Pneumatics

Pneumatics in Pressurization and Air Conditioning Systems

Pneumatic systems on pressurized aircraft use high-pressure, high-temperature bleed air—primarily from engine compressors in flight, with the APU or a ground air cart as alternatives—to power air conditioning, pressurization, and anti-ice functions. A core concept for ATPL students is cabin differential pressure (ΔP), defined as cabin pressure minus ambient pressure. In level flight, if cabin altitude rises (meaning cabin pressure decreases), ΔP decreases. During a normal pressurized climb, ambient pressure falls rapidly while the cabin is scheduled to descend more slowly, so cabin altitude increases at a controlled rate and ΔP changes according to the controller’s schedule and structural limits.

Cabin pressure is controlled primarily by regulating air outflow, not inflow. The cabin outflow valve(s)pack inlet flow valve (pack valve) to ensure sufficient mass flow for ventilation and cooling. Instrumentation includes cabin altitude, cabin vertical speed (V/S), and differential pressure indicators. If, in level flight, a controller malfunction causes the cabin V/S to show a descent (cabin altitude decreasing), cabin pressure is rising while ambient stays constant, so ΔP will increase until the safety relief valves open at the certified limit (typical maximum ΔP for large transport jets is approximately 7–9 psi). Negative relief protects against excessive cabin-to-ambient suction. Note that compressing air for pressurization does not change the oxygen percentage; it remains roughly 21% by volume, an important point for FAA ATP/EASA ATPL theory.

Most transport-category aircraft use a bootstrap (air cycle) machine within each air conditioning pack. In this system, bleed air is first cooled in a primary heat exchanger, compressed by the air cycle machine (ACM) compressor, cooled again in a secondary heat exchanger, and then expanded through a cooling turbine to deliver low-temperature, low-pressure air to the mix manifold. The ACM both reduces temperature and, across the turbine, reduces pressure. A main water separator is located downstream of the turbine to remove moisture (preventing icing and fog in the cabin). This arrangement gives efficient, turbine-driven refrigeration without chemical refrigerants and is fundamental knowledge in aircraft systems training and exam procedures.

What this question bank covers

• Pressurization fundamentals: definitions of cabin altitude, cabin vertical speed, and differential pressure; typical ΔP limits and structural considerations.
• Control and protection: roles of the pressure controller, outflow valve(s), pack flow control, safety relief and negative relief valves, and normal versus malfunction indications.
• Pneumatic sources and distribution: engine compressor bleed air, APU supply, and ground pneumatic support; ventilation mass-flow requirements.
• Air cycle (bootstrap) systems: ACM components, primary/secondary heat exchangers, compressor-turbine flow path, water separation, and the effect on air pressure and temperature.
• Operational procedures: behavior in climbs, level flight, and abnormal events; interpretation of cabin instruments per aviation regulations and ATPL exam standards.
• Human factors and gas laws: oxygen percentage unchanged by compression and implications for cabin altitude management.