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Old 03-21-2006, 12:26 PM   #1  
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Question Wake Turbulence

I'm interested in hearing about wake turbulence from you pilot types.

As Air Traffic Controllers, we have had the dangers of wake turbulence drilled into our heads since Day-1 at the FAA Acadamey in OKC. Our instructors showed us films, pictures and diagrams relating to the effects of wingtip vortices. The rules and procedures for applying wake turbulence separation criteria were constantly discussed and written tests given. We still have "Quarterly Refresher" briefings to remind us of the dangers surrounding wake turbulence.

What are some of your thoughts on this subject?

Could the departure or arrival separation standards be reduced?

Should there be more spacing on departure or arrival based on aircraft types and/or max takeoff weight?

Do you believe that any of our wake turbulence separation standards for parallel or intersecting runways are sufficient or excessive?

Have you had any unusual or interesting encounters with wake turbulence that you can share?

Thanks,

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Old 03-21-2006, 12:53 PM   #2  
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Default Wake Turbulence effects as seen from the Tower

My first encounter with wake turblence was one night working the UPS inbound at SDF when Runway 19 and 29 were still in use.

Approach had sequenced and cleared a Metroliner for a visual approach to follow a Heavy UPS DC8 to Runway 29. As the DC8 approached the numbers, the Metroliner was just crossing the marker inbound. I looked out the Tower Cab window just in time to see the Metroliner wingtip lights rotate 360 degrees, and then heard the pilot yell "Jesus Christ... we just did a barrel roll!"

My second encounter was again at SDF. A DAL B737 was on short final to Runway 19 with a Bell Jet Ranger departing from the base of the Tower Cab turning northeast bound VFR. I exchanged traffic with the helicopter and DAL. The helicopter lifted off and banked sharply just east of the area where the DAL B737 was about to touch down. Suddenly the B737 banked sharply to the right... away from the runway... over the parallel taxiway... banked sharply to the left... and then finally landed.

The DAL pilot said, "Son... I want you to know that it was partly my years of experience that allowed me to land this aircraft, but it was mostly luck that we weren't all killed from that helicopter's prop-wash." Apparently the helicopter's prop wash was directed down towards the runway as he banked and then upwards... underneath the left wing of the B737!

In both instances, traffic had been exchanged and the appropriate separation rules had been applied... but wake turbulence and prop wash almost took a life.

These two instances sure made a believer out of me! Scary stuff to see from the Tower Cab.

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Old 03-21-2006, 01:18 PM   #3  
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Wake turbulence can kill ya if you don't give it some respect. The worst cases that I've been through have been where we least expected it...

One was about 11 miles behind a 747-400 (in a CRJ). We were flying on the STAR into IAD, and this traffic was ahead of us on the same routing. Because we had departed a field that was relatively close to IAD, the 747 descended through our altitude on the arrival. We hit the wake, and it rolled us about 60 degrees each direction as we passed from one side to the other.

The other time was following a Smurf Jet (BAe-146) into ORD. We were about 2-2/1 or 3 miles behind and I noticed that I needed more and more aileron to keep the plane level. Nothing abrupt, but just this slow roll which required control deflection. At first I thought it was a flight control malfunction, but then realized that we were just in the wake and climbed out of it. I would guess that at it's worst, the yoke was turned about 45 degrees.

Overall though I think that if all the factors are known, pilots can take actions to minimize the risk and close some of the gaps. I know that many controllers get nervous with the Citation that I fly- flew into atlanta a few times last week and I don't think that we had any closer than 4 miles separation, and the controllers kept slowing us down. As long as I'm not behind a heavy or a 757, I have no problem with 3 miles as long as I can see the traffic and have an ILS to the runway.
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Old 03-21-2006, 04:13 PM   #4  
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Default c-5

this past weekend coming out of Worcester, MA a C-5 galaxy crossed about two miles ahead of us.. i was at 3,000, climbing for 5,000 and he was at 4200 according to ATC... the only problem was that i was in a warrior and he was most definitely heavy.. we crossed over his path about 400 feet above it.. thought i was out of it, but the updrafts from the wind (lots of chop that day) probably pushed it a few hundred feet in my direction, with my little PA28 kickin around like a rag doll.. not a good feeling at all
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Old 03-21-2006, 04:40 PM   #5  
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Default FAA 7110.65 Wake Turbulence Separation Standards

FlyerJosh,

Quote:
Originally Posted by FlyerJosh
I know that many controllers get nervous with the Citation that I fly- flew into atlanta a few times last week and I don't think that we had any closer than 4 miles separation, and the controllers kept slowing us down. As long as I'm not behind a heavy or a 757, I have no problem with 3 miles as long as I can see the traffic and have an ILS to the runway.
According to our rulebook (FAA Order 7110.65R), we must ensure that 4 miles radar separation exists between your Citation and any large aircraft at the time that the large aircraft is over the landing threshold. This separation standard applies even if the preceding traffic is landing on a parallel runway that is separated by less than 2,500 feet.
Paragraph 5-5-4(f) from the 7110.65 describes the separation standard for small aircraft behind other Large, B757 and Heavy aircraft:

TERMINAL. In addition to subpara e, separate an aircraft landing behind another aircraft on the same runway, or one making a touch-and-go, stop-and-go, or low approach by ensuring the following minima will exist at the time the preceding aircraft is over the landing threshold:

NOTE-
Consider parallel runways less than 2,500 feet apart as a single runway because of the possible effects of wake turbulence.

1. Small behind large- 4 miles.

2. Small behind B757- 5 miles.

3. Small behind heavy- 6 miles.
Of course, if ya see the traffic and want the Visual Approach, then all bets are off!

Paragraph 5-5-4(e) covers the standards for other aircraft conducting an instrument approach:
Separate aircraft operating directly behind, or directly behind and less than 1,000 feet below, or following an aircraft conducting an instrument approach by:

NOTE-
Consider parallel runways less than 2,500 feet apart as a single runway because of the possible effects of wake turbulence.

1. Heavy behind heavy- 4 miles.

2. Large/heavy behind B757- 4 miles.

3. Small behind B757- 5 miles.

4. Small/large behind heavy - 5 miles.
This document may be viewed at the following FAA site: http://www.faa.gov/atpubs/ATC/INDEX.HTM

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Old 03-21-2006, 06:07 PM   #6  
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MEM_ATC-

Thanks for the info. It's been a long while since I looked at the 7110, but refresh my memory what classifies as small / large and where the line is. IIRC its 41K right? For some reason I thought it was 3 behind large, 5 behind heavy. But now that you mention it 4 seems to ring a bell.

Guess it's all the years of flying a "small" large category airplane (CRJ) that muddles things up...
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Old 03-21-2006, 08:27 PM   #7  
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Whats the story with the 757 being listed by its self? Does it not classify as a heavy but have a helluva wake or what?
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Old 03-21-2006, 08:32 PM   #8  
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Exactly. It's not heavy enough to make the "heavy category" but has very similar wake characteristics as the 767.
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Old 03-22-2006, 11:10 AM   #9  
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Default Aircraft Classes for Wake Turbulence Separation

FlyerJosh,

Quote:
Originally Posted by FlyerJosh
Refresh my memory what classifies as small / large and where the line is. IIRC its 41K right? For some reason I thought it was 3 behind large, 5 behind heavy. But now that you mention it 4 seems to ring a bell.
From the Pilot/Controller Glossary:
AIRCRAFT CLASSES- For the purposes of Wake Turbulence Separation Minima, ATC classifies aircraft as Heavy, Large, and Small as follows:

a. Heavy- Aircraft capable of takeoff weights of more than 255,000 pounds whether or not they are operating at this weight during a particular phase of flight.

b. Large- Aircraft of more than 41,000 pounds, maximum certificated takeoff weight, up to 255,000 pounds.

c. Small- Aircraft of 41,000 pounds or less maximum certificated takeoff weight.
It's interesting to note that these classes have changed over the years. It seems that certain aircraft companies have had some influence over the process. Two that come to mind are the SF340 and the B757. Both have been in different classes for wake turbulence separation at one time or another, however I don't recall if the actual class criteria was changed, or if the aircraft maximum certificated takeoff weight was changed.

Also, the B757-300 is considered a Heavy.

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Old 03-22-2006, 12:25 PM   #10  
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Check out this short video.\
http://www.alexisparkinn.com/photoga.../Jetblast.mpeg
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