Hello, and Welcome back. Today we are going to talk about Wake Turbulence and Wing Tip Vortices. Wake turbulence is best explained as a spinning mass of air, originating from the wingtips of a large heavy airliner, trailing backwards that may cause following aircraft to enter an "unusual attitude" of which recovery is doubtful. This usually occurs during still or light air conditions but WestWind Captains should be aware of it at all times.
As the wings of a large airliner pass through the air, lift occurs when the high pressure under the wing tries to fill a vacuum of the low pressure on top of the wing. As a byproduct of this lift, some air will circulate causing a "rolling effect" to take place behind the wingtips. What happens is two vortices (the left wing clockwise and the right wing counterclockwise as viewed from behind the airliner looking forward) develop and continue to spin until their energies are reduced. Most of the energy is within a few feet of the center of each vortex but pilots should avoid a region within 100 feet of the vortex core. Airliners which are heavy and in the clean configuration give off the most intense vortices. WestWind Captains, flying an airliner, with a shorter wingspan and lighter weight, trailing a "heavy", should be alert of the possibility of wake turbulence.
Wing tip vortices start at the moment a "heavy" leaves the ground. Prior to takeoff or landing WestWind pilots should note the point of liftoff or touchdown of a "heavy"(you can see this using SB and MP). What is needed, in this situation, is to takeoff or land before wingtip vortices can become a threat to your airliner. To do this we must takeoff before the rotation of the "heavy" ahead of us and, (if landing) land at a point on the runway further down from where the "heavy" has touched down.
Wingtip vortices trail upward and outward from the wingtips of a "heavy" and spin in a counterclockwise motion. When they reach their highest point (depending on weight and speed of the "heavy") they start to sink towards the ground. Once they hit the ground they will roll laterally at about 2 or 3 knots. A crosswind may keep the upwind vortice in the runway area for an extended time. The worst condition is a light quartering tailwind. In this case the upwind vortice will move towards and up the runway making the judgment of where to touchdown difficult.
If you were following a "heavy" on an ILS approach, it is possible to experience wake turbulence as you proceed down the glideslope. In this case you would fly your approach descending "one dot" below the glideslope center position (fly above the glideslope). In a previous post I stated that the autopilot should be used down to minimums. Here is one case where you would "hand fly" the approach - keeping your glideslope path "one dot below" to avoid any wake turbulence.
It is possible to experience wake turbulence while you are on the ground waiting to take off. For example, if you were at the hold short line on the taxiway leading to the runway threshold and it was downwind of an approaching "heavy" then you could experience wake turbulence as the "heavy" lands. In this case the hold short lines of the taxiway are 280 feet from the centerline of the runway (for designated runways for "heavies").
Another case, that could "ruin your day", is doing a "missed approach" following a "heavy" that also "missed". In this case it would be wise to fly your missed approach at your best angle of climb airspeed to get to your initial missed approach altitude.
The controllers responsibility (for VFR Flight) is to allow for a two minute "hold time" before clearance to takeoff behind a "heavy". In cases where the Captain accepts a "visual" approach, it is the Captain's responsibility to stay clear of any wake turbulence condition. In all cases the Captain is the final authority for safe flight.
OK, this concludes the presentation. How about a test? Now, you know I wasn't going to let you leave so soon!
"Captain Ed's" 10 Question Test.
1. You are a Captain of a WestWind Beech 1900 and a voice controller gives you this clearance: "WestWind 1838 (or use your own pilot number) number two to land behind Company Boeing 747". The winds are light and variable. Given the circumstances you should be concerned about?
a. Clear air turbulence
c. Wing tip vortices and wake turbulence
d. None of the above
2. The danger of Wake turbulence and Wing tip vortices are MOST prevalent when?
a. The wind in gusty
b. The wind is right down the runway
c. The wind is calm or "light and variable"
d. All of the above.
3. Wing tip vortices are the strongest?
a. Behind a large heavy transport in a clean
b. Behind a large heavy transport with flaps down
c. Behind any corporate business jet
4. You are flying a WestWind Beech 1900 and are following a "heavy" Boeing 747-400. The winds are "light and variable". You should land?
a. In front of his touchdown point
b. Further down the runway than his touchdown point
c. On his touchdown point
d. Any of the above.
5. You are the Captain of a WestWind Boeing 747-400 and are concerned about your wake turbulence. In consideration of your fellow pilots you should?
a. Land as far down the runway as is safe.
b. Land as close to the approach end of the runway as is safe
c. Land on those little arrows that lead to the runway threshold.
d. Land at, or in the touchdown-landing zone.
6. You are about to takeoff behind a "heavy" that has just departed. You should?
a. Takeoff after the liftoff point of the
b. Takeoff before the liftoff point of the preceding "heavy"
c. Takeoff at the liftoff point of the preceding "heavy"
d. None of the above.
7. You are on an ILS approach and are following a "heavy". There is a chance of wake turbulence you should?
a. Follow the glideslope needle at the normal
b. Follow the glideslope needle at "one dot above" the middle point
c. Follow the glideslope needle at "one dot below" the middle point
d. None of the above.
8. Great danger from Wake turbulence or Wing tip vortices can occur when?
a. The wind is a light and variable and you are
following a "heavy".
b. You are not "holding short" at the "hold short line" but have crossed it in the direction of the active runway.
c. If you get too close to a "heavy" on a visual approach.
d. All of the above.
9. The direction of the wing tip vortices is (As viewed from behind looking forward towards a "heavy".)?
c. The Right one is Clockwise and the Left one is Counterclockwise.
d. The Left one is Clockwise and the Right one is Counterclockwise.
10. If you suspect a condition of wake turbulence and are on a "missed approach" you should?
a. Fly your missed approach at Vfs
b. Fly your missed approach at V2 plus 10 knots.
c. Fly your missed approach at Vref minus 10 knots
d. Fly your missed approach at your Best Angle of Climb Airspeed.
The Correct Answers are:
OK, how did you do? If you got 100 percent, then consider yourself an "Ace". If you made less than 100 percent you must do the test over again. : <(
Just kidding, LOL
Ed Ward, Jr.
Executive Vice President,
Head Of Training
NOT TO BE USED IN REAL WORLD FLIGHT. NO PARTS OF THIS ARTICLE MAY BE REPRINTED WITHOUT THE WRITTEN PERMISSION OF THE AUTHOR.
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