Wake turbulence timing

[BMM389EC]

At the holding point and a heavy starts its take-off roll, when do you initiate the 2/3 minute timing? From when it rolls or rotates?

[Gonzo]

From an ATC point of view it's the airborne time. That's why we sometimes clear you to go 1 imnute and 30 seconds behind a heavy, to allow for the 30 second (ish) roll.

Gonzo.

[Code Blue]

Interesting comments:

Canadian AIP AIR 2.9.1 states that Vortex generation starts with rotation (and ends with nosewheel touchdown). It suggests 2 minutes wait, though other Transport Canada docs give the more specific times related to wake turbulence categories for a/c involved.

I personally like to start my waiting times from rotation of preceding a/c when flying my little spam can.

Happy Hols
CB

[woofer]

Gonzo it appears that you are an Air Traffic Controller. I have a question for you if you do not mind. How much seperation do you give after a Boeing 757 ?

[acm]

My company JAR OPS manual specify that: "minimum spacing at time aircraft are airborne".

[GotTheTshirt]

Another question for Gonzo,

Is the separation time (2 mins) the same for all types of aircraft say from 747 to 737 ?

[hannibal]

I believe the 2 mins is applied to B757 due its wing design and vortex generation properties (this is despite it not being categorised as a "heavy")

[Feather #3]

Something missing here is the distance standard.

There is either a time or distance airborne requirement. While pilots work in the time [from liftoff in my case], ATCO's, in the US at least, may use distance provided that the two a/c meet the distance standard when the second a/c lifts off.

In the 80's [and it would appear that not much has changed since, except in the UK] when we did a study to attempt to increase flow rates, the UK were the most consevative, the USA the least, and Aust was in the middle, where we've stayed.

There are elements of both judgment and luck in wake problems; sometimes it will still bite!!

G'day <img src="smile.gif" border="0">

[cossack]

Separation behind a 757 - Our manual says 2 minutes for lighter types from the same departure point, or 3 minutes from an intersection. For other Medium vortex types or Heavies following there is no vortex minima, just route separation. This can sometimes be "wheels up".

We are very aware of the possible problems caused by the 757 and that some companies require a 2 minute minimum when following a 757 regardless of type - even a 767 pilot asked me for 2 minutes once.

If we intend to use a 1 minute separation behind a 757 it has become good practice to ask you first if you will accept it before you are lined up. It would be nice if you could tell us in advance if you need more than 1 minute in case we forget!

In the UK there have been no reports of wake vortex encouters behind a 757 on departure. Is this because we use 2 minutes sometimes or because there is less of a risk on departure than on arrival?

I believe that in Oz the 757 is categorised as a Heavy if leading and a Medium if following.

In the US it has its own wake vortex group and is considered to have the properties of a Heavy. US departure separations can be distance based (4 miles) rather than time. When time is used it is based on start of roll. Confusing!?

See <a href="http://www.pprune.org/cgibin/ultimatebb.cgi?ubb=get_topic&f=41&t=002107&p=" target="_blank">Vortex Wake and the 757</a> from a few months ago.

Edited to add link

[ 28 December 2001: Message edited by: cossack ]</p>

[nice line-up]

Is it correct that no separation minima exists between a departing heavy and a landing medium/light. And if, isn't that somewhat inconsequent?
<img src="confused.gif" border="0">

[cossack]

nice line-up
No minima exist in your scenario unlessthe predicted flight paths will cross, i.e. a very displaced landing threshold.

This is because no vortices are being produced by the departing aircraft until its nose wheel lifts. Yes there will be turbulent air behind it, but no vortex wake.

[Gonzo]

woofer,

After a 757 on departure, which we class as a Medium, I would give any other M 1 minute. However, we appreciate that elsewhere it is classed as a heavy, and many airlines want 2 minutes. We generally know who'll take 1 minute or 2 minutes, although if it is looking a bit tight with a lander at 4 miles, we'll ask before committing ourselves!

On landing, the 757 is a Upper Medium, and other medium get an extra mile behind it.

Got the T Shirt: I'm not quite sure what you're asking, but I could roll a Cessna 150 2 minutes behind a 747 if I wanted to, if they departed from the same point.

H followed by M gets 2 mins, H followed by S gets 2 mins, H followed by L gets 2 mins etc etc.Gonzo.

[Propellor]

Is there an difference in the wake separation if the preceding plane departs from the beginning of the runway, and the following plane from an intersection further down on the same runway.
Also, what is the criteria if both ends of the runway are being used, say in nil wind conditons on a rather lonely airport. What criteria is applicable in tht case?
Thanks

[Gonzo]

Propellor,

If both depart from the same point, then if it's, say, a Heavy 747 followed by a Medium 737, then it's 2 minutes. If the 737 departs from an intersection, then it's 3 minutes. However, at Heathrow it's a bit more complicated, because some of our intersections, 102 on 09R, 86 on 27L and 18 on 27R, are not considered as intersections for vortex purposes.
You mean if a 747 took off from, say runway 18, then a 737 took off from runway 36? Got no idea! The thing you'd have to watch out for is the crossing of airborne tracks.

Gonzo.

[Slasher]

In my 737 FLUF I start timing from the point of rotation.

* 2 minutes for 747 and similar

* 3 minutes for 757 MD-11 and DC-10 (and not a second less!)

All born out from hard experience! <img src="eek.gif" border="0">

I put more emphasis on time rather than distance. ATC uses ground distance rather than air distance. Its the air distance between aircraft thats the factor here. What appears as a good 2 minutes ground-wise can be less than 40 secs in time given unfavorable airspeeds and wind.

[GotTheTshirt]

Gonzo,
I think my question got answered in the flurry of answers but I was wondering who makes the definitions of heavy light etc and how is it evaluated ? Why is the B757 singled out?
Is this a Airworthiness definition (like noise certification) or just an operational one ?
Is there a listing of aircraft votex profiles?

<img src="confused.gif" border="0">

[Gonzo]

The Vortex category is determined by a/c Max take off weight:

In the UK:

Up to 17000kg is light
17000 - 40000kg is small
40000-136000kg is medium
more than 136000kg is heavy,

The 757 was found to produce more violent vortex wake than other mediums, so we classify it as an 'upper medium' along with Ilyushin 62, VC10, 707 and DC8.

I don't think this has anything to do with airworthiness, it's purely an operational concern.

Gonzo.

[ 06 January 2002: Message edited by: Gonzo ]</p>

[Willit Run]

A few years ago , here in the USA, the "heavy" classification was changed from 300,000 lbs to 225,000 lbs so the 757 would be included in the "heavy" catagory and so ATC would give enough space to avoid the 757 wake.

[Gonzo]

Willit run, So what's the spearation between a 757 following a 747?

Is it a case of if the 757 is in front it's a Heavy, but if it's No.2 it's a Medium?

Gonzo.

[Binary]

It is well known that different sttes use different separation minima and different weight categories. This has arisen because the ICAO provisions (DOC 4444) don't provide adequate separation, e.g. when following a 757. Isn't it about time all states agreed on a safe standard and applied it uniformly. UK NATS seem to have a good system that is based on research; are any others better?

[cossack]

We all seem happy about the extra mile behind a 757 on approach, but the main thrust of this thread is departure separation timing.

Every ATC provider has their own minima. Couple that with differing company minima and we've got confusion. Confusion must be avoided and this should be done by deciding whether or not a 1 minute separation following a 757 on departure is appropriate or not. Either way, once a decision is made we will all be singing from the same hymnsheet.

Until that decision is made, if you're in the UK (where 1 minute separation is approved for ATC) and you're behind a 757 in the queue and you want/need 2 minutes separation, please inform ATC prior to line up. You never know you may just depart first!