Module 11B.4 and 11B.15 — Piston aeroplanes: pressurisation, heating, ventilation and oxygen

What's in scope for 11B

Item	A2	B1.2
11.4(a) Pressurisation, controllers, safety valves, indication	1	3
11.4(b) Air supply (bleed/APU/ground cart)	—	—
11.4(c) Air conditioning (air/vapour cycle)	—	—
11.4(d) Safety and warning devices	1	3
11.4(e) Heating and ventilation systems	1	3
11.15 Oxygen	1	3

So for B1.2 you must know piston-aircraft pressurisation, cabin heating (exhaust and combustion heaters) and ventilation, and oxygen systems in detail.

11.4(a) Pressurisation on piston aeroplanes

Some piston twins and singles (e.g. pressurised Cessna twins, Piper Malibu/Mirage) are pressurised to a modest differential — typically about 3.5–5.5 psi — keeping cabin altitude around 8 000–10 000 ft at cruise.

Air source

• Turbocharger compressor bleed: compressed induction air is tapped downstream of the turbocharger compressor (before the throttle) and passes through a sonic venturi (flow limiter) — the venturi throat chokes at sonic speed, limiting the mass flow taken from the engine regardless of pressure. A heat exchanger/intercooler and a shut-off/dump valve may follow.
• Older types: separate engine-driven cabin superchargers (Roots-type blowers).
• Inflow is roughly constant; cabin pressure is controlled by the outflow valve.

Controller and valves

• Cabin pressure controller (pneumatic or electronic): pilot selects cabin altitude and rate of change; it modulates the outflow valve via a reference chamber.
• Safety valve (often combined in a similar valve body): limits maximum differential (positive relief), provides negative pressure relief, and acts as a dump valve (cabin depressurisation switch).
• Squat switch keeps the safety/dump valve open on the ground so the cabin cannot be pressurised with the doors closed on the ground.
• Door seals: inflatable seals fed with pressurised air when the door is closed and the aircraft is airborne, or compression seals.

Control and indication

Cabin altimeter, cabin differential pressure gauge, cabin rate-of-climb indicator (often combined in one instrument); controller setting knobs.

11.4(d) Safety and warning devices

• Cabin altitude warning (light/horn at about 10 000–12 500 ft depending on type).
• Over-differential protection by the safety valve; negative relief.
• Door-ajar warning, pressurisation fault annunciators.
• Carbon monoxide detection (see heating).

Key fact: On turbocharged piston aircraft, pressurisation air comes from the turbocharger compressor through a sonic venturi that limits the flow taken from the engine.