Aircraft Rain Control and Removal Systems

Aircraft rain control systems improve pilot visibility during wet weather operations by removing water from the windshield and preventing visual distortion. Various mechanical, pneumatic, chemical, and surface treatment methods are used to maintain clear forward vision during flight.

There are several different ways to remove rain from the windshields. Most aircraft use one or a combination of the following systems: windshield wipers, chemical rain repellent, pneumatic rain removal (jet blast), or windshields treated with a hydrophobic surface seal coating.

Windshield Wiper Systems

In an electrical windshield wiper system, the wiper blades are driven by an electric motor(s) that receive power from the aircraft’s electrical system. On some aircraft, the pilot’s and copilot’s windshield wipers are operated by separate systems to ensure that clear vision is maintained through one of the windows should one system fail.

Each windshield wiper assembly consists of a wiper, wiper arm, and a wiper motor/converter. Almost all windshield wiper systems use electrical motors. Some older aircraft might be equipped with hydraulic wiper motors. [Figure 1]

Figure 1. Windshield wiper assembly/installation on a transport category aircraft. The motor-converter is mounted under the aircraft skin

Maintenance performed on windshield wiper systems consists of operational checks, adjustments, and troubleshooting. An operational check should be performed whenever a system component is replaced or whenever the system is suspected of not working properly. During the check, make sure that the windshield area covered by the wipers is free of foreign matter and is kept wet with water. Windshield wiper system adjustments include setting the wiper blade tension, sweep angle, and proper parking position of the blades.

Chemical Rain Repellent

Water poured onto clean glass spreads out evenly. Even when the glass is held at a steep angle or subjected to air velocity, the glass remains wetted by a thin film of water. However, when glass is treated with certain chemicals, a transparent film is formed that causes the water to behave very much like mercury on glass.

The water forms into beads that cover only a portion of the glass and the area between beads is dry. The water is readily removed from the glass. This principle lends itself quite naturally to removing rain from aircraft windshields. The high-velocity slipstream airflow continually removes the water beads, leaving a large part of the window dry.

A rain repellent system permits application of the chemical repellent by a switch or push button in the flight deck. The proper amount of repellent is applied regardless of how long the switch is held. On some systems, a solenoid valve controlled by a time delay module meters the repellent to a nozzle which sprays it on the outside of the windshield. Two such units exist — one unit for each forward windshield panel of the pilot and copilot. [Figure 2]

Figure 2. Cockpit rain repellant canister and reservoir

This system should only be used in heavy rain conditions. The rain repellent system should not be operated on dry windows because heavy undiluted repellent restricts window visibility. Should the system be operated inadvertently, do not operate the windshield wipers or rain removal system as this tends to increase smearing. Also, the rain repellent residues caused by application in dry weather or very light rain can cause staining or minor corrosion of the aircraft skin.

To prevent this, any concentrated repellent or residue should be removed by a thorough fresh-water rinse at the earliest opportunity. After application, the repellent film slowly deteriorates with continuing rain impingement. This makes periodic reapplication necessary. The length of time between applications depends upon rain intensity, the type of repellent used, and whether windshield wipers are used.

Windshield Surface Seal Coating

Some aircraft models use a surface seal coating, also called a hydrophobic coating, on the outside of the pilot’s/ copilot’s windshield. [Figure 3]

Figure 3. Hydrophobic coating on windshield

The word hydrophobic means to repel or not absorb water. The windshield hydrophobic coating is on the external surface of the windows (windshields). The coatings cause raindrops to bead up and roll off, allowing the flight crew to see through the windshield with very little distortion. The hydrophobic windshield coating reduces the need for wipers and gives the flight crew better visibility during heavy rain.

Most new aircraft windshields are treated with surface seal coating. The manufacturer’s coating process deeply penetrates the windshield surface providing hydrophobic action for quite some time. When effectiveness declines, products made to be applied in the field are used. These liquid treatments rubbed onto the surface of the windshield maintain the beading action of rainwater. They must be applied periodically or as needed.

Pneumatic Rain Removal Systems

Windshield wipers characteristically have two basic problem areas. One is the tendency of the slipstream aerodynamic forces to reduce the wiper blade loading pressure on the window, causing ineffective wiping or streaking. The other is in achieving fast enough wiper oscillation to keep up with high rain impingement rates during heavy rainfall. As a result, most aircraft wiper systems fail to provide satisfactory vision in heavy rain.

The rain removal system shown in Figure 4 controls windshield icing and removes rain by directing a flow of heated air over the windshield. This heated air serves two purposes. First, the air breaks the raindrops into small particles that are then blown away. Secondly, the air heats the windshield to prevent the moisture from freezing. The air can be supplied by an electric blower or by bleed air.

Figure 4. Windshield rain and frost removal system

Frequently Asked Questions

Why should chemical rain repellent never be used on a dry windshield?

Chemical repellent is highly concentrated and designed to be diluted by heavy rain. If applied to a dry window, the undiluted liquid creates a thick, smeared residue that severely restricts visibility. If inadvertently applied, you should rinse the area thoroughly with fresh water rather than using wipers, which would worsen the smearing.

What is a hydrophobic surface seal coating and how does it work?

A hydrophobic coating is a surface treatment that repels water at a molecular level. It causes raindrops to bead into small spheres that are easily carried away by the slipstream. This significantly reduces the need for mechanical wipers and provides a clearer, less distorted view during heavy rainfall.

How does a pneumatic (jet blast) rain removal system operate?

This system uses high-velocity heated bleed air directed across the outside of the windshield. The air blast breaks raindrops into tiny particles and blows them away before they can impact the glass. The heat also serves as a secondary anti-icing measure to prevent moisture from freezing on the pane.

What are the primary maintenance requirements for electrical wiper systems?

Maintenance involves regular operational checks (ensuring the glass is kept wet during the test), and adjusting the blade tension, sweep angle, and park position. Technicians must ensure the blades park completely out of the pilot's line of sight to prevent visual obstruction during clear weather.

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