by Captain Joe
Let’s imagine you´re sitting in your car, and as you get onto the motorway it starts pouring with rain. One little move with your finger and you are windscreen wipers comes on, and you have a clear sight onto the road again. But how about airplanes, how do they get rid of the rain?
Put yourself into your passenger seat and imagine that we are slowly taxing towards the runway and it´s raining heavily. A passenger jet taxi´s (straight) with a speed at about 20 – 35 mph (30-60 km/h), which is not fast enough for the air stream to clear the windshields from raindrops.
The visibility reduces dramatically which can be dangerous on airport aprons. So with one little hand movement, like in your car, every pilot can switch on his windscreen wiper. There is one wiper on each side which can be turned on separately and has two speed settings, slow and fast.
During a rainy take-off roll we keep the wipers switched on, cause as you gain speed a lot of the water from the nose cone runs over the windshields and the visibility is reduced even more, so you have the option to increase the wiper speed . Similar as you go along the motorway at 60 mph, you may use wiper speed „1“, but if you accelerate to 100 mph, at the same amount of rain, you’re gonna have to adjust your wiper speed.
Okay let’s imagine we are in flight:
During en route cruise the wipers are inoperative. They are aerodynamically and mechanically limited to the speed of the aircraft so that the switches are inhibited. The limit varies between aircraft manufactures. The limiting speed for example on an Airbus A320 is 230 kts (260 mph). On final approach (approx 20 miles from the runway) you fly at speeds of 260 mph, which is fast enough to clear the windshields from rain.
But three miles prior to landing you will have reduced the speed down to 140 kts and in heavy rain the wipers are absolutely essential to get clearly visibility onto the runway.
In the event that one wiper is inoperative, may only the pilot with the functional wiper fly the aircraft. In the event that both wipers are inoperative, which is very unlikely, the aircraft is limited such as it may only take-off and land in visual meteorological conditions and it´s capability to land within certain approach minimas (height and visibility) is reduced and limited.
Single engine airplanes don’t necessarily need wipers cause the propeller air stream is strong enough to clear the windshield of precipitation and most of them are only certified to fly in visual meteorological conditions anyways.
Many aircraft manufacturers also installed a switch or a button which allows you to spray “rain repellent” onto the windscreen in heavy rain, similar to your wiper washer. Unfortunately this liquid was banned in many countries due to its highly chemical substances so that on most airplanes this switch is set inoperative.