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Old 02-28-2011, 06:40 PM   #1  
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Default Wake turbulence and prop/jet blast

When an aircraft is heavy, clean, and slow, wake turbulence caused by wing tip vortices poses an extreme hazard for aircraft. We all learned that in ground school. Since the illustration in textbooks usually has a large airliner, I always thought this was a phenomenon related only to "big" airplanes right? Suppose you are flying a 172, and can prop blast and wake turbulence of another 172 (say on final approach) be a hazard on a calm day? As a student pilot on a cross country, a Saab 340 departed and the tower warned me about wake turbulence from the departing Saab. I though it was interesting. So I executed a textbook take off and rotated prior to the Saab's rotation point. Only yesterday a Diamond Eclipse Jet took off and the tower immediately gave me clearance to go (while the baby jet was lifting off). Hmm...I said I need about a minute for the wake to dissipate. The tower acknowledged (with a slight hint of a chuckle) "no problem, just let me know when you are ready." I felt a little silly, but as a PIC it is ultimately my responsibility so you can laugh at me all you want, I thought. If a 747 took off, well then it's a different story. All in all, I have to admit, I am a little confused here at least when it comes to wake turbulence and "small" planes. Does it exist to such an extent as to pose a threat?
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Old 02-28-2011, 07:32 PM   #2  
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Originally Posted by PearlPilot View Post
When an aircraft is heavy, clean, and slow, wake turbulence caused by wing tip vortices poses an extreme hazard for aircraft. We all learned that in ground school. Since the illustration in textbooks usually has a large airliner, I always thought this was a phenomenon related only to "big" airplanes right? Suppose you are flying a 172, and can prop blast and wake turbulence of another 172 (say on final approach) be a hazard on a calm day? As a student pilot on a cross country, a Saab 340 departed and the tower warned me about wake turbulence from the departing Saab. I though it was interesting. So I executed a textbook take off and rotated prior to the Saab's rotation point. Only yesterday a Diamond Eclipse Jet took off and the tower immediately gave me clearance to go (while the baby jet was lifting off). Hmm...I said I need about a minute for the wake to dissipate. The tower acknowledged (with a slight hint of a chuckle) "no problem, just let me know when you are ready." I felt a little silly, but as a PIC it is ultimately my responsibility so you can laugh at me all you want, I thought. If a 747 took off, well then it's a different story. All in all, I have to admit, I am a little confused here at least when it comes to wake turbulence and "small" planes. Does it exist to such an extent as to pose a threat?
All aircraft generate wake turbulence. Generally speaking, aircraft are not adversely affected by wake turbulence created by a similar aircraft. However, when in doubt, wait as you did.

Also remember that wake turbulence is a by-product of lift. Prior to lift being generated at rotation there is no wake turbulence. So, even if you are behind a larger aircraft, if you can rotate before he did you will be above his wake turbulence. Just be cognizant of possibly flying through it once airborne. The easiest way to avoid that is stay above his climb path or alter your ground track slightly upwind of his.
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Old 02-28-2011, 07:33 PM   #3  
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Originally Posted by PearlPilot View Post
When an aircraft is heavy, clean, and slow, wake turbulence caused by wing tip vortices poses an extreme hazard for aircraft. We all learned that in ground school. Since the illustration in textbooks usually has a large airliner, I always thought this was a phenomenon related only to "big" airplanes right? Suppose you are flying a 172, and can prop blast and wake turbulence of another 172 (say on final approach) be a hazard on a calm day? As a student pilot on a cross country, a Saab 340 departed and the tower warned me about wake turbulence from the departing Saab. I though it was interesting. So I executed a textbook take off and rotated prior to the Saab's rotation point. Only yesterday a Diamond Eclipse Jet took off and the tower immediately gave me clearance to go (while the baby jet was lifting off). Hmm...I said I need about a minute for the wake to dissipate. The tower acknowledged (with a slight hint of a chuckle) "no problem, just let me know when you are ready." I felt a little silly, but as a PIC it is ultimately my responsibility so you can laugh at me all you want, I thought. If a 747 took off, well then it's a different story. All in all, I have to admit, I am a little confused here at least when it comes to wake turbulence and "small" planes. Does it exist to such an extent as to pose a threat?
Well to be short the wake defiantly still exist in small planes. Of course the bigger the aircraft the more pronounced the wake is going to be. Is the wake from a little VLJ going to be enough to really create a hazard? Hmmmmm probably not. I do totally agree with you you are the PIC and it was your decision to hold on and wait, and who cares if the controller thought it was silly ? I personally am not really concerned with wake turbulence coming off anything smaller than a mid size jet: falcons challengers regionals jets etc...
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Old 02-28-2011, 08:00 PM   #4  
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I always think F=Ma. The force is depended on the mass since the acceleration is usually similar due to friction and compressibility. Mass is the size of the aircraft. So if the mass is equal to or greater than the one your in, it can have an influence. You might feel a sudden jolt from a smaller aircraft. My primary concern is the actual size of the wake itself. A large aircraft capable of a large wake and a light quartering tailwind are usually conducive to a threat.

The real threat to any aircraft is when you get sort of stuck in the vortex. When you fly through it, it's just like violent turbulence. But when it follows your flight path and your control-ability is affected, it may last a long time causing a loss of control.

Rare, but a not so fun situation.
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Old 03-01-2011, 06:34 AM   #5  
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You can feel the wake of your own airplane in a steep turn in light winds, I am sure you already have. It takes a pretty large airplane to make this wake strong enough to interfere with small airplane control. On the other hand, you have the right to ask for wake clearance any time you feel it is worth waiting for, and I would be more worried about someone who is unaware of it than someone who is. It is not impossible to be vectored 500ft under a heavy jet in a terminal area. Had it happen many times, and it could lead to a very bad day.
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Old 03-01-2011, 07:54 AM   #6  
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It takes a pretty large airplane to make this wake strong enough to interfere with small airplane control.
Don't forget about helicopters, their wake turbulence can flip a small aircraft. A 152 flipped over and crashed while on final approach to landing after flying through the wake turbulence of a helicopter a few years back.
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Old 03-01-2011, 08:01 AM   #7  
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Originally Posted by Cubdriver View Post
You can feel the wake of your own airplane in a steep turn in light winds, I am sure you already have. It takes a pretty large airplane to make this wake strong enough to interfere with small airplane control. On the other hand, you have the right to ask for wake clearance any time you feel it is worth waiting for, and I would be more worried about someone who is unaware of it than someone who is. It is not impossible to be vectored 500ft under a heavy jet in a terminal area. Had it happen many times, and it could lead to a very bad day.
Yep on the checkout flight on the 150 while doing a steep turn, I experienced a jolt. It was one of those really calm evenings.
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