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Some aircraft (A380, Concorde,...) are equipped with a trim tank in the tail. Transferring fuel from or to this tank help controlling the CG. This feature can only be achieved if there is fuel to transfer. If tanks are nearly empty, and CG must be kept aft, this fuel could not be usable, except if there is another mechanism to compensate for the CG.

Is the fuel from trim tank always usable? Does it count when calculating minimum fuel requirement for a travel?

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Here's a good pilot write up of the A380's fuel systems:

https://captaindave.aero/2017/09/07/the-a380-fuel-system/

Of particular note is this paragraph:

Towards the end of the flight there are two additional fuel transfers. Any fuel remaining in the trim tank when the time remaining to destination drops below 80 minutes is pumped forward. Similarly, when the time remaining to destination drops below 30 minutes any fuel left in the outer tanks is moved.

So, clearly, on the A380 not only is the fuel usable but it's also a standard operational procedure to utilise it.

I would expect that any fuel trimming is either

  1. Contingent on having more fuel outside of the trim trank. As that fuel depletes, the trim will change, necessitating (or at least allowing) the movement of fuel back away from the trim tank

or

  1. The change in trim is about optimisation, rather than keeping the aircraft within safety tolerances. On a long trip in a large airliner, small changes in trim could have real impacts on efficiency and fuel usage without necessarily impacting control and safety.
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    $\begingroup$ +1 for the reference. Also OP wrote "CG must be kept aft", note that fwd CG is preferable before and during descent. The lighter the plane (after burning fuel), the more fwd CG become preferable for drag reduction (lower AoA is smaller stab trim with fwd CG). $\endgroup$ – ymb1 Sep 26 '19 at 15:26

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