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2

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 ...

6

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 ...

3

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 ...

25

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: Emirates 7 (A380) inbound to Heathrow: FL320 → 360 → 380 → 400. As 757toga mentioned, there are variables to ...

11

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 ...

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