Well that is why you must tune the engine with more fuel in the tank when you perform the nose down tests.

The idea is to keep the clunk in the fuel.

Do this on a tank that is less than almost full and you may indeed expose it, leading to poor results and/or tuning.

The problem with this, is that it is effectively a "best case" test for a nose down state.

In lieu of the presence of a header tank, it's better than nothing though.

What offsets the above is that the engine will run a bit leaner in flight anyway, so this test is safe.


However the nose up test can be made with far less fuel in the tank, so you can tune the engine for it's worst case condition...

e.g. nose up, running leaner as the tank empties.