Pneumatic Closure Control
Do you know how the bleed air is controlled by a modern day aircraft engine? If not don’t worry, let’s have a deep look into the system and get things cleared up.
Bleed air is commonly used for cabin pressurization, wing anti-icing, engine anti-icing, engine starting, water tank pressurization and hydraulic reservoir pressurization. For these purposes bleed air is taken from the cross bleed duct. Bleed air outlets from both engines and APU bleed outlet are connected to the cross bleed duct plus a ground air service cart can be connected to the ground. The in-flight cross bleed duct can receive bleed air from all sources except the ground cart. During the flight, bleed air from the engines is used hence APU is kept out of the drama until needed.
Going back to the topic, at what time bleed air from an engine is cut off? There are few instances where engine bleed valve is kept closed. Operation of the bleed valve is controlled by the BMC(Bleed Monitoring Computer) and a twin-engine aircraft consists of two BMCs, one for each engine. If one BMC of an engine is inoperative, the other one takes over and oversees the related operations of both engines.
Bleed valve position is decided by a control logic and the control logic comprises only with OR gates. If one condition agrees, bleed valve will be driven in to close position.
A pneumatic starter is used to start the engines and the starter receives bleed air for engine starting from the APU. But an already started engine’s bleed or air from a ground service cart also usable for this task. When the bleed air is available for engine starting, starter valve is opened and let air pass into the starter. The system is designed in a manner that bleed valve can’t be kept open when the starter valve is opened. If these two valves are kept open at the same time, the starter might not be able to rotate the engine up to the required rpm. So here comes the first logic, (1)if the starter valve is open engine bleed valve should be in closed position. BMC looks into the position of starter valve and adjusts the position of the bleed air valve in accordance with that.
As said earlier APU bleed too can be used to drive the starter, hence if the (2)APU bleed is ON, BMC will command the bleed valve to close. If the upstream (3) pressure or (4) temperature from the bleed valve is exceeded, BMC will command a valve closure. If there is a leak (5) in the bleed air system the process of tapping bleed from the engine should be terminated to avoid huge energy loss and to maintain the rated thrust from the engines.
Out of 8 ways which close the bleed valve we have discussed up to 5. Reverse flow is a phenomenon that should be avoided to protect the engine from surging and due to that bleed air valve is designed as a one-way valve and air can only flow out from the engine. Hence bleed valve is driven to a closed position during a (6)reverse flow situation.
Other two ways of closing the bleed valve are simply by pushing the ENG 1 BLEED push button or by pressing ENG 1 FIRE push button.