitude v/s distance? $\endgroup$ Commented Jan 3, 2022 at 12:41
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    $\begingroup$ What do you mean by "you can always lower your pitch angle, and get less drag and save fuel"? If you pitch down (in level flight at a given airspeed), the AoA decreases and thus lift decreases as well. As that lift is no longer enough to counteract gravity, you will start to descend. $\endgroup$
    – TooTea
    Commented Jan 3, 2022 at 13:55
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    $\begingroup$ @TooTea: as the plane loses weight, a smaller angle of attack with lower drag is indeed what happens with the speed constant. $\endgroup$
    – user14897
    Commented Jan 3, 2022 at 14:26
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    $\begingroup$ @ymb1 Oh, right, of course. I didn't realize the question doesn't assume trimmed level flight at an optimal cruise airspeed to begin with. $\endgroup$
    – TooTea
    Commented Jan 3, 2022 at 14:32

5 Answers 5

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The term for it is step climb. It's more common on longer flights. On busy transatlantic routes (a non-radar environment) it's not as common (it may change as satellite ADS-B surveillance is being implemented).

This is live as of writing this:

enter image description here

Emirates 7 (A380) inbound to Heathrow: FL320 → 360 → 380 → 400.

As 757toga mentioned, there are variables to consider. Typically the flight plan will have the step climbs included. Updates/changes can also be sent by the airline's operations center to the pilots (more common in transpacific flights as newer weather observations are processed). Given the variables, a step descent can also be requested (example below).

enter image description here
Delta 40 on Jan 1st

From an Airbus publication (Getting to Grips with Aircraft Performance) the relation looks like this:

enter image description here

With stable weather above the tropopause and no traffic to speak of, the high-flying Concorde used to cruise-climb (not in steps, but continuous).

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There are a number of variables that influence a decision whether or not to climb to a higher altitude as fuel is burned and the airplane becomes lighter. Burning less fuel, as you note in your question, is one consideration.

Considerations for not climbing to a higher altitude may include:

The wind at a higher altitude may result in more of a headwind (or less of a tailwind); perhaps there is reported turbulence at a higher altitude; the type of airplane and its associated optimal performance characteristics may make flying at a higher altitude (than its current altitude) less efficient; Air Traffic Control may not permit the aircraft to climb to a higher altitude because of traffic or other issues related to the route being flown, etc.

Fortunately, many modern aircraft have best-performance and optimum altitude data continually available using on-board Flight Management Computer systems to aide in the decision whether or not a climb to a higher altitude as fuel is burned is appropriate as the aircraft becomes lighter.

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Yes, this is both practical in the real world and very common. Almost all trans-Pacific flights do this (called "step climbs") because they start off much, much heavier than they land. Their initial cruising altitudes need to be relatively low due to their high initial weight, but they're able to climb up into the thinner air once they've burned off hundreds of thousands of pounds of fuel weight and that enables them to fly the remainder of the flight more efficiently.

This is much less common on short 2-hour domestic hops, though. Those typically aren't altitude-limited by their fuel weight in the first place, so they have no need to start off at a lower altitude and then climb later. They just climb straight to a cruising altitude that they'll use all the way to top-of-descent unless they or ATC need them to change altitude for something like weather, turbulence, or traffic. The fuel weight is a much smaller fraction of the overall aircraft takeoff weight on 2-hour domestic hops than it is on 15-hour trans-Pacific long-hauls. This is also why those aircraft can typically land immediately after takeoff if needed and typically don't even have fuel jettison capabilities.

Here's an example of a Tokyo-Narita to New York-JFK flight on a Japan Airlines Boeing 777-300ER earlier this week:

NRT-JFK Flight Altitude Profile
Source: FlightAware

This flight is a bit over 12 hours in duration. It started off with an initial cruise altitude of 32,000 feet. Interestingly, it seems to have initially received a block clearance, taking nearly an hour and 40 minutes to climb from 30,000 to 32,000. It then did a step climb to 33,000 at roughly 3 hours, 34,000 at 4.5 hours, 35,000 at 6.5 hours, and finally 37,000 at 9.5 hours. If you are paying attention as a passenger, you can hear when these step climbs take place, as the engines will get noticeably louder for several minutes or so during the step climbs. Of course, you can also see it on the flight tracker on the in-flight entertainment screens on flights that have those.

Because you can always lower your pitch angle, and get less drag and save fuel. In this way, you don't need climb to consume additional fuel. And you don't mess with other airway traffics.

This is what you'll do to maintain an altitude assignment as you burn off fuel (generally done in very small steps constantly and automatically by the autopilot.) It does reduce fuel burn somewhat, of course, but it doesn't reduce it as much as climbing into the thinner air does.

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Yes, the step climb is done to conserve fuel, because the air is thinner there and takes less fuel to push through. (Like the way you'll get an MPG bump driving your car in Wyoming).

They already take the lowest AOA that will result in level flight, and the AOA is high because the airplane is heavy with fuel. In that condition, pushing the nose down to reduce AOA would result in losing altitude.

it raises the issue "why step-climb, why not start at the higher altitude?"

The answer is they can't.

The even higher AoA required to fly in the thinner air would result in stall.

The first steps are lower because the airplane is too heavy to be able to fly at the higher altitudes. Only after it loses some weight in fuel is it possible to safely attain the next step upwards. Then it loses more and takes the next step if able to get clearance.

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In horizontal flight, lift L = weight W = $C_L \cdot ½ \rho V^2 \cdot S$ (in incompressible circumstances). If W decreases continuously because of fuel burn, lift will need to decrease accordingly as well. Since wing area S is a fixed value in cruise, lift can be reduced by:

  • Lower $C_L$ by changing trim;
  • Lower air density $\rho$ by flying higher;
  • Lower airspeed V by adjusting throttle;

enter image description here

It turns out that the optimum way to adjust for the reduced weight is by increasing altitude, linearly and continuously, according to the green line in above pic, from this document page 10.. But there are quite a few aeroplanes cruising between continents and they need to be separated in flight corridors, that is why subsonic jetliners perform the cruise climb in steps. Indeed Concorde did not have this issue and could climb continuously, as @ymb1's answer mentions.

enter image description here

The referenced MIT-document is quite an interesting one. From page 13 above, which mentions that density altitude should increase linearly but aircraft fly pressure altitude, which can lead to steps in pressure altitude.

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