S+ vs. Small Wake Turbulence

whegner Junior Member

We had a discussion today regarding this.

For the purpose of wake turbulence separation how or would you apply WT hold for a small departing an intersection behind a full length Small+? (i.e. C-172 behind a L35) Keep in mind the 7110 (3-9-7) only discriminates between small/large/heavy.

Basically, for WT would you treat the S+ in the same way as a Large?

Please cite any supporting documentation (7110, AIM, etc..) for your argument.

ATC_MacGyver MacGyver with the bears.

oops. i meant 3-9-7 paragraph 3.

1. Separate a small aircraft taking off from an
intersection on the same runway (same or opposite
direction takeoff) behind a preceding departing large
aircraft by ensuring that the small aircraft does not
start takeoff roll until at least 3 minutes after the large
aircraft has taken off.

i meant to post paragraph 3. but for some reason i posted paragraph 1. and i dont feel like changing it.

Roddy_Piper Resident Knucklehead Vegas baby

3 minutes waiverable. the discussion we used to have is when do u start the 3 minutes? when the S+ passes the intersection that the S is at, or when the wheels on the S+ leaves the ground. i always went with the S+ leaves the ground. any opinions?

3-9-7. WAKE TURBULENCE SEPARATION FOR INTERSECTION DEPARTURES

a. Apply the following wake turbulence criteria for intersection departures:

3. Separate a small aircraft weighing 12,500 lbs. or less (S) taking off from an intersection on the same runway (same or opposite direction takeoff) behind a preceding small aircraft weighing more than 12,500 lbs. (S+) by ensuring the following small aircraft does not start takeoff roll until at least 3 minutes after the preceding aircraft has taken off.

whegner Junior Member

Concur on the point of rotation. Also concur on the 3 min PW rule. The discussion we had came about do to the thought process of treating the S+ the same in that situation as a large (i.e. 3 Min pilot waiver ). Frankly, both controllers were saying the same thing to each other but on different freqs. One felt you could not refer to the S+ as a Large as it's a small (Which it is) and the other was saying "I know but the rule is the same".

It was all about semantics. I was just curious to see if anyone else has had the same tussle.

CLEATCT2010 Senior Member Ohio

I'm sure others have had similar thoughts or questions. Yes the .65 only classifies weight classes as Small/Large/Heavy however for Wake Turbulence purposes, they have the Small + (Plus) category. For those that don't know, a Small is any aircraft weighing 41,000 lbs or less. A Small Plus is any aircraft that weighs greater than 12,500 lbs to 41,000 lbs.

While it sounds weird to separate a technically small aircraft from another small aircraft for wake turbulence, it does make some sense. If you have a Learjet or Citation take off full length and a Cessna 172 or Piper Cherokee wanting an intersection departure, you apply the wake turbulence separation. You're putting a heavier aircraft ahead of a really light aircraft. Further, the small departing from an intersection means that the aircraft has a higher probability of lifting off near where the other aircraft (your Learjet or Citation) also lifted off. While I personally don't believe a Small Plus aircraft creates a lot of wake turbulence, it's a rule that errs more on the side of safety. It is 3 minutes and completely waiverable by the pilot (remember the pilot must ask to deviate from the rule).

As for the rule, no the small behind a large is a different rule. While the wake turbulence application is the same (3 minutes), the rule assumes both aircraft are taking off from the departure end of the runway (not full length).

Notice that if you have a Small Plus ahead of a Small and both take off from the same point on the runway (the departure end, or, FULL LENGTH), wake turbulence separation DOES NOT APPLY. All you need then is 6000 ft and the preceding aircraft to be airborne and you can clear the small for takeoff. Of course, you can always issue a wake turbulence cautionary advisory to that aircraft but there is no wake turbulence separation.

Weird? Yes, but, what else do you expect?

polo708 Epic Member tower

Wayne, S+ is basically a small when it takes the turbulence and a Large when it gives it. And, the 3 minutes starts when the preceding departure rotates. 2 minutes wake turbulence starts from departure roll.

You only issue WT cautionaries to arriving aircraft. The only time you issue a cautionary to a departing aircraft is when the pilot requests to waive the 3 minute WT rule.



you can clear the aircraft before that as long as you can anticipate that it wont begin take off roll prior to the preceding a/c passing 6000 ft and being airborne

whegner Junior Member

Polo! You broke down the fourth wall...people know my name now I agree with what you stated. The conversation came up as S+ is a fairly recent "group" for a/c (at least to me it is since it didn't exist when I last worked 15 yrs ago) so I was trying to relate it to what I already knew vis-a-vie small/large intersection departure WT application. As I said, it was all about semantics anyway.

CLEATCT2010 Senior Member Ohio

You only issue WT cautionaries to arriving aircraft. The only time you issue a cautionary to a departing aircraft is when the pilot requests to waive the 3 minute WT rule.

you can clear the aircraft before that as long as you can anticipate that it wont begin take off roll prior to the preceding a/c passing 6000 ft and being airborne[/QUOTE]

The .65 also gives the controller discretion that, in their judgment, the situation warrants an advisory. Yes it's not required but you can still give it if you feel it's needed. Otherwise yes, you would only advise arrivals with an advisory or any aircraft that is deviating from the WT hold.

And yes you can also anticipate that 6,000 ft of separation. I'm just trying to keep it simple. Either way you have to come up with 6,000 ft. and airborne, regardless if you're anticipating the separation.

polo708 Epic Member tower

well this job is full of detailed rules... might as well post them correctly.

You can throw around the word discretion all you want. Hell, I can tell an a/c to "use caution" every single time he taxis to the runway but I don't. Places with decent traffic don't really allow you to use all the extra chit-chat verbiage you want. If you feel the need to tell a Cessna "caution wake turbulence" for a departing King Air you have way too much time on your hands.

Sorry, im just tired... and sick of hearing people say caution wake turbulence for every damn departure. It makes a real cautionary seem meaningless when it truly may be needed. Pilots hear it so much that it becomes meaningless.

GLF5 Senior Member

I'll play. 3 minute hold for the original post. Everything polo said is correct. I also agree with him/her about the cautionary advisories.

Where I work, when an aircrafts 3 minutes is up everyone says "N123, caution wake turbulence, [insert aircraft] departed more than 3 minutes ago, runway 36 cleared for takeoff". I cringe every time I hear it. What a waste of radio time. But of course I'm the A-Hole because I don't do it.