applying wake separation

[chillie]

Been having a discussion about how, exactly, Tower applies the 2 or 3 minutes wake separation and where is this laid down in docs?.

Seems some airports use a stop-watch to measure time from rotation until next is cleared for take-off.

This also brought us onto how times are recorded and where this is laid down.

Sounds like Europe record to the nearest minute but ICAO used to say the current minute.

Any deffinative answers appreciated.

chillie

[Marcus Maricus]

In Jepessen introduction ATC chapter you can find it written down. However, due to dense traffic on major airports this is rarely followed through.

[HEATHROW DIRECTOR]

Timing - controllers have clocks in front of them which show seconds so it's fairly straightforward to work out the separation.

Times are usually recorded using the 30 second point as the minute changeover. So if an aircraft takes off at 18:22:28 it's logged at 18:22. If it goes at 18:22:32 it's logged as 18:23. The timing for the log is at lift-off, not start of roll. However, for separation, timing usually starts at commencement of roll with appropriate allowance made for the type of the next a/c for vortex wake purposes.

[vespasia]

Depends! Vortex separation has to be 120 seconds (2mins) or 180 seconds (3mins) as appropriate. However, vortices are not considered to begin until the aircraft rotates, so we're trying to apply 120 ( or 180 ) seconds from rotate of aircraft 1 to rotate of aircraft 2. We normally allow about 40 seconds for the take-off roll (30 secs if the aircraft is particularly light or there's a good headwind), so I can clear aircraft 2 for take-off approximately 1min 20 to 1min 30 after aircraft 1 rotates.

Who said Air Traffic wasn't an art form!

Personally I use the ATM (radar) and I clear aircraft 2 for take-off when aircraft 1 is 4 miles on the climbout on a calm day and 3 to 3.5 miles when there's a headwind. This guarantees me 120 seconds vortex separation and is easier than trying to subtract 1m30secs from 1354h14secs when I'm busy.

[Spitoon]

The good book, in this case the UK's Manual of Air Traffic Services Part 1, says that the 2 or 3 minute spacing for departing aircraft is the minimum spacing at the time the aircraft are airborne. So if looking into the future is an artform, aerodrome control is an art. And I guess it explains why tower controllers have crystal balls!

[niknak]

Vortex wake application is applied to the second, ie if its a two min dep separation and the preceding a/c departs at 18:29:29 secs, the subsequent a/c may not depart until 18:31:29 secs.
That's the way it's taught in all the colleges and that's the way it should be applied in the field.

[chillie]

Hi everyone

Thanks for the replies.

niknak: When does the acft depart?

We, in our training enviroment record the seconds when the first acft rotates. The second acft can then start its roll after the 120 or 180 seconds.

This means the the separation is always a bit bit greater than required which could mean you are operating at less than 100% efficiecy.

There does not seem to be a laid down method of application??

HD: Times are usually recorded using the 30 second point as the minute changeover.??? Does this mean some countries record the current minute?

Cheers

[simfly]

In NATS college, we are tought to record the mm:ss, then the nearest minute in a box below. As previously mentioned, a/c 2 requires the full 120/180 seconds.

[Gonzo]

Similar to vespasia, we use a distance on climb out from the ATM to judge when to launch the next one, so we get 120 seconds rotation to rotation. Of course, that means we clear no.2 for take off about 80-90 seconds later to allow for no.2's take off roll.

[JustaFew]

Been taught, and teach, that for departure separation based on time, an aircraft is considered to have taken off when there is no contact between plane and ground.

[rata2e]

Vortices are a consequence of producing lift and my understanding was that they commence as soon as the NLG leaves the runway ie prior to the whole aircraft defying gravity.

[Neptune262]

Chillie.....

Wake seperations start from first aircraft's wheels up to the following aircraft's wheels up. Timed to the second, i.e. no rounding up or down of minutes.

Operationally we try to achieve the closest to the required minimum by clearing the following aircraft to take-off prior to the minimum time required allowing for take-off roll required.

However, from a training perspective how can you guarantee the take-off roll time? That is probably why the instructors are getting you to issue take-off clearances after the required time has elapsed.

Hope this helps

[chillie]

262 and others

Thanks. In this case it is I who is doing the coaching :-)

I think the new students are probably not in a position to guesstimate the landing run of the following acft so what we do is safe plus.
chillie

[Van Demon]

Wake turbulence is something that can definitely kill you. The prescribed times in minutes should always be applied as the full number of seconds. Application of wake turbulence separation on departure is a doddle. The real art is to apply it correctly for arrivals timed to crossing the threshold.

[NZScion]

All I know is that it seems like a bloody long time when sitting in the cockpit (not being able to see behind) while waiting on the runway, and knowing there is an aircraft on final behind you...

[slackie]

Scion...You can always request your own WT sep (in NZ)...ATC aren't allowed to prompt you to do this.

[ALATOWER]

Hi everyone!


We have 2 displaced (2000 ft) parallel RWs separated by less than 2 500 ft. Taxing to the line-up for RW2 goes through threshold RW1. What minima separation should we have between a/c when a/c#1 is landing on the RW1 and a/c#2 requests line-up for the RW2 through threshold of the RW1? Is it considered only by wake separation like that departing and landing a/c? Where I might get info?

Thanks in advance!

[Wojtus]

There's no wake turbulence problem at all. Vortexes are following landing aircraft while it's in the air and they stop to form on touchdown. So even there might be some wake turbulence near the touchdown zone, threshold is free of them. What's more, touchdown zone turb there is no danger for taxiing aircraft unless it is huuuge difference of weights.

[ALATOWER]

…might U right, but where can I get info about minima separation for crossing RW clearance in this case? What safety distance should we applies?

[Wojtus]

…might U right, but where can I get info about minima separation for crossing RW clearance in this case? What safety distance should we applies?
Standart "runway occupation" rules should apply. If you cleared a/c to land - you can't clear anybody else to touch the runway. If you cleared a/c to cross the threshold, you can't clear anybody to land unti crossing one vacates. All of the details should be stated in local Operations Manual. "Safe distance" is usually 60m from runway edge and should be well marked on ground.

[ALATOWER]

…thanks, U right I know what u mean…to be correct according to doc.4444 ICAO “at RWY-holding position should be 50m from RWY edge where RWY length is 900m or more” ….but I can give landing clearance at any distances from the RWY- 15 km, 9 km or 1 km. Providing safety separations between departing and landing a/cs we apply the wake separation and not issue line-up clearance if it less than established. So I asked about minima safety separation between landing and crossing RWY a/cs. What different from lining-up a/c preparing for take-off and crossing RWY? I hope we both understand that we talk about case except LVP. Our local doc. not prescribes it.

[jumpseater]

Vortex wake application is applied to the second, ie if its a two min dep separation and the preceding a/c departs at 18:29:29 secs, the subsequent a/c may not depart until 18:31:29 secs.
That's the way it's taught in all the colleges and that's the way it should be applied in the field.

I can confirm that's what I've been taught. Recently. When the nose wheel lifts on the first a/c, the clock starts eg 18:29:29. At college we were given 'fails' if we started the phrase 'cleared for T/O' before 18:31:29.

[Wojtus]

So I asked about minima safety separation between landing and crossing RWY a/cs.Well, that's just the pure art of aerodrome control! ;D
Unless you have strict values published in your Manual, you're on your own judgement here. So it is all safe if you decide it is... until it's too short! Use the licence :)


What different from lining-up a/c preparing for take-off and crossing RWY?A taxiing aircraft is not going airborne, so if you screw it up - just send approaching traffic for go-around and you're fine. But if we are speaking about departure/arrival separation - you have to consider the separation between departing a/c and a/c possibly executing a missapp. So the difference is huge.

[ALATOWER]

Thanks for discussing.

Cheer guys!