Potable Water and Drain Systems
The potable water system stores drinking water on board and delivers it, under air pressure, to the galleys and lavatories. Used water from sinks and wash basins is discharged overboard through heated drain masts, while separate engine drain masts carry small leaks of fuel, oil and hydraulic fluid out of the nacelles.
An airliner carries its own supply of drinking water. The potable water system stores it in a tank, pressurises it with air and delivers it to the galley taps and the lavatory wash basins and, on types such as the Boeing 737 NG, the toilets. The water that runs down the sinks then leaves the aircraft through heated drain masts under the fuselage. The engines have drain masts of their own, which carry small leaks of fuel, oil and hydraulic fluid overboard and give the walk-round inspection a useful clue.
None of this is flight-critical in the way that the fuel or flight control systems are, but it touches the pilot's world in several places. The water tank is a consumer of the pneumatic system, the water on board counts in the dry operating mass, the drain masts must be heated to stay clear, the water and drain heating appear among the electrical loads the crew can shed, and an engine drain mast that drips fuel during the start has its own procedure.
Potable water system
Potable means fit to drink. The water is held in a tank in the fuselage and must reach outlets throughout the cabin. Rather than pumping it, jet transports typically pressurise the tank with air. ATPL texts describe a tank pressurised with air from the pneumatic system, which drives the water through the distribution pipes to the galley sinks and lavatory taps without separate water pumps; a regulator and a relief valve keep the pressure within limits, and the water is filtered before delivery.
The water system therefore appears among the consumers of the aircraft's bleed air (see bleed air and pneumatic systems). Three types show the variations:
- Boeing 737 NG: one tank, behind the aft cargo compartment, pressurised by engine bleed air or by a water system air compressor.
- Airbus A320: water pressurisation is one of the five consumers of the pneumatic system, alongside the packs, engine starting, wing anti-ice and hydraulic reservoir pressurisation.
- Embraer E190: the E1 carries 90 l, with the quantity shown only on the aft flight attendant panel; the E2 shows the level on both the forward and aft panels, and its pneumatic system supplies water system pressurisation.
From the tank the water runs to each galley and lavatory. On the 737 NG, shutoff valves are fitted on each galley and below the sink in each lavatory, so that each outlet can be isolated on its own. The lavatory water supply shutoff valve has four positions: SUPPLY ON, FAUCET ONLY, OFF and TOILET ONLY. When the lavatory supply lines are drained, the valve must be at SUPPLY ON, or the water will not drain from the tap or toilet.
Hot water comes from small electric heaters. The 737 NG's lavatory water heater heats a fresh charge of water in about four minutes after it has been emptied; its amber light is on when it is operating normally, and an overheat switch turns off the heating element if the temperature becomes excessive.
Exam tip: potable water tank pressurisation and hydraulic reservoir pressurisation are both bleed air consumers, and potable water and lavatory chemicals belong to the dry operating mass, not to the traffic load.
The drinking water has one further use on some long-haul aircraft: a cabin humidifier atomises it into the air conditioning supply to raise the very low cabin humidity (see cabin air distribution).
Electrical loads and aircraft mass
The electrical loads of the water and drain systems, such as water heaters, the 737's potable water compressor and the drain mast heaters, are cabin loads. On both the A320 and the 737 NG they are grouped with the galleys and other cabin equipment under a single switch.
- A320: the COMMERCIAL pushbutton supplies the cabin and cargo lights, the water and toilet system, drain mast ice protection, the galleys, passenger entertainment and semi-automatic cargo loading. Selecting it OFF removes power from all of them, but not from flight deck loads such as the instrument lighting or the avionics fans.
- 737 NG: the CAB/UTIL switch at OFF removes power from cabin and utility loads including the 115 V AC galley buses, the recirculation fans, the forward and aft door area heaters, the drain mast heaters, the lavatory water heaters, the logo lights, the potable water compressor and the shaver outlets.
The water also has mass. Potable water and lavatory chemicals are part of the dry operating mass, together with the crew and their baggage, catering and removable passenger service equipment, and they are included in the dry operating index from which the load and trim sheet starts (see aircraft mass definitions).
Waste water and drain masts
Water from the galley sinks and lavatory wash basins is not kept on board. On the 737 NG this waste water is drained overboard through two heated drain masts on the bottom of the fuselage, one forward and one aft. A drain mast, called a waste water outlet horn or drain horn in some ATPL texts, is a small projection under the skin through which the water is discharged overboard.
The drain system has its own valves. On the 737 NG a drain valve in the forward lavatory must not be opened in flight.
Lavatories and galleys also need ventilation. Their air carries odours and moisture, so it is not recirculated: on the A320 an extraction fan, running whenever electrical power is available, draws cabin air through the lavatories and galleys and exhausts it near the outflow valve. Lavatory smoke detection and the automatic extinguishers in the waste bins are covered in cargo and lavatory fire protection.
Drain mast heating
At cruising levels the outside air is far below freezing, and water leaving a drain mast would freeze in the outlet and block it. ATPL texts list waste water outlet horns among the parts of an aircraft that need ice protection, and, like other small protrusions such as aerials and probes, they are protected by electrical heating rather than hot bleed air (see ice protection and ice detection). Where the heating runs whenever power is available, as on the A320, it works as anti-icing: the outlet is kept clear rather than freed of ice once it has formed.
The drain mast heater is not always controlled like the probe heaters:
- A320: the drain masts are heated whenever electrical power is available, not through the probe heat controls. The heating runs at a low level on the ground, to avoid injury to ground personnel, and at a high level in flight. Drain mast ice protection is one of the loads of the COMMERCIAL pushbutton.
- 737 NG: both fuselage drain masts are heated, and the drain mast heaters are among the loads removed by the CAB/UTIL switch.
Exam tip: drain masts, probes and windscreens are heated electrically; wings and engine intake lips on jets are anti-iced with hot bleed air. On the A320, drain mast heating does not depend on the probe heat selection.
Engine and nacelle drains
Every engine nacelle contains seals and components that can weep small quantities of fluid. Rather than let these collect inside the cowlings, the nacelle drains lead them to an engine drain mast: on the A320, a small aerodynamic fairing on the underside of the nacelle, typically at the aft lower part, which drains fuel, oil and hydraulic fluid overboard.
What comes out is a clue to the engine's health:
- traces of fuel or oil during and after operation can be normal;
- how much is acceptable is type-specific: the 737 NG's exterior inspection accepts fluid leaks from the engine drains provided they are less than a continuous stream, so a continuous stream is a matter for maintenance;
- staining around the drain mast is one of the items checked on the walk-round inspection.
The A320 has a normal procedure for a fuel leak at the engine drain mast reported by the ground crew during engine start. The engine is run at idle for 5 minutes. If the leak stops within that time, the aircraft can be dispatched without maintenance action; if it is still present after 5 minutes, the engine is shut down and maintenance investigates the source (see ground handling, pushback and engine start and gas turbine engine fuel system).
Engine drain masts should not be confused with the other drains on an aircraft:
| Drain | What it removes | When it is used |
|---|---|---|
| Fuselage drain masts | Waste water from galley sinks and wash basins | Continuously, heated |
| Engine drain mast | Fuel, oil and hydraulic fluid weeping inside the nacelle | Continuously; checked on the walk-round |
| Fuel tank water drains | Water that has settled at the lowest point of a tank | On the ground, during the fuel contamination check |
| Pitot probe drain hole | Rain or condensation entering the pitot inlet | Continuously |
The fuel tank drains are opened deliberately: a sample is drawn from the low point of each tank into a clear container and checked for water, which is denser than fuel and settles at the bottom, and for sediment or the wrong fuel grade. On light aircraft the pilot does it before flight; on transport aircraft it is normally done by the ground crew (see fuel additives and contamination). The pitot drain hole lets water that enters the probe escape, so that it neither blocks the line nor damages it by freezing (see pitot-static system).
Note: the system details in this article are type-specific. The aircraft's FCOM and the operator's procedures take precedence for servicing limits, valve positions and leak criteria.
Frequently asked questions
How is water supplied to the toilets and galleys on an airliner?
Drinking water is carried in a tank and pushed through the pipes by air pressure rather than pumps. On the Boeing 737 NG a single tank behind the aft cargo compartment is pressurised by engine bleed air or by a water system air compressor, and the A320 and E190-E2 also pressurise their water systems from the pneumatic system. On the 737 NG, shutoff valves at each galley and under each lavatory sink allow each outlet to be isolated.
What is a drain mast on an aircraft?
A drain mast is a small projection under the fuselage or engine nacelle through which liquids are discharged overboard, clear of the aircraft. Fuselage drain masts carry waste water from the galley sinks and lavatory wash basins; the Boeing 737 NG has two, one forward and one aft. Engine drain masts carry small quantities of fuel, oil and hydraulic fluid that collect inside the nacelle. Staining around an engine drain mast is checked during the walk-round inspection.
Why are aircraft drain masts heated?
Waste water leaves the aircraft into air that is usually far below freezing at cruising levels, and a drain mast that ices up stops the water draining. Drain masts are therefore heated electrically, like other small protrusions such as probes and aerials. On the A320 they are heated whenever electrical power is available, at a low level on the ground to avoid injuring ground staff and at a high level in flight, and not through the probe heat controls.
Is fuel dripping from an engine drain mast normal?
Some fluid, or light staining, during and after operation can be normal, because the drain mast carries away small leaks from seals inside the nacelle. Limits are type-specific: the Boeing 737 NG accepts engine drain leaks that are less than a continuous stream. On the A320, if the ground crew report a fuel leak from the drain mast during start, the engine is run at idle for 5 minutes; if the leak persists, it is shut down for investigation.
Does the potable water on board count in the aircraft's mass?
Yes. Potable water and lavatory chemicals are part of the dry operating mass, together with the crew and their baggage, catering and removable passenger service equipment. The dry operating mass and its index are the starting point of the load and trim sheet, so the quantity of water carried is part of the operator's mass and balance figures, while fuel and traffic load are added separately.
Test yourself on Potable Water and Drain Systems
The v1prep banks cover this topic in Aircraft General Knowledge (021), with a worked explanation for every answer. EASA ATPL, PPL, IR and CPL, the FAA written tests and A320/B737 type ratings.
Start practising →Sources and further reading
- EASA, Explanatory Note to ED Decision 2018/001/R, Part-FCL theoretical knowledge learning objectives (021 Airframe and Systems)
- FAA Aviation Maintenance Technician Handbook, Airframe (FAA-H-8083-31B), Chapter 15, Ice and Rain Protection
- EASA Easy Access Rules for Air Operations (Regulation (EU) No 965/2012), definitions and CAT.POL.MAB
- 14 CFR 25.1455, Draining of fluids subject to freezing
Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.