Learning Objective 021.11.02.03.06: State the function of the drain valves.
VENT/DRAIN VALVE
In a gas turbine engine combustion chamber, the fuel and air mixture usually burn at the chemically correct mixture, in order to burn hotter and more efficiently, in theory burning all of the fuel perfectly. Sometimes, however, there can be deposits of unburnt fuel that build up on the walls of the combustion chamber, which will need to drain from the engine somehow. A much more common usage of a vent/drain valve is after a "wet start", where a start malfunction has occured that stopped the engine from igniting the fuel, and the fuel has been sprayed out of the nozzles but not burned, so is now sat in the bottom of the combustion chamber.
This situation could be made very dangerous if the engine is started again, this time with a pool of fuel at the bottom, and could lead to a "hot start", with the potential for large flames to be sent out of the back of the engine. This could also overheat the turbine, as there is not enough cooling airflow during start to counteract the large fire. In aircraft with no vent/drain valve, the pilots would usually run the starter without running the igniters to blow the fuel away, but a vent/drain valve is much easier and can drain the fuel out of the combustion chamber quickly and safely.
Note: we would usually refer to this as a drain valve, and so do EASA, but in this question, the feedback says that they have called it a "vent valve", which is the same thing.
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