Module 11B.16–11B.17 — Piston aeroplanes: pneumatic/vacuum systems and water/waste

Levels

Sub-module	A2	B1.2
11.16(a) Pneumatic systems: layout, sources, pressure control, distribution, indications, interfaces	1	3
11.16(b) Pressure and vacuum pumps	1	3
11.17(a) Water and toilet systems	2	3
11.17(b) Corrosion aspects	2	3

11.16 Pneumatic / vacuum systems (ATA 36)

On piston aeroplanes the "pneumatic system" usually means the instrument air (vacuum or pressure) system driving gyros, plus the de-icing boot air supply. On turbocharged aircraft, compressed induction air is also used for pressurisation (see 11B.4).

Sources

Source	Description
Engine-driven dry-vane pump	Carbon rotor and vanes in a steel/graphite housing; self-lubricating; driven from the accessory case through a shear (drive) coupling that breaks to protect the engine if the pump seizes
Engine-driven wet-vane pump	Steel vanes lubricated by engine oil; outlet air carries oil → oil separator returns oil to the engine sump; the clean outlet air can be used for boots
Venturi tubes	On the fuselage side; produce suction in flight only; no ground operation — older/simple aircraft
Engine intake manifold suction	Used as standby source through a check/shuttle valve on some aircraft (only effective at reduced power)
Electric standby pump	Backup on some modern installations
Ground supply	Not normally used; tests use a vacuum test set

Vacuum (suction) system layout — typical single-engine

1. Central (inlet) air filter — paper element cleaning cabin air before it enters the gyros.
2. Gyro instruments — air jets spin the rotors (attitude indicator, heading indicator).
3. Suction relief (regulator) valve — admits cabin air to limit suction to the set value; has its own foam garter filter.
4. Suction gauge — between instruments and pump; typical 4.5–5.5 inHg (some instruments 4.8–5.2) — read at the specified rpm.
5. Vacuum pump — exhausts overboard (dry) or via oil separator (wet).

In pressure systems the pump outlet pushes filtered air through the gyros instead (with an inline filter and pressure regulator), useful at high altitude where vacuum systems lose effectiveness.

Twin-engine systems

Two pumps (one per engine) connected through check valves (often in a manifold check valve) so the failed pump's side is isolated and the remaining pump powers the instruments; failure indicators (red buttons on the gauge or annunciator) show which source has failed.

Pressure control and indication

• Regulator adjusted at the specified engine rpm during maintenance checks.
• Low-vacuum annunciator (pressure switch) on many aircraft.
• Gyro failure symptoms: slow erection, tumbling, drifting heading indicator — check suction, filters and pump condition.

Pumps — failure and maintenance (11.16(b))

Point	Detail
Dry pump failure	Usually sudden (vane/rotor fracture) without warning — the reason for standby sources and electric turn coordinators
Contamination	Oil or solvent entering a dry pump destroys it — never clean with solvents or allow oil from leaking seals to reach it
Installation	Correct gasket, no thread sealant debris in fittings, correct drive rotation, flush lines and replace filters when a pump fails (carbon debris)
Life	Some manufacturers set a recommended replacement time
Wet pumps	Check oil separator function and drain line

Interfaces with other systems

• De-icing boots: pump outlet pressure inflates boots via a timer/distribution valve; suction holds them down.
• Pressurisation door seals and some autopilot servos (older pneumatic autopilots) used vacuum/pressure.
• Indicators: boot inflation gauge/light.

Key fact: Dry-vane vacuum pumps fail suddenly and are destroyed by oil or solvent contamination; a failed pump should be followed by filter replacement and line flushing.

11.17 Water / waste (ATA 38)

11.17(a) Systems on piston aeroplanes

Most light piston aircraft have no fixed water/waste system. Larger piston twins and some business piston aircraft may have:

• Potable water: small tank, electric pump or pressurised container, cabin sink/refreshment unit; drain valve; freeze protection by draining during cold parking.
• Toilets: chemical (portable or self-contained) toilets with a pre-charge of disinfectant fluid; flush by electric pump (recirculating) or manual; removable waste tank for servicing.
• Relief tubes (older types) venting overboard — risk of corrosion at the outlet.
• Servicing: drain and rinse waste tanks, refill pre-charge; potable water disinfection; drain lines heated or drained in winter.

11.17(b) Corrosion aspects

• Waste and chemicals (urine is highly corrosive to aluminium alloys and magnesium) leaking into floor structure, seat rails and lower skin.
• Water from galleys/sinks pooling in the bilge — keep drain holes clear and floor sealing intact.
• Protection: corrosion-inhibiting compounds, sealed floors and liners, prompt clean-up of spills with approved cleaners, inspection of structure under toilets and sinks during scheduled checks.
• Outflow points (drain masts, relief tube outlets) cause local skin corrosion and staining — inspect and treat.

Key fact: Areas under toilets, sinks and relief tube outlets are corrosion hot-spots — keep sealing and drainage intact and clean spills immediately.