Masts are typically associated with marine vessels like ships and boats. Consisting of vertical posts, they are used to hold sails or equipment. Airplanes may not have these same vertical posts, but many of them do have drain masts. What are drain masts exactly, and why do airplanes have them?
Overview of Drain Masts
Drain masts are small, short protrusions on an airplane that provide an escape point for water and fluids. They are typically found on the underside of the fuselage, near the forward and aft sections. Rather than accumulating within the airplane, fluids will drain safely out through the drain masts. They are known as “drain masts” because they consist of short posts that stick out of the belly of an airplane where they provide a drain point.
How Drain Masts Work
Drain masts are designed to release a variety of fluids, including condensation, grey water and hydraulic fluid. Condensation consists of water produced by air conditioning systems and other equipment. Not to be confused with blackwater, grey water consists of wastewater from gallery sinks and lavatories. Hydraulic fluid is a type of fluid used to transfer power to hydraulic systems like flight control surfaces, landing gear and flaps.
Most drain masts work in conjunction with a valve. An air stop valve is connected to each drain mast. These valves remain closed until sufficient pressure has built up. As fluid builds up behind an air stop valve, pressure will increase. Upon reaching the pounds per square inch (PSI) for which the valve is rated, the valve will open, thereby releasing the fluid.
Why Airplanes Have Drain Masts
Drain masts serve several purposes, one of which is protection from ice buildup. Water and condensation can freeze and turn to ice. If left unchecked, all this ice can change the airplane’s aerodynamic properties while simultaneously adding unnecessary weight to it.
Drain masts help to protect airplanes from ice buildup by automatically draining condensation. Many drain masts, in fact, are heated. They feature electric heating elements that warm the fluid as it passes through the drain mast. Therefore, there’s little or no risk of the fluid freezing within the drain mast.
Drain masts also help to regulate cabin pressure. Cabins are typically pressurized to the equivalent of 6,000 to 8,000 feet. Even when flying at 30,000 feet, the cabin pressure “feels” like 6,000 to 8,000 feet. Drain masts feature valves that only open when sufficient fluid has collected behind them.
