Wake Turbulence - Interpretation

[one25six]

Interpretation - what a can of worms that word can mean with regulations.

I have my own (strong) view on this - but in the interests of seeing others, please tell me what you think in the following scenario (only one of a few!).

Background - Australian Airspace (pretty much ICAO standard v.v. wake turbs) - Assume IMC (no vis sighting possible)

Like to know pilot as well as ATC's interpretations.

PLAN VIEW

[A] ------------------------->[B]B763 3000FT HDG090
^
|
|
|
|
|
|
[C] C402 2000FT HDG360

Excuse the crude graphics.

We are using 6NM Sep Light (C402) with Heavy (B763).

At what point can B763 descend? Assume radar separation of 3NM?

a. When B763 is 3nm from C402 and 6nm from A.
b. When B763 is 3nm from C402 and 3nm from A.
c. When B763 is 3nm from C402 and 0.5 nm from A.
d. When B763 is 6nm from C402.
e. Other.

[ASD]

I go with answer "d"

You can only descend the B763 through the level of the light a/c once the you have acertained that the distance is 6nm apart and that the distance will not reduce at any point while you dont have vertical seperation.

[Lock n' Load]

I go with c
You only need provide wake turbulence separation within the path of the C402, and that is the absolute answer in Canada as far as I've been taught.
Despite the fact that wake does fall through the air, hence the 1,000ft when within 6nm ahead of the path of the light, it won't fall on faster on the path of the light if descent is delayed until through that path.
We regularly use such separation here against VFR traffic, against whom the radar separation is target resolution (or 1nm which amounts to the same thing). Cross over the top of the VFR a/c at close to 90 degrees, and descend as soon as target resolution is achieved, so long as the VFR's path is predictable or steady.

[DFC]

Agree with Lock n' Load.

There is no need to provide lateral saparation for wake purposes bewteen aircraft that are separated vertically by 1000ft even though wake sinks.

Provided the 3nm radar separation is maintained, the B767 can be descended as soon as it has cleared the path of the C402 because when the wake is generated at [A] in the diagram, the vortex are 1000ft above the C402. During the time it takes the C402 to reach [A], the wake will - sink, diverge from the track of the B767 (depending on wind) and partially disipate. Provided the C402 does not track east of the cutrrent track, it makes no difference what wake exists to that side of track.

Regards,

DFC

[ASD]

Sorry, speed reading again

didnt see Assume radar separation of 3NM

My bad - that changes everything. :E

[one25six]

OK.

Further scenario: (ELEVATION VIEW this time)



B763-H 4000FT----------->>[A]------------------------->>

<<---------------------------[A]<<------------- C402-L 3000FT


Aircraft have achieved definite passing at [A]. Aircraft are on exact reciprocal tracks (no lateral offset)

Basic radar sep is 3NM.

When can B763 Descend?
a. at definite passing.
b. at definite passing +3NM
c. at definite passing+6NM

When can C402 Climb?
a. at definite passing
b. at definite passing+3NM
c. at definite passing+6NM

(Worth considering answer to earlier scenario together with this one)

[2 sheds]

Are we talking en route situation or sequencing onto final approach?

[Lock n' Load]

In the second example, must be terminal/approach airspace to have 3 mile sep, I suppose.
Heavy can descend as soon as they have passed under Canadian rules (reciprocal tracks, diverging by more than 135 degrees, when you have tail to tail you don't need any further separation).
Light can climb when it is 6nm from the heavy's PRESENT position, which will probably be A+2 given the speed difference.
You don't need A+6 (separating by 6nm from where it was 2 minutes ago? why would you do that?) because that would be the same as giving about 15 miles of wake separation if it was passing behind the 767 as opposed to underneath. The wake 6nm behind the 767's PRESENT position will be exactly the same regardless of the direction of flight of the C402.

[DFC]

Again agree with Lock n' Load.

What has to be remembered is that radar or lateral separations are designed so that provided the minima are complied with, there is practically no chance of a mid-air.

Separations for wake turb. are not deigned to ensure that the following aircraft will have no consequent effect. They are designed to ensure that when the other aircraft receives the effects of the wake turb., it's effects are reduced to an acceptable level.

To really look at this question, put your procedural hat on. You establish the pass point and confirm it by position report. You then count to 10 and swap levels. Wake anyone?

To make this question more interesting, make the light aircraft a C150 flying at 80Kt into a 50Kt headwind. Would that change your opinion? Waiting until the C150 is 6nm beyond [A] is going to take 12 minutes by which time the B767 will be some 100 miles past [A].

The wake of the B767 is created in the parcel of air that the B767 is passing through. That parcel of air in the case I have given is moving at 50Kt (the windspeed).

Let's put a helicopter at A, 1000ft below the path of the B767.
At the instant the B767 passes [A], the helicopter climbs vertically at 500ft per minute to the same level as the B767. It takes 2 minutes to complete the climb. With that wind, the instant turb encountered at [A] (ignoring sink etc) would be that which was produced by the B767 1 2/3nm before [A] (distance traveled at 50Kt in 2 minutes. That turb will have been subjected to disipation, divergence etc. (Remember that a light can take-off behind a heavy with 2 minute separation! and low speed =high induced drag = strong wake vortex!

So think of the helicopter at [A] if it travels in the same direction just behind at the same speed as the B767, it gets the full whack of wake. If it as 0 groundspeed (climbs vertically it gets a lot less).

If it travels at a significant groundspeed in the opposite direction.............

Regards,

DFC

[5milesbaby]

Being an area controller wake vortex separation is somewhat overlooked, however I do give consideration in certain circumstances. In both examples given I would have to wait for 5 miles laterally constant or increasing and could then go for it. Likewise if they were both going in the same direction on the exact same track my book just says wait for the B767 to overhaul by 5 miles and then I'm free to descend/climb it through the level of the Cessna. In this case however I would normally use vectors to keep the tracks separate so wake turb cannot come into it. Is it bad that wake isn't considered in area control? I probably encounter maybe one separation a YEAR that may fall below the minima for approach.

I get more complaints about vortex on climb outs usually following B757's or MD11's, however I have also had requests to change course slightly from a B737-300 following a B737-800, it was 16 miles behind!!!!!!!! Strange stuff this vortex... :ouch:

[one25six]

ok.

My answer is:

Case 1

c. Wake Turb 'volume' is theoreticallyonly behind the aircraft generating the wake to a width of 1nm (therefore 0.5 nm either side of track). As long as an aircraft separation standard is kept, in this case 3nm, the wake turb 'volume' only descends with the aircraft as it desends.

Climbing up through through the wake trail is of course a different matter, and the appropriate wake turb standard then needs to be applied unless you wait until once again 0.5nm clear on the other side.

Case 2

a.

and

c. for the same reasons.

The reason I pose the question is I am amazed by the number of people (controllers as well as pilots) who insist the standard (theoretical, not practical) requires 6 miles before descending the heavy. This suggest that somehow the whole volume of wake 6 miles behind descends in unison with the aircraft ahead, instead of on a sloping trajectory.

[3 slips and a gully]

1.C, 2.A, 3.C .

Wake turbulence (136,000kg or more) with a light (7,000kg or less) is 6 behind, 0.5 laterally, 1000 below (and the vortices are descending not going up).

You descibed it to a tee 125.6.

[Lock n' Load]

one25six and 3slips, you're both wrong on the last one.
The rules say 6 miles behind a heavy FROM WHERE THE HEAVY IS RIGHT NOW, not from where he was 2 minutes ago!
If you had a light 6nm in trail behind the heavy and at the same speed, you'd climb or descend him through the heavy's altitude without worrying. The C402 in the illustration is going in the opposite direction and the point at which you'd climb or descend him were he in trail is NOT A+6!!! It's 6nm from where the heavy is right now!!! That will most likely be about A+1.5 or so. You don't need to provide extra separation just because they're going in opposite directions.

[one25six]

I think we are talking about the same thing Lock n Load -prob posed the answers in a confusing way. The actual point of climb is probably better described as when 6nm exists after definite passing rather than DP+6.

Thanks.

[Lock n' Load]

okay-dokey one25six. How about redrawing the diagram with a point B for where the heavy is 6nm from the light and increasing? ;)
A wee point for 3slips, ICAO wake catagories don't apply in the UK obviously enough, and also not in Canada of all places! Let's see... where is ICAO's HQ??? Silly, but there you go. Canadian lights are 5,700kg / 12,500 lbs or less, which is a might confusing coming from the UK where, even with an extra catagory, there are lights which are Canadian mediums.
Cases in point, JS31, B350, D328, C550 and many more.
I just had a look in MANOPS and there's a good digram showing a heavy with a line going out behind it, and aircraft shown going in all directions at a point where wake turbulence separation exists. Section 533 for equally geeky...

[3 slips and a gully]

LNL

125.6 posts on the Dunnunda forum and 125.6 is a Brisbane App frequency so I presumed Australian standards. :ok:
Section 533 for equally geeky...
Yes, MANOPS 533.2 Diagram 1 shows 6 miles Tail to Tail

Here is a pearler from Australia's MATS I bet you wish you had:
4.7.2.5 Wake turbulence separation is not required:

a. when a LIGHT aircraft will cross the track or follow the track of a
MEDIUM fixed-wing aircraft of less than 25,000 KG maximum
certificated take-off mass (includes B190, E120, SH36, SF34 and
A748);

With so many different global wake turbulence stnadards one wonders what ICAOs role is. :hmm:

Happy New Year :ok: :ok:

[topdrop]

Definite passing - for those that think you need 6NM you're way out.
You don't apply wake turb when you have 1000 ft between them.
Therefore wake turb is achieved at definite passing - point A. You then only have to keep the 1000ft until the lateral tolerance can be applied - 0.5NM. So the 767 can descend when the distance between the 767 and 402 is 0.5NM. You have traffic separation as they have definitely passed by radar.

Think of it this way - the wake turbulence from the point where there is less than 1000ft vert sep is not going to move back past A and catch up with the 402 heading away from A.

[Lock n' Load]

topdrop - we're also considering when the C402 can climb to less that 1,000ft below where the heavy was, and this is where the 6nm comes in. Not 6nm from the point of passing, as we've discussed to death now, but 6nm behind the heavy's present position. The heavy, as you rightly say, can descend as soon they have tail-to-tail.

[topdrop]

In Oz, once two acft have passed by radar you can climb him. We just need the 0.5NM in this case for wake turb.

[one25six]

Top Drop - if the tracks are exactly opposite (no lateral offset) you will still need 6nm before climbing in to the wake in the example above. If they are opposite and diverging, then wait for 0.5 and climb.

I think this is what you meant(?).

[topdrop]

125.6
Agreed, I was originally talking about dropping the 767, but then stupidly put in about climbing the 402.