Water and Waste Systems practice questions

Answer: A) Air pressure in the tank, supplied by bleed air or an onboard air compressor

Potable water systems on transport aircraft push water to the galleys and lavatories by pressurizing the tank with air from bleed air or an onboard air compressor. A relief valve protects the tank.

• ACorrect. The potable water tank is pressurized with air, either bleed air or an onboard air compressor. A pressure relief valve protects the tank from overpressure.
• BIncorrect. Hydraulic fluid systems are not used to pressurize potable water, because of the contamination risk and system separation.
• CIncorrect. The vacuum blower creates suction to move waste into the waste tank. It does not pressurize the potable water supply.
• DIncorrect. Nitrogen bottles are used to charge items such as struts and accumulators. Potable water tanks are pressurized with air from bleed air or a compressor.

Answer: A) To keep draining water from freezing at the mast and building hazardous ice

Drain masts are heated so gray water drained overboard does not freeze, block the mast, or form hazardous ice, as 14 CFR 25.1455 requires.

• ACorrect. Gray water drained overboard can freeze at altitude, blocking the drain or forming ice that may break off and strike the airplane. 14 CFR 25.1455 requires drains for fluids that can freeze to prevent hazardous ice from forming.
• BIncorrect. The mast heater only keeps the outlet free of ice. The water still drains overboard as liquid and is not boiled off.
• CIncorrect. Drain mast heat is for anti-icing, not for treating water. Gray water is not sterilized before it is drained.
• DIncorrect. Toilet waste stays in the waste tank until ground servicing. Drain masts carry gray water from sinks and galleys, not tank contents.

Answer: C) Frozen lavatory waste leaking outside

• AIncorrect. That is not blue ice.
• BIncorrect. That is normal icing.
• CCorrect. Leaking drain valves can let fluid freeze on the fuselage.
• DIncorrect. That is fuel icing.

Answer: A) Leaking fluid can freeze into blue ice that breaks loose and damages engines, airframe or property below

A leaking lavatory dump valve, drain valve or service panel cap can let waste fluid freeze on the outside of the fuselage as 'blue ice'. That ice can break loose and damage engines or the airframe, or endanger people and property on the ground.

• ACorrect. FAA airworthiness directives on lavatory drain systems address leakage at the service panel. Such leakage forms 'blue ice' that can break loose and cause engine damage, airframe damage, or a hazard to people or property on the ground.
• BIncorrect. Waste and potable water systems are kept separate and do not share a fill line. The hazard of service panel leakage is ice forming outside the aircraft.
• CIncorrect. Heated drain masts discharge gray water so that ice does not form (see 14 CFR 25.1455). Toilet waste leaking at the service panel does not pass through the drain mast.
• DIncorrect. The cap and seals are there to stop leakage, not to hold disinfectant against the fitting. The hazard the FAA identifies is blue ice, not corrosion.

Answer: A) Indicate waste quantity and stop toilet flushing once the tank reaches full

Sensors on a vacuum waste tank tell the crew how full the tank is. When it is full, they stop further flushing so the tank cannot overfill.

• ACorrect. The sensors give a quantity or full indication to the crew and servicing panel. When the tank is full, they stop the toilets from flushing so the tank cannot overfill.
• BIncorrect. A barometric or altitude switch controls the vacuum blower, because cabin-to-ambient pressure difference provides the vacuum at altitude. The tank sensors do not do this.
• CIncorrect. The flush control unit sequences the rinse valve when the flush switch is pressed. That timing does not depend on the tank sensors.
• DIncorrect. Waste tanks are emptied through the servicing panel on the ground. They are not dumped overboard in flight.

Answer: B) The difference between cabin pressure and the lower pressure in the vented waste tank

Vacuum toilets move waste using the pressure difference between the cabin and the overboard-vented waste tank. At low altitude or on the ground, a vacuum blower supplies this difference.

• AIncorrect. Vacuum toilets do not need a pump at each toilet to push waste. The waste is drawn through the line by lower pressure at the tank end.
• BCorrect. The waste tank is vented overboard. At altitude, the pressurized cabin is at a higher pressure than the tank, and this difference pulls waste through the line when the flush valve opens. On the ground and at low altitude, a vacuum blower creates the difference.
• CIncorrect. Potable water system pressure only supplies the small amount of rinse water used in the bowl. It does not move waste to the tank.
• DIncorrect. Vacuum systems are used because waste must travel long, often horizontal or rising lines to a remote tank, which gravity cannot do.

Answer: C) Through a fill fitting on the exterior potable water service panel

Potable water tanks are filled from a water truck or cart through a fill fitting on an exterior potable water service panel. This panel is separate from the lavatory waste service panel.

• AIncorrect. The lavatory waste service panel is used to drain and rinse the waste tank. Potable water is serviced at a separate panel to prevent contamination.
• BIncorrect. Galley and lavatory faucets are outlets of the distribution system, not fill points. Tanks are not serviced by back-feeding through them.
• CCorrect. A water truck or cart hose connects to the fill fitting on the exterior potable water service panel. An overflow or fill indication shows when the tank is full.
• DIncorrect. Condensate from the water separator is used for cooling or drained overboard. It is not a potable water source.

Answer: A) The heating element can overheat because no water surrounds it to absorb the heat

Water heaters depend on surrounding water to carry heat away from the element. If one is powered with the system empty, the element can overheat and burn out.

• ACorrect. The water around the element carries its heat away. When the heater is powered dry, the element temperature rises quickly and it can burn out or damage nearby wiring.
• BIncorrect. A powered heater adds heat, so it cannot cause freezing. Freezing is the reason for draining the system, not the reason for keeping the heater off.
• CIncorrect. A water heater's thermostat or overtemperature switch controls the heater element. It does not control the water pump.
• DIncorrect. A heater does not pump or draw air. Air in drained lines is removed during refill through the system vents and faucets.

Answer: B) A manually operated drain valve reached through the exterior service panel

The waste tank is emptied during ground service by opening a manually operated drain valve at the exterior service panel. The drain port is also sealed by a cap.

• AIncorrect. Drain masts discharge sink and galley gray water. Toilet waste is held in the tank and removed by the ground service cart, not dumped overboard.
• BCorrect. The service cart connects at the service panel, and the drain valve is opened to empty the tank. This manual valve can only be operated on the ground and is backed up by a sealing cap on the drain port.
• CIncorrect. The vacuum blower creates the pressure difference that moves waste into the tank at low altitude. It does not empty the tank during servicing.
• DIncorrect. The flush motor and its timer run the flush cycle at the bowl. They do not open the tank for draining.

Answer: A) A level-sensing probe in the tank sends a signal to quantity indicators at the service panel and attendant panel

Airliner potable water quantity comes from a tank level sensor that drives indicators at the water service panel and the attendant panel. Any measuring device must not let contamination into the tank.

• ACorrect. A tank level sensor drives remote quantity indication. The ground crew uses it while filling at the service panel, and flight attendants see it on the attendant panel.
• BIncorrect. The tank is pressurized to a regulated pressure to move water to the faucets. That pressure stays about the same whatever the water level, so it cannot show quantity.
• CIncorrect. Putting a dipstick into the potable tank would risk contamination. Sanitation guidance requires any quantity-measuring device to be built so contamination cannot get into the tank.
• DIncorrect. Airliner potable tanks are monitored remotely through a sensor and indicator, so nobody needs to open access panels and look at the tank.