Pneumatic, Air-Conditioning and Pressurisation Systems
Sources of pneumatic air
Know the sources: engine bleed air, an engine-driven compressor, or a ground cart. Bleed air is usually taken from the compressor stage and must be regulated in pressure and temperature before use. Understand the roles of the bleed valve, pressure regulator, precooler and shut-off valve. Traps include thinking bleed air is taken from the turbine; it is taken from the compressor, and the precooler uses fan or ram air to protect downstream components.
Air conditioning
Air-cycle machines cool bleed air by expanding it through a turbine after passing it through a heat exchanger. Vapour-cycle systems use a refrigerant and compressor. Be able to say why an air-cycle machine is common on turbine aircraft: it uses bleed air directly and needs no separate refrigerant. Know the function of the water separator and the temperature control valve.
Pressurisation
Pressurisation uses an outflow valve to control cabin altitude, with the inflow set by bleed air and the outflow modulated by the controller. Know the safety valves: positive pressure relief, negative pressure relief and the dump valve. Traps include the relationship between cabin altitude, aircraft altitude and differential pressure, and the fact that the outflow valve controls cabin pressure, not the inflow.
Worked example
If cabin altitude climbs while the aircraft climbs, the outflow valve may be stuck open or the controller may be commanding a higher cabin altitude. Rule of thumb: more inflow or less outflow raises cabin pressure; the outflow valve is the primary control.