Aircraft lavatory systems are equipped with a vacuum generator that creates differential pressure. This differential pressure forces waste from the toilet bowl to the waste storage tank.
Let’s explore the inner workings of an Aircraft Lavatory System for better understanding. The toilet system is a sub-system of the water and waste system (ATA 38) in any airplane.
Water and Waste System of Aircraft A320
The water and waste system supplies potable water to the lavatories and galleys of an airplane; and removes the sink and toilet waste.
Potable water is stored in a tank capacity of 200 liters tank in an airplane.
The airplane has two drain masts. The wastewater drains overboard through these two anti-iced drain masts. This wastewater comes from the aircraft galleys and from the lavatories sinks. The forward mast drains wastewater from the forward cabin and the aft drain mast drains wastewater from the aft cabin.
Differential pressure (created by the vacuum generator) discharges the wastewater from the toilet bowl to the waste storage tank in flight, and gravity does so on the ground. Differential pressure created by the vacuum generator forces the waste from toilet bowls into the waste storage tank of aircraft.
A vacuum generator produces the necessary pressure differential while the aircraft is on the ground, and at altitudes below 16000 ft.
The waste system uses clear water from the potable water system to flush the toilets.
Waste tank in the airplane has a usable capacity of 170 liters.
A water heater is installed under the washbasin in each toilet unit to supply hot water to the water tap.
The wastewater is discarded outside through the drain valve (air stop valve) and the heated drain mast. Air stop valves avoid constant aircraft cabin depressurization by opening only when a certain amount of water is collected.
Toilet System Operation in Airbus A320 Aircraft
The overall toilet system operation in Airbus A320, monitoring, and fault indication are controlled by the Vacuum System Controller Function (VSCF) integrated in the Cabin Intercommunication Data System (CIDS). It is connected to the Centralized Fault Display System (CFDS).
When the toilet flush switch is pressed, the Flush Control Unit (FCU) starts the flush sequence. The rinse valve and the flush valve open in sequence, controlled by the FCU, to evacuate the waste material. At the same time, the Flush Control Unit (FCU) sends a signal to the VSCF, which will operate the vacuum generator to generate differential pressure.
The vacuum generator creates the necessary differential pressure between the cabin and the waste holding tank to move the waste from the toilet bowl. Above 16,000 ft, the vacuum generator will not be started by the VSCF as the differential pressure is sufficient.
Aircraft Water Service
Water Servicing Cart is used to fill the water in the airplane. The water servicing panel in the airplane contains a fitting for the attachment of a fill hose. Once the hose is attached, a fill valve is positioned to allow the water to flow into the tank. The quantity indicator on the panel is used to fill the tank. An overfill drain line is plumbed from the top of the tank to the aircraft exterior. When the water reaches the level of the overfill fitting it spills into the overfill line and is drained overboard. Often, tanks are filled until water comes out of the overflow.
Aircraft Lavatory Service
Lavatory Service Cart is used to drain the waste from the aircraft waste tank. The aircraft waste tank is drained by pulling the waste drain valve handle on the toilet servicing panel after attaching the ground lavatory service cart to the 4-inch drain outlet and removing the drain plug. The tank is cleaned by attaching water pressure to the ground flush connection.
How does the pressurization of the aircraft affect the waste system? Can a plane be pressurized if the 4-inch waste dump valve is stuck open? This is an issue with a Pilatus sitting on our ramp today. The valve handle is stuck open.