Wake turbulence - Radar enviroment

[alexious85]

Hello
I would like to ask if there is a reason for pilots to bother counting the time since the preceding traffic has taken off , instead of just adhering to the take off clearance . If in a radar controlled airport , wouldn't it be the tower's responsibility to adopt distance-based separation and issue take off clearance accordingly ?

[alexious85]

TangoAlphad you may be right but error / omission set aside , my question leans more on the procedural side . The reason i am asking is because quite frequently in LHR , a place were controllers very rarely ever make mistakes , take off clearance is issued in what seems to be less then the prescribed radar wake turbulance separation minima . It gets me thinking that there are other factors that come into play that we do not know .
I have to admit i did not pose the question in the manner i wanted but i was hoping i would get many replies that would resolve my question

[HEATHROW DIRECTOR]

LHR used to base separation on time but I don't know what happens now.

[Gonzo]

alexious85

At LHR we anticipate the duration of the reaction time and take off roll.


For a two minute wake turbulence separation, we time the rotation of the first aircraft, add two minutes to that time, and then take away what we anticipate will be the time taken from issuing the take off clearance to the rotation of the follower aircraft (45-50s for a Medium, 60-65s for a Heavy).


So we would give take off clearance 1:15 after the lead aircraft rotates, in an effort to achieve 2:00 at rotation.

[Talkdownman]

...which becomes compounded by the crew doing its own timing on the stopwatch.

Quite frankly the tower controller has quite enough to do rather than clocking 120 seconds, whatever, on behalf of the crew, so since the crew are the first at the scene of the turbulence they might just as well do the fine tuning themselves. They know the required minima too...don't they...

The crew would certainly have to do it at an aerodrome with no ATC!

[GASA]

Controllers at all airports anticipate take off roll in issuing clearances, that's not special to LHR. Probably just the timings change slightly airport to airport depending on their airlines and aircraft and how quick they are :P

[Crazy Voyager]

I'm not convinced all crew know the minima, or they know but don't always like them. For example we regularly get crews asking for more time from intersections that are notified as the same for wake purposes. I do realise you can't ask every crew who operates in once every 3 years to know this, but when operating on single runway you really do not want that surprise after they've lined up in a gap which only gives you 2 minutes separation before the next lander.

Also less common is pilots asking for extended routings after certain aircraft, we all know the 757 is common to ask for more after but for example, A330 after B777 asking for 2 minutes for wake. There is nothing wrong with that at all (the rules clearly say a commander can ask for whatever they want and they will get it) but we need to know it before lining up (again, in the AIP).

Finally I've had crews ask for more time, which I've provided, and then say they're ready to go when less of that time is elapsed, which makes me wonder how they actually decided they needed more in the first place. Key here again being that as said above, we anticipate roll to time rotations. It is not a case that we start timing at rotation and can not clear you until the 2 minute have passed.

On the topic of wake, be aware that out of Paris it seems you now get new timings as well:
RECAT-EU, optimising Europe's airports capacity | Eurocontrol (http://www.eurocontrol.int/articles/recat-eu)
Pair Wise Separations (PWS) / RECAT-2 | Eurocontrol (http://www.eurocontrol.int/articles/pair-wise-separations-pws-recat-2)

[Gonzo]

GASA, no. It's not special to LHR, but it's generally special to the UK.

Most other countries that I know of will mark the time of the start of roll of the lead, then wait 120seconds, then begin passing the take off clearance to the follower. The theory being that the read back, reaction time etc will cover any differences in take off roll time.

[Cough]

As flight crew we time as I think its fair to say, most of us have been subjected to the occasional mistime on wake. I've personally been cleared for takeoff immediately following an A330 when in an A320... Needless to say the controller was very humble when he noticed the error. Then asked if we had a clock running...

Anyhow, I think the issue in this thread is interesting, in that we time differing things. I almost always see colleagues time the start of the T/O roll whereas I time the rotation ever since an ex atc colleague mentioned the difference in techniques. I simply feel the more people who are aware of the differing timing techniques (and the reasons) the better it would be...

[cossack]

In a radar environment we provide distance separation when required behind everything except the A380.

[LEGAL TENDER]

behind everything except the A380.

And what do you do behind the 380?

[cossack]

And what do you do behind the 380?
Time, rotation to rotation.

[fujii]

The OPs question was about towers applying radar standards yet most answers are time related. 2 miles upwind plus two miles of RWY you'll easily get 5 miles airborne. I had the occasional query but just explained I am using radar, not time.

[cossack]

The rule of thumb to provide a distance on departure is to roll the second aircraft when the previous departure is 1 mile less than the required distance away. Due recognition has to be given to aircraft type and differing performance characteristics (short take off roll and/or fast acceleration) in order to avoid short-changing the required separation.
There is no need to use a stop watch.
If you need a time based minimum, advise tower before accepting a line up clearance with the required time. Asking for another 30 or 60 seconds once on the runway is more often than not, in a mixed mode operation, going to result in a missed approach. In non- mixed mode you'll probably just incur the ire of the controller.

[cossack]

H-H according to ICAO is 1 minute. My manual says I need to provide 4 miles which is more than 1 minute. How is this "a bit dodgy" or "bending the rules"?

In Memphis, Fedex and the FAA agreed on different wake turbulence separation criteria after much research. MD-11, A300 and B767 are separated there by only 2.5 miles.
New Wake Separation Standards Boost Capacity at Memphis | Air Transport News: Aviation International News (http://www.ainonline.com/aviation-news/air-transport/2013-02-04/new-wake-separation-standards-boost-capacity-memphis)

[sheepless]

<LookingForAJob>
DOC 4444, (2015) - Possibly relevant extract..

8.7.3.4 The following distance-based wake turbulence separation minima shall be applied to aircraft being provided with an ATS surveillance service in the approach and departure phases of flight in the circumstances given in 8.7.3.4.1.
HEAVY / HEAVY 7.4 km (4.0 NM)
HEAVY / MEDIUM 9.3 km (5.0 NM)
HEAVY / LIGHT 11.1 km (6.0 NM)
MEDIUM / LIGHT 9.3 km (5.0 NM)
8.7.3.4.1 The minima set out in 8.7.3.4 shall be applied when:
b) both aircraft are using the same runway, or parallel runways separated by less than 760 m (2 500 ft);

It refers to the "approach and departure phases".

My guess - some individual countries might include Landing and Take-off in these phases whilst other countries might not. :*

Although why distance is universally accepted for landing (where radar is used) but not for take-off beats me..... Anyone have an explanation ?

[terrain safe]

Sheepless.

I guess because they were developed before ATMs or DFTIs were in common usage.