Aircraft Pneumatic Systems — Page 203, Lesson 262

Let’s start with the big picture. A pneumatic system is fitted in most modern aircraft to supply some or all of a long list of aircraft systems. I want you to hear that list, because it tells you why pneumatics matter. The pneumatic system supplies air conditioning, pressurization, aerofoil and engine anti-icing, air turbine motors, engine starting, hydraulic power, thrust reverse, leading and trailing edge flap and slat operation, pneumatic rams — for example thrust reverser actuation — hydraulic reservoir and potable water tank pressurization, and cargo compartment heating. Now, the key point: most of these systems use high volume, low pressure airflow bled from the compressor stages of a gas turbine engine. That is the heart of it. You take air that has already been compressed inside the engine, tap it off, and use it to drive all these other systems. That is why we call it bleed air. Other sources of supply are engine driven compressors or blowers, auxiliary power unit bleed air — that’s the APU — and ground power units. So when the main engines aren’t running, you can still get pneumatic supply from the APU or from a ground cart. There is an important contrast here. Some older turbo-propeller and piston engined aircraft use high pressure pneumatic systems for the operation of landing gear, brakes, flaps, and so on — the Fokker F.27 is the example given. But those aircraft are a minority. Hydraulic power has become the normal method of operation for those systems. So don’t confuse the two: pneumatics today is mostly about high volume, low pressure bleed air for environmental and anti-icing duties, while hydraulics does the heavy actuation work. Now let’s move into the air conditioning system itself. The air conditioning, or environmental control system, is fitted to an aircraft to regulate the temperature, quantity, and quality of the air supply to the passengers and crew. So three things: temperature, quantity, and quality. That conditioned air is also used, with additional components, for ventilation and pressurization of the aircraft. One thing to note carefully: humidity within the cabin is not generally controlled. Water is removed by the air conditioning system, and a certain amount is introduced, but humidity is not regulated to a specific level. So don’t think of the ECS as a humidifier — it removes water and adds some, but it does not hold humidity at a set value. Then we come to why modern aircraft are pressurized. There are two reasons given. First, the aircraft can fly at an altitude where it can operate efficiently and economically, and avoid the worst of the weather conditions, whilst maintaining cabin pressure at a comfortable level. Second, aircraft can achieve high rates of climb and descent with small corresponding rates of cabin pressure changes. That second point is important — pressurization lets you climb and descend fast without subjecting the passengers to rapid pressure changes in the cabin. Finally, the requirements of an air conditioning system are laid down in the BCARs — that’s the British Civil Airworthiness Requirements. We’ll go into those requirements next, but for now, know that the design standard for the air conditioning system is set by BCARs. Let me show you the bleed air arrangement so you can see how the compressor stages feed these systems. So to summarise what we’ve covered: pneumatics supplies a wide range of systems using high volume, low pressure bleed air from the engine compressors, with APU and ground units as alternates. Older aircraft used high pressure pneumatics for actuation, but hydraulics took over. The air conditioning system regulates temperature, quantity, and quality of cabin air, does not regulate humidity, and pressurization exists so the aircraft can fly high and efficiently while keeping the cabin comfortable and allowing fast climbs and descents with gentle cabin pressure changes. And the whole thing is governed by BCAR requirements.