Why does the Typhoon vent fuel in displays

[RetiredF4]

I'll bite and try to add to the various explanations.
First the humorous one: It' s a wet system machine and as long as it is venting fuel there' s everything ok, enough of it still available.

Now to the logic explanation. The tanks are pressurized, to prevent the collapsing of the rubber type self sealing fuselage tank cells during high g maneuvering (at least that was the case in aircraft like F4 and other pre typhoon jets).

In normal operation the tanks are kept full by the transfer pumps (external and internal transfer), and when no transfer fuel is available those tank cells will go empty and stay empty, therefore there will be no repeated filling and emptying of single tank cells. They are either full or while getting emty they are pressurized with bleed air and stay pressurized until landing.

During display asociated with high powersettings the fuel volume of some cells will decrease, as the internal or external transfer pumps are not able to refill the tanks in the same time as fuel is used by the burners. The depleting fuel volume is replaced by pressurized bleed air. When the tank cells fill up again the bleed air is vented overboard, thus taking some fuel with it into the vent lines, especially just prior full fuel state of the respective cell is reached again.

Hope that helps.