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I'm a noob so pardon my ignorance. So my understanding is that as the plane gets lighter during the flight, its mass reduces therefore reducing the lift needed to maintain the altitude. At those altitudes and speeds, my understanding is that we can't engage slats or flaps. As a result, we would have to reduce thrust to reduce the lift to account for the loss in fuel and maintain the same flight level

So what can one use to continually reduce lift coefficient at those altitudes to maintain constant airspeed and flight levels? I understand that ATC also clear aircraft to higher FL thereby reducing the air density and therefor reducing lift and enabling higher airspeed but that still leaves us with the problem of flying at constant airspeed at cruise altitudes.

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    $\begingroup$ One word: autopilot. $\endgroup$ – CatchAsCatchCan Dec 10 '19 at 0:49
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    $\begingroup$ Note that for this reason (and because it flew higher than other airplanes so it didn't have traffic to avoid above it) the Concorde did climb during cruise. $\endgroup$ – Manu H Dec 10 '19 at 9:01
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    $\begingroup$ @ManuH Concorde was also an extreme design that was atypically (for a passenger jet) susceptible to huge efficiency spoiling by just about anything that deviated from its ideal flight configuration. A normal subsonic tube jet is not punished so severly for maintaining a fixed pressure altitude during cruise, so really the reason that Concorde did not was more to do with its particular and highly constrained flight parameters than anything else. $\endgroup$ – J... Dec 10 '19 at 12:40
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    $\begingroup$ @JonathanWren: Some of it's mass is always going out the back of the engines, though (burnt fuel). That's far more significant than any change in gravity due to altitude. $\endgroup$ – Fred Larson Dec 10 '19 at 19:27
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    $\begingroup$ In fact the difference due to additional height only lowers the force due to gravity by about .33%, and as @FredLarson says, is completely negligible when compared to the mass change due to fuel losses $\endgroup$ – eps Dec 10 '19 at 19:31
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The autopilot pitches to hold the flight level when it captures the level at the top of the climb, so later on as the aircraft gets lighter and wants to climb further, the A/P will lower the nose as required to hold the flight level (the A/P is able to move the elevator through its servo's link to the elevator controls; it can also work the trim if the servo has to hold too much out-of-trim force for too long, to avoid wearing out the servo - in effect it does what the human pilot does, more or less). It happens very slowly and significant amount of pitch over is not apparent until some time has passed.

Effectively, AOA is being reduced by the A/P to compensate for the reduction in wing loading. Because induced drag is being reduced, if thrust is not reduced to compensate for the drag reduction, the A/C will accelerate. If your clearance requires you to maintain a certain Mach #, this is a problem.

If the jet has an autothrottle system that can be configured to hold a Mach # while the A/P holds the altitude, the thrust is adjusted automatically. If not, the pilot monitoring at some point will notice the Mach # creeping up, and will reduce thrust in very small increments compensate and maintain the flight planned/cleared Mach #.

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    $\begingroup$ Which control surfaces does the autopilot use? I guess the elevator? This is probably obvious to pilots but maybe worth adding here but with this being an elemantary question ... after all, the autopilot can't "think the plane" level! :-) $\endgroup$ – Dan Sheppard Dec 10 '19 at 1:34
  • $\begingroup$ Thanks for the answer John. So the A/P is lowering the nose by moving the elevators? Is that a correct reading of the problem? But the other question I had was, by lowering/raising the nose, aren't you also changing the direction of the engine thrust? It just seems a little counter intuitive to me that the A/P would lower the nose. May be you could throw a little more light. $\endgroup$ – Sriram Subramanian Dec 10 '19 at 1:52
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    $\begingroup$ Text added thanks. The other thing that happens, much more suddenly, is you will fly into some mountain wave that is rising maybe 50 or 100 fpm, and you will observe the nose pitching down on your PFD because the A/P wants to hold the alt and starts applying down elevator, and you have to reduce thrust maybe a percent or so to keep the same speed. Then you fly out of the wave and it pitches back up and you have to put the thrust back the way it was. $\endgroup$ – John K Dec 10 '19 at 1:55
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    $\begingroup$ @SriramSubramanian no the pitch change doesn't induce a climb or descent just from the orientation of the thrust line. I added some text to my post. The A/P pretty much does what a human would do to hold an altitude, but is a bit less fussy about working the trim and will tolerate holding a bit of force continuously, but otherwise it's the same control inputs. $\endgroup$ – John K Dec 10 '19 at 1:59
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    $\begingroup$ Awesome! That really sums it up neatly. Thanks a lot! $\endgroup$ – Sriram Subramanian Dec 10 '19 at 2:06
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There are a lot of things that affect lift beyond just the speed and air density. For instance, the angle of attack. As the weight of the aircraft decreases, the pilot (or autopilot) will pitch the nose down slightly to reduce the AoA, and therefore lift.

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As pointed out by HiddenWindshield, the pilot will pitch the nose down slightly as the fuel load is burned off. The pilot does this using the elevator trim control and will from time to time in a long flight add nose-down trim to maintain zero vertical speed. With less load, less lift is required, and trimming away the unnecessary lift will cause the plane's speed to rise slightly unless the pilot reduces power.

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That's easy. Just push the nose down a teensy bit. (or rather, pull the nose up less.) That will reduce AoA, reduce lift slightly, prevent you from climbing and will hold altitude.

"But then, we'll speed up!" Correct. So, reduce thrust as needed.

Another option is to intentionally climb. Then, you enjoy the fuel economy benefit of higher altitude. A large plane heavy with fuel is unable to reach its max design altitude. So, as the aircraft sheds fuel, it can seek higher and higher altitudes, to gain the fuel efficiency of pushing through thinner air. It's common for a long-haul flight to start at 31,000 feet, then climb to 33, 35, 37, 39,000 as fuel is burned off.

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As pointed out in other responses, there are many factors that influence how and why an aircraft will need constant adjustments made to maintain speed and altitude while cruising.

You are correct in assuming that as the mass decreases as you burn off fuel, there will be less lift required to counteract gravity. It will also take less thrust to counteract the drag of the air, so either the pilot or the autopilot will need to make continuous adjustments inflight, small though they may be.

One thing that has not yet been mentioned is Air Traffic Control. Each aircraft at "cruise altitude" (which is usually above 18,000 feet, or Flight Level 180, and most often above 29,000 feet, FL290) is in controlled airspace, and has an altitude assigned to them by Air Traffic Control (ATC). You must maintain this altitude to maintain proper separation from other aircraft. Of lesser importance, but still something to keep in mind, is that ATC expects you to maintain your airspeed while cruising, and not to significantly increase or decrease your speed unless you notify them. Again, this is to allow them to maintain separation between you and other aircraft.

With that in mind, I hope you can better understand why the constant pitch and throttle adjustments mentioned above, even if small, are essential.

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