CRJ200 Airline Pilot

The personal experiences, thoughts of an CRJ captain

Wingtip Vortices and Wake Turbulence Explained

Posted by Jeffrey on October 24, 2008

The other night we were flying the CRJ700 into Chicago O’hare (KORD). It was a clear night with a beautiful view of downtown. About 30 miles from final approach, as we were gradually descending to intercept the final approach course, air traffic control advised us that we were 10 miles in trail of a Boeing 777.

“Oh…great!” was my comment.

The whole flight had been relatively smooth, so what was about to happen was going to be a big surprise to me, my crew, and my passengers.

This video shows how wingtip vortices generate. It is simple and to the point:

Now here is a little history about wingtip vortices. If you aren’t familiar with the wake turbulence of a Boeing 777, it generates more wake turbulance than almost all airplanes except for some military airplanes. It is a big, heavy airplane and therefore, generates a lot of lift.

But how is lift generated?

Think about a time when you were taking a shower in a tub with a shower curtain. Has the pesky shower curtain ever kept coming into your leg? You think, “what the heck, is the door open?”

Free Pilots Tip of the Week from PilotWorkshops.com

Well that’s not the problem. The problem is that the “pressure” inside the shower curtain is actually lower than the pressure on the outside of the shower curtain. This happens because the water coming out of the showerhead accelerates the air thus decreasing the pressure in the tub. Therefore, it seems like there is something pushing on the shower curtain from the outside, when it is actually the higher pressure air.

In the 1700’s, Daniel Bernoulli formulated the equation that explains how this happens, but that goes way beyond this blog entry. If you would like to read more about the Bernoulli equation, click here.

But that’s only half the story. Have you ever watched water flowing down a stream with a big boulder in the way? If the boulder is perfectly round, the water “splits” when it hits the rock and some of the water goes to the left of the rock and some of the water goes to the right meeting up on the other side. What we don’t see is that the water accelerates for a few seconds as it goes around the rock because it has to meet up with it’s other half on the other side which also accelerated. And when the water accelerates, the pressure per se, decreases.

Now take a wing.

The bottom of a wing, for all practical purposes and for this discussion, we will say it is flat. The top of the wing will have a bend in it or a camber. As the wind hits the wingtip, some of the air goes over the top and some goes underneath. Since the air mass that was split has to meet at the back of the wing at the same time, the airflow over the top has to accelerate to meet up with the same air mass going under the wing. The accelerated air over the top, because it has been accelerated, has to give something up, and that would be pressure. The result is lower pressure over the top of the  wing and higher pressure under the wing, hence lift. Ta Da!

Wingtip Vortices

Wingtip Vortices

Now this explanation is VERY general in nature. And you should get a good book on aviation that discusses all the aspects of wings and lift to have a thorough understanding of these principles.

OK, now back to wingtip vortices. In a perfect wing, the air flows from the front of the wing to the back of the wing. But we now know that there is low pressure air on top of the wing and high pressure air on the bottom. Well the high pressure air needs somewhere to go because it just can’t meet up with the low pressure air and go on it’s way. The high pressure air travels to where the low pressure air is and vice versa and thus creates the (wingtip) vortices.

So back to my flying story.

There were were, flying along minding our own business. The wind was calm and all of a sudden, a little shudder here, then a big upset there and before we knew it the airplane had lurched more than 20° noseup and banked 30° laterally. The autopilot kicked off and as fast as it came it was gone and once the airplane was stabilized we re-engaged the autopilot and I talked to the passengers and let them know what happened. THEN, five minutes later, IT HAPPENED AGAIN! Not my day! I ended up hand-flying the rest of the flight.

Note: One of the most fatal results on record of wake turbulence that resulted in a crash was American Airlines flight 587 on November 12, 2001. To read more about this accident, click on the NTSB report here. See if you can figure out what actually went wrong.

Now, when landing or departing, a similar thing can happen but worse. The airplane in front is slow, heavy, and at a large angle-of-attack and this is when an airplane generates its most lift. It is important to remember that in calm air, vortices tend to move outward from the aircraft. So if you are behind a departing aircraft, the vortex from the right wing will tend to move to the right and the vortex from the left wing will tend to move to the left.

If we have a crosswind, the wind will tend to influence the movement of the vortices. A crosswind of about 3 knots will hold the upwind vortex pretty much in place at the runway where it was created, while the downwind vortex will rapidly move away from the runway. Therefore, light crosswinds require the most caution during takeoff and landing. 

Wingtip Vortices with Crosswind from the right
Wingtip Vortices with Crosswind from the right

However, crosswinds greater than approximately 5 knots will tend to break up the vortices. So stronger crosswinds are a good thing, as far as vortices are concerned.

One Final Note

During landing behind a heavy airplane, a lot of pilots will fly abovethe glideslope in order to avoid any possible wake turbulence created by the preceding airplane. To me, there are two things wrong with this: 1) you may land long, which, in a Land and Hold Short (LAHSO) operation may cause you problems and 2) you are not flying your normal procedure and the possibility of a missed approach is possible. I’ve never seen pilots who fly above the glideslope not fair well during landing.

Consider this, wake turbulence falls typically about 500 feet per minute, so even in ORD on a no wind day with 2.5 miles separation, that wake turbulence is most likely already gone by the time you get to where that airplane was in front of you. Also, if you consider the wind for that day, the wake turbulence has probably blown clear of the runway as well. With that said, there is always the chance it is right along your path of flight.

But, it is your call. Just fly safe and use good judgement and consider all the variables.

Till next time…

Jeffrey

Jeffrey is a captain at a regional airline and flies the CRJ200, CRJ700, and the CRJ900. He has over 4000 hrs of flying experience in many different airplanes and is a Gold Seal flight instructor to his credit. He has recently written “The CRJ200 Quicknote Study Guide” that simplifies the systems of the CRJ200 into a downloadable eBook. Click here to get your copy today!

P.S. Encountering wake turbulence should be considered an emergency. Knowing what to do and when to do it is as important as knowing how to avoid it because most in-flight emergencies can be safely resolved if the pilot has the proper training and mental attitude. If you want to brush up on your emergency procedures (and all good pilots should), I want to recommend this book: Handling In-Flight Emergencies or sign-up for a f.r.e.e. audio on “Emergency Landing: In-Flight Engine Failure” and newsletter from PilotWorkshops.com .

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2 Responses to “Wingtip Vortices and Wake Turbulence Explained”

  1. falcon124 said

    The best description of how lift is generated can be found at this NASA article. It shows how both Bernoulli’s and Newton’s laws are involved, plus it points out that the “equal transit” concept is incorrect.

    Not a short read, but really really worth reading 🙂

  2. Rolling Garment Bags…

    Wingtip Vortices and Wake Turbulence Explained « CRJ200 Airline Pilot…

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