Aircraft Pneumatic Systems — Page 211, Lesson 270

Let’s start with the big picture, because this is the heart of how a jet aircraft gets its "air power." I’m talking about the pneumatic system — the network that takes compressed air from the engines and routes it to do useful work around the aircraft. Here we have a typical bleed air system. "Bleed air" is simply air that is bled, or drawn off, from the engine’s compressor. In this system, air is ducted from two stages of the compressor: a low pressure stage, which we call the LP stage, and a higher pressure stage, the HP stage. In this particular example, those stages are the 5th and the 9th compressor stages. The 5th gives you lower pressure air, the 9th gives you higher pressure air. Now, these two sources are combined together at a component called the High Pressure Shut-off Valve, abbreviated HPSOV. Let me explain what this valve does, because it’s clever. It is pressure sensitive and pneumatically operated — meaning it’s driven by air pressure itself, not by electricity. The valve is open when there is insufficient air pressure from the LP system to maintain the required flow. Think about that: at low engine speeds, the 5th stage isn’t producing enough pressure, so the valve opens to let the higher pressure 9th stage air join in and keep the flow going. As the engine speeds up, the LP air pressure increases. Eventually it reaches a point where it’s sufficient on its own, and that rising pressure closes the high pressure shut-off valve. So in all normal stages of flight, bleed air comes from the LP stages — the 5th stage. The HP stage is only a backup for when the LP pressure is too low. Now, there are two important design characteristics of these high pressure shut-off valves, and both are about timing. First, they are designed to open relatively slowly — on engine start up, or when air conditioning is selected. Why? To minimize the possibility of a surge of air pressure. A sudden rush of high pressure air could cause a pressure spike, and that’s what they’re avoiding. Second, they are designed to close very quickly. Why? To prevent an ingress of fumes or fire to the cabin in the event of an engine fire. If an engine catches fire, you want that valve slammed shut instantly so that smoke and flames can’t travel down the ducting into the cabin. So: slow to open, fast to close. Next, we come to the bleed air control valve. This is the separation point between the engine and the pneumatic system manifold. In other words, it’s the boundary — the valve that allows the bleed air to enter the pneumatic system. It is controlled electrically from the flight deck, so the pilots have direct control over whether bleed air flows into the system. Then we have non-return valves, abbreviated NRV. These are installed in the LP stage ducts. Their job is to prevent HP air entering the LP stages of the engine when the high pressure shut-off valve is open. Think about the flow path: when the HPSOV is open, HP air is flowing. Without a non-return valve, that high pressure air could flow backwards into the LP stage ducting and into the engine’s 5th stage, which isn’t designed for that pressure. The NRV acts like a one-way door — it lets air flow out of the LP stage toward the system, but blocks any reverse flow back into the engine. Finally, let’s talk about how multiple engines are kept separate. Most multi-engine aircraft keep the supplying engines, or sides, separate — each engine supplies its own user services. These are kept independent by isolation valves. These isolation valves are normally closed, which keeps the two sides independent. But they may be opened if an engine supply is lost, to feed the other side’s services. So if the left engine fails, you can open the isolation valve and the right engine’s bleed air will feed the left side’s services too. That’s your redundancy. So, to tie it all together: bleed air comes off the 5th and 9th compressor stages, combines at the HPSOV which manages pressure, passes through the bleed air control valve into the pneumatic manifold, non-return valves protect the LP stages from backflow, and isolation valves keep the engine sides independent with the ability to cross-feed in an emergency. That’s the architecture of a typical bleed air system.