Wake Turbulence

Wed May 24, 2000 3:29 am

Recently on a flight from ORD to MKG, our small but dependable EMB120 lined up behind a slew of heavies. Just as a UA 763 started rolling, we began turning on to the runway. Not much longer, couldn't have been more than 20-30 seconds we began rolling too. Had a big chop and dip just after t/o, but nothing major. I doubt it was wake turb because we would have had a much bigger problem, but it got me curious.
My question, is whether wake turbulence is a big concern on t/o or just when landing? I would think if you are the pilot of a turbo prop, you would want to wait a good bit before departing even if the runway is backed up backed to the terminal. Thoughts? Experiences? Wisdom?

RE: Wake Turbulence

Wed May 24, 2000 4:14 am

ChunkyP, if you were rollinging an EMB120 just 20-30 seconds after a heavy you most certainly experienced wake turbulence. Keep an eye on the topic ATC heavy as well for more info.
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RE: Wake Turbulence

Wed May 24, 2000 4:22 am

You should of felt slight chop, nothing major considering you probrobly only used like 2,000 feet when a 7673 uses like 7500 feet, but in almost any plane u feel wake turb. I was in an Western Pacific a couple years ago and they put us on top of an a340-300 of lufthansa, i guess its not a huge deal.
Go big or go home
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RE: Wake Turbulence

Wed May 24, 2000 4:52 am

It is not uncommon practice to launch a light behind a medium or a heavy. As stated above, the larger jet will use up about 4000 to 5000' of runway prior to rotating, the point where wing tip vortex turbulence becomes severe. The light probably is airborne in under 2000' and probably has a turning instruction in his/her departure clearance. In combination with a crosswind, the practice can be quite safe.

There is still turbulence created by the aircraft wake and jet efflux, but it is not nearly as strong as wing tip vortex turbulence.

To answer the first question, it can be as dangerous on takeoff if not more so, as on landing approach.

Lots more can be/has been written on this subject in Johan's archives.

Best Regards,


RE: Wake Turbulence

Wed May 24, 2000 2:25 pm

Chunky, almost anything that flies causes wake turbulence, a helicopter's wake can be deadlier to a small airplane, than the opposite. Every airplane's worse wake turbulence is produced when :
1. It is slow.
2. It is heavy (not referring to "heavy" callsign)
3. It is clean (no flaps, no gears out, nothing out)
W.T. not only occurs at takeoff or approach, but at any time the airplane is at this conditions.
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RE: Wake Turbulence

Wed May 24, 2000 3:48 pm

The worst wake turbulence actually occurs at points where production of lift is relatively high thus takeoff, landing and high speed cruise and heavy a/c all produce heavy wake turbulence. So I think HP-873's third point was referring to high speed cruise clean config and not low speed clean config.


RE: Wake Turbulence

Thu May 25, 2000 12:34 pm

Well, what actually happens when it is said that the cleaner, slower and heavier an airplane is, it is because (relating to the clean config) :
When an airplane is in slow flight at clean configuration, stall speed increases, and this is because the wing is needs a higher angle of attack to generate sufficient lift than when the airplane is in full flaps. The higher angle of attack is one of the reasons of the tremendous vortices created. When the flaps are down, the airplane doesnt need such a high angle of attack, because the difference in airspeeds below and over the wing is already high, of course air reaches the trailing edge at the same time. The need for a higher angle of attack when clean is to accelerate further more the flow of air over the wing to increase lift. A living and very famous example of a cleaner wing compared to other planes even when flaps are down is the 757, also the Concorde, and the Valkyrie (which was involved in an accident where a spot plane was grabbed by the valkyrie's vortices and the spot plane collided with the bomber) their wing is so clean that they need such a high angle of attack that creates superpowerful vortices. Now about that strong vortices are created on high speed cruise, they are not going to be as strong, because the airplane wont need big angles of attack because of the fast flow of air on the wings, if the airplane remains at a fixed angle of attack, lets say 10 degrees, if speed doubles, lift will be 4 times higher and this is why the airplane climbs.Am not saying that vortices are not present at high speed cruise but just that they are smaller, even an airplane with winglets will have vortices at it's tips, really small but they will still be there.

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