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I've never bothered to check the weight and balance when the tanks are empty. I always assume that if the Center of Gravity is within range when the tanks are full, then it will remain within range as the flight progresses.

Am I correct? Should I compute the W&B on both ends?

I know some airliners have complex fuel systems which require fuel to be moved around during flight (e.g. B747-100, Concorde). I am interesting in small aircraft in general, i.e. General Aviation and perhaps personal and up to regional jets.

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  • $\begingroup$ I was recently flying a DA20, fully fueled up for 4 hours of flight time, where the endurance ended up being 3:30h, because of exactly this problem with the CG being too far FWD if the fuel tank was totally empty. $\endgroup$
    – Florian
    Commented Dec 18, 2018 at 12:10

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Should I compute the W&B on both ends?

Yes, you should generally always compute W&B on both ends of the flight. If for no other reason that you may want to know where your CG falls when you land as even within limits it can greatly change handling of the aircraft. If you are currently training you should encourage your instructor to setup situations where you can fly the aircraft at different loads, perhaps bring a friend along to ride in the back and fill the tanks or you and your instructor a very light fuel load and some pattern laps.

I always assume that if the Center of Gravity is within range when the tanks are full, then it will remain within range as the flight progresses.

This depends on the airframe and you should consult the POH that accompanies the airframe. It really depends on the tank configuration, the fuel load, and the aircraft load. For example the Bonanza has tip tanks that actually increase useful load which can have a pretty big CG impact. While some GA planes have their tanks mounted fairly center mass and the burn has little impact.

In the various PA-28's and 172's I have flown, generally with myself and a passenger, they are fairly hard to push out of CG limits just by burning fuel. However there are plenty of bigger GA planes that have more complex fuel systems, and extra bag compartments that allow potential CG issues.

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  • $\begingroup$ C172 is certified for both the normal and utility categories, but with more stringent weight and CG limitations for the latter. It is easy to stray from one category to another as the fuel burns, although in practice you'll usually move into the utility area. Also, from memory, on C152 it is possible to have a too-forward CG with two heavier pilots and nearly empty tanks. $\endgroup$
    – Zeus
    Commented Dec 17, 2018 at 23:41
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With a straight wing aircraft with all the fuel in the wings, the fuel is close enough to the C of G across the span of the tanks that the C of G shift as fuel is consumed is insignificant. When this is the case, the weight and balance chart won't have any special requirements to show C of G shift, relative to the fore/aft limits, from fuel burn.

If there is an aux tank that is forward or aft, the effects of its fuel level on C of G will be included in the airplane's weight and balance, and there may be limits on loading of cabin or cargo incorporated into the weight and balance data, and special instructions for C of G calculations when it's being used along with curve added to the wight and balance graph.

On swept wing airplanes, sweep and dihedral in wing tanks causes C of G to move forward as fuel is burned (level drops in the tips first, which are aft). On swept wing airplanes with a center tank, the center tank is almost always burned first and it causes an aft shift of C of G as its fuel is burned, followed by a forward shift as the wings are consumed.

The C of G charts of swept wing airplanes will include a fuel burn curve to show the C of G location variation with fuel burn. If no center tank, the curve will slant or curve to represent the forward shift, and if there is a center tank it will have a kind of sideways V shape as the C of G moves aft and then forward as the pounds of fuel load declines.

Bottom line is, do whatever the weight and balance chart for the airplane says to do. If it's something like a 172, there isn't going to be any major shift. If it's something like a Taylorcraft with a nose tank, C of G will shift aft as fuel is burned. To keep things simple, there may be a baggage restriction that is intended to prevent exceeding the aft limit when the tank is empty at the end of a trip.

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It's not general aviation, but the P-51 had enough of a CG shift as fuel as used to be worthy of mention in the Pilot's Handbook (reference)

The fuselage tank should be used for takeoff and climb to to a safe altitude as it is the most direct system to the engine and is on a higher plane in relation to the engine. Use of the fuselage tank will also move the C.G. of the airplane forward to a more desirable position for flight.

and also

The stability of the plane improves rapidly as fuel is expended from the fuselage tank.

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