Do a bit of searching on the forums before posting please. It is an old bug.
The ONLY thing you need to do when FF disappears(usually after firing the guns for the first time) is to reduce speed to ca 250km/h and the FF will come back and stay for the rest of the duration until you exit the game.
My trick is to fire all guns (MG and cannons on 109/110) before even taking off with a short bursts as when you climb up the speed is anyway below 250. Never had the issue come back since.
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