What is Wing Mist?

While I started a series a little while ago on the inner workings of an airplane, I think it might be time to pick back up on the intricacies of aircraft, how they work and more.

Hundreds of millions of passengers take to the skies every year, and most people are not that familiar with the craft they are flying on or how any part of it works. Nowhere was this more apparent to me than during my last flight from London to Chicago, where I was upgraded to business class for no cost. After a quick taxi to the runway, our 777-200 lined up for takeoff. Outside of the aircraft was a fairly typical London afternoon… so in other words it was wet, and the air was full of moisture.

Hearing the engines spool up, I prepared for takeoff. Lumbering forward, our giant plane gained speed and rotated into the air. As the aircraft pulled back, a stream of mist came off the top of the engine near the support strut. As soon as this began to happen, from the back of the business class cabin came the voice of a surprised woman. She was worried, asking what was happening to the engine. While she was surprised and seem slightly worried, she didn’t freak out, most likely in part due to the rest of the passengers paying no attention to it.

Personally, I was a bit surprised to hear this from a business class passenger. While I know some inexperience travellers sometimes use the cabin, usually business class is filled with very experienced travellers who have seen this happen many times. It does make a point to me though that there are people who don’t have my background in Aerospace Engineering, who have no idea what's going on outside that window.

If you travel long enough, you will see this quite often. Not only does this happen over the engine, but more commonly over the entire wing. Usually the air needs to be quite moist in order for this to happen, but it is something you will see pretty frequently.

So what is happening here?

Mist clouds forming over a wing, in an engine or sometimes over the body of an aircraft, is due to a process called the Prandtl-Glauert singularity, or the Prandtl-Glauert effect. In essence, we are creating a cloud over the wing due to the moist air. When air flows over a wing, it flows faster over the top surface than the bottom, this is how a plane lifts into the air. When the aircraft pulls back on an accent, or pulls back to increase lift during a landing, the air over the wing sees an immediate and increased reduction in air pressure.

The rapid reduction in pressure over the wing causes moist air to rapidly lower in density. When this occurs, the temperature of the air over the wing drops for a split second. Temperature drops cause the moist air to condense into a cloud. This is very much how a normal cloud forms, water rises in the air until the temperature lowers to a certain point, and then the vapor condenses into a standard cloud. The same thing is happening on the wing, just rapidly and due to a quick pressure drop in the air.

So the next time you’re flying into or out of a rainy city, take a look out the window and see if you can catch a cloud forming on the wing. It’s not dangerous in any way, but to those who don’t know what it is, it can be startling. No need to fret, everything's shiny captain.