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so when a plane is in cruise, it loses weight and then the weight begins to exceed lift.when this happens, the plane begins to climb.

My question is this please: The plane would be trimmed up or down to maintain the straight flight, but how do you know when to use which one?

Also, wouldn’t it mean that if the elevator for example was kept in its new position, the plane would continuously climb? How do pilots stop this continuous climb?

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    $\begingroup$ Doesn't this answer to another question of yours answers your question already? $\endgroup$
    – sophit
    Oct 22 at 12:29
  • $\begingroup$ Hi, I looked through those and they don’t answer this question sadly. But I think I got it, can some some check my answer please? $\endgroup$
    – James
    Oct 22 at 12:33
  • $\begingroup$ Have a look to this basic NASA guide. There are also interactive simulations to understand how each control surface works. $\endgroup$
    – sophit
    Oct 22 at 12:37
  • $\begingroup$ Hi sophit, I’ll take a look now $\endgroup$
    – James
    Oct 22 at 12:53
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    $\begingroup$ In your different questions you're focusing on secondary elements (cruise phase, elevator trimming, fuel weight influence on altitude) trying to understand how altitude is controlled. But the main factors controlling altitude are lift coefficient, angle of attack and velocity. You should focus on understanding that first before entering into specifics like how to keep altitude constant or to trim the elevators. $\endgroup$
    – mins
    Oct 22 at 14:13

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Trim affects the attitude of the aircraft in flight. To fly straight and level typically the aircraft would be in a slight nose-up attitude. If the aircraft had a good cg (slightly tail heavy on most types) you would not need much stab trim to acheive this. If the load-planning was screwed up and the aircraft is too nose/tail heavy, this would require more stab trim to get the aircraft level. More stab trim leads to more drag so will increase your burn-off. Hence the smart fellows at MDD (MD11) and Boeing (747-400) came up with the tail-tank as a means to correct the trim without using the stab trim (ie no drag penalty). Getting extra fuel capacity was a bonus.

To keep it level at different weights require different power settings, ie less thrust for xxx weight. Trim is not a factor.

There is also trim change due fuel burn off as the fuel cg shifts as the fuel is used up.. previously when doing manual loadsheets we would have a template that would mark out the cg curve.. interesting as the cg would move fore and aft as fuel was used.

On widebodies on long flights, initially it will be very minor changes in cg as the fuel in the centre tank is used first. As this is in the centre section it has very insignificant impact on centre of gravity (ie not much trim correction required) but once wing tanks start supplying fuel, the cg changes as the fuel tanks are quite spread out due to the sweep of the wings.

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