Chapter 18: Pneumatic, Vacuum, Water and Waste Systems

Part 1 — Pneumatic and vacuum systems (ATA 36)

On a turbine aircraft the "pneumatic system" is the network that collects compressed air — mostly bleed air from the engines' compressors — and distributes it to the systems that need it. Light aircraft have much simpler vacuum or pressure systems for their gyro instruments and de-icing boots, and a few aircraft use high-pressure pneumatic systems instead of hydraulics for some services.

Sources

• Engine bleed — from an intermediate (low-pressure) stage through a check valve, or from a high-pressure stage through the HP valve when engine power is low.
• APU — its load compressor (or the compressor of a single-shaft APU) supplies air through the APU bleed valve and a check valve, for ground air conditioning and engine starting, and in flight within limits.
• Ground supply — a high-pressure air cart connected to the ground connection (with a check valve) for engine starts and ground conditioning when the APU is unavailable.
• Compressors and reservoirs — some aircraft have dedicated electric or engine-driven compressors (for example cabin air compressors on bleedless designs, or the high-pressure compressors and storage bottles of high-pressure pneumatic systems).

Pressure and temperature control

• PRSOV (pressure-regulating and shut-off valve) — regulates manifold pressure (typically around 40–45 psi) and shuts off the bleed; closed by the bleed switch, the fire handle and protection logic.
• HP valve — opens only when intermediate-stage pressure is too low; the IP check valve stops HP air flowing backwards into the IP stage.
• Precooler and fan-air valve — the precooler is an air-to-air heat exchanger cooled by fan air; the fan-air valve modulates to hold the outlet near 200 °C.
• Overpressure and overtemperature protection — a bleed monitoring computer closes the valves and alerts the crew if limits are exceeded.

Distribution, leak detection, indication and interfaces

• Ducts are thin-wall titanium or stainless steel, insulated, with expansion joints (bellows/compensators) and V-band couplings. A cracked duct or loose coupling releases air at 200 °C or more into wing, pylon or body areas — so bleed-leak detection loops (overheat sensing elements like those of fire detection, Chapter 11) run along every duct and automatically isolate the leaking section and warn the crew.
• Isolation (cross-bleed) valves separate or join the left and right manifolds; APU and ground air reach both sides through them.
• Indications: duct pressures, valve positions on the bleed/air system page, and cautions or warnings for bleed fault, overheat, overpressure and duct leaks.
• Users (interfaces): air-conditioning packs and pressurisation (Chapter 6), wing and engine anti-ice (Chapter 15), air turbine starters, hydraulic reservoir and potable water pressurisation, cargo heating, the nitrogen generation system (Chapter 13) and air-driven hydraulic pumps (Chapter 14).

Light-aircraft vacuum and pressure systems

• An engine-driven vane pump creates suction; air enters through a central filter, flows through the gyro instruments (driving their rotors) and a regulator (relief) valve, which holds the suction at about 4.5–5.5 inHg as shown on the suction gauge.
• Dry pumps have carbon vanes and need no lubrication, but fail suddenly if contaminated by oil, solvents or debris — never use oil-based fluids near them. Older wet pumps are oil-lubricated and need an oil separator in the outlet.
• Very old or simple aircraft used venturi tubes on the fuselage side to create suction (a small venturi for the turn indicator, a larger one for the attitude and heading gyros).
• Pressure systems blow air through the instruments with the pump's outlet instead; they work better at high altitude, where suction from a vacuum pump is weaker. The pump's pressure side can also inflate de-icing boots.
• Low suction warning lights or gauges alert the pilot, because attitude and heading gyros slowly become unreliable after a pump failure.

High-pressure pneumatic systems and pumps

Some aircraft, notably the Fokker F27/F50 family, use engine-driven multi-stage piston compressors charging storage bottles (reservoirs) to around 3 000 psi or more, with moisture separators, chemical driers, pressure regulators and relief valves, to operate the landing gear, brakes and nose-wheel steering. Many hydraulically powered aircraft also carry nitrogen or air bottles for emergency gear extension or emergency braking. Pumps in pneumatic systems are therefore of two families: vane-type pressure and vacuum pumps for low-pressure duties, and piston compressors for high-pressure duties.

Part 2 — Water and waste (ATA 38)

Potable water

• Tanks — usually filament-wound composite tanks under the cabin floor, pressurised by bleed air or a small compressor (typically about 25–35 psi) to push water to the users.
• Distribution — lines to the galleys (water boilers, coffee makers) and lavatories (taps and small water heaters, toilet rinse valves). Lines and fittings in cold areas, drain masts and service panels are electrically heated; filters or UV units may treat the water.
• Grey water from basins and galley sinks drains overboard through heated drain masts, pushed by cabin differential pressure in flight.
• Servicing — the tank is filled at a potable water service panel (fill/overflow valve, quantity indication) from a dedicated water truck. When the aircraft will stand in freezing conditions without heating, the system is drained to stop pipes and tanks bursting. Water systems are disinfected periodically to the operator's and health authority's procedures, and samples are tested.

Toilet systems

• Vacuum toilets (modern transports) — pressing FLUSH opens a flush valve, a short rinse of potable water cleans the bowl, and the pressure difference between cabin and outside pulls the waste through pipes to a waste tank. On the ground and at low altitude (below about 16 000 ft), a vacuum generator (blower) supplies the suction. Level sensors stop the toilets being used when the tank is full.
• Recirculating (chemical) toilets (older aircraft) — each toilet has its own tank of blue flushing fluid; a motor-pump unit filters and recirculates it for each flush.
• Servicing — waste tanks are emptied through a waste service panel into a lavatory service vehicle, rinsed, and pre-charged with disinfectant. Potable water and waste equipment, hoses and vehicles must never be interchanged, and staff wear gloves and eye protection. Leaking waste drain valves can form "blue ice" on the fuselage, which can break off — the panel's valves and seals are checked for leaks.

Corrosion