Wake turbulence time

[ORICHETTI]

Wake turbulence separation by time on take off. When do you take time? i´ve searched almost evereywhere and I don´t find it. thanks guys :D:ugh:

[underfire]

One can use distance or time based separation. Are you looking at some of the time-based scenarios, or what is the context, that would help.

http://i.imgur.com/zk4XVLZ.jpg

[Sino]

We take the time at rotation

[RAT 5]

The separation is required because of vortex. These are created by lift. Thus I've always started my clock when the preceding a/c rotates. I haven't read anywhere a specific 'must do' moment. This just seemed appropriate.

[de facto]

If you are a M following another M…when the aircraft gets airborne(i.e. starts to make wake) then you count time (1 min- your time to get airborne..40-50)…about 10secs.
So aircraft get airborne,10 secs time check then off you go…by the time you get airborne,the leading aircraft will be 3NM in front of you.

[ORICHETTI]

I´ve always taken time at liftoff of the traffic, but somebody "corrected me" and i was searching for the exact moment wrotten on an official document. nothing on 8168 or on 4444 as far as i saw

[de facto]

i will find the docs and give you the reference,,give me 2 days…not at home now.

[good egg]

"Wake turbulence separation minima on departure shall be applied by measuring airborne times between successive aircraft. Take-off clearance may be issued with an allowance for the anticipated take-off run on the runway; however, the airborne time interval shall reflect a difference of at least the required time separation."

[RAT 5]

UK MATS Part 1
That's an interesting one. It would suggest ATC is responsible for separation on takeoff. Years ago I was at LGW, holding. An M was given "cleared for takeoff" behind an H. The captain of M spouted up that "You should know I need 2 mins!" Humph. ATC replied, "I only cleared you for takeoff, I didn't order you to takeoff. Take your time."
So which is it? I'd always assumed it was the pilots' responsibility on takeoff and ATC on arrival.

[FlightDetent]

I time the preceding's positive start of rolling. Probably not scientifically exact, but good enough for me in practical terms, to reach its wake in later than two minutes. Most likely my liftoff point comes earlier as well.

The ATC has been always helpful as far as I can remember. Either delaying the take off clearance or amending it with an advisory. I suppose individual ATC's procedures may differ.

"Caution wake turbulence, behind departing XY triple-seven, cleared for take-off Rnn".

[good egg]

UK MATS Part 1
That's an interesting one. It would suggest ATC is responsible for separation on takeoff. Years ago I was at LGW, holding. An M was given "cleared for takeoff" behind an H. The captain of M spouted up that "You should know I need 2 mins!" Humph. ATC replied, "I only cleared you for takeoff, I didn't order you to takeoff. Take your time."
So which is it? I'd always assumed it was the pilots' responsibility on takeoff and ATC on arrival.

UK MATS Part 1
"Note: ATC shall apply the minima as prescribed above, irrespective of any pilot request for reduced wake turbulence separation. ATC does not have the discretion to reduce wake turbulence separation minima."

Unless the rules have changed since your quoted incident I'd suggest the ATCO was at fault.

[ORICHETTI]

so answe will be "liftoff" then? thanks!

[Gonzo]

RAT5,

Or perhaps the lead a/c was a 757, which if followed by any other M in the UK does not require wake turbulence separation. We used to come across that at LHR quite a bit when BA, BMI and Aer Lingus 320/737s would go wheels up behind a 757, but those same types from countries which applied the ICAO '757 is a H if followed by a M on departure' separations used to ask us for/tell us they needed 2 mins.

As has been said, UK applies rotation to rotation, and ATC cannot reduce the minima, even if the pilot wants to do so.

[FCeng84]

As SLF it has always seemed to me that similar sized aircraft are allowing almost exactly 2 minutes between departure brake releases. Doesn't that simplify things? For some departures the visibility will be such that one cannot see the airplane ahead in the takeoff sequence at the time that it rotates. Time is time is time and takeoff speeds will be similar for similar sized planes. Allowing 2 minutes between brake releases translates very closely to the same amount of time separation between rotations and even more closely to that amount of time separation when initially airborne. Am I missing something here?

[underfire]

Many airports use a rolling departure. Depending on the ac, some of the 4 engine variant take some time to balance.
Same issue on roll time, some take a bit of time to get moving. Yes, of course, there are plenty of airports where the follower cannot see the leader rotate. Pretty sure that ATC can see the aircraft rotate, and there is the weight off wheels broadcast.

There is wake sep, and there is the min sep to consider.

[DaveReidUK]

As SLF it has always seemed to me that similar sized aircraft are allowing almost exactly 2 minutes between departure brake releases. Doesn't that simplify things? For some departures the visibility will be such that one cannot see the airplane ahead in the takeoff sequence at the time that it rotates. Time is time is time and takeoff speeds will be similar for similar sized planes. Allowing 2 minutes between brake releases translates very closely to the same amount of time separation between rotations and even more closely to that amount of time separation when initially airborne. Am I missing something here?

The assumption that two "similar sized aircraft" will accelerate at the same rate on the runway and rotate at the same point at the same elapsed time after brake release isn't necessarily a valid one.

[Ian W]

The WTC separations are all based on worst case estimates of the stability of wake vortices remaining as hazards after aircraft takes off or lands.

This is a good ad-hoc system but it can be extremely wasteful of resource if there is a crosswind that is effectively clearing the wake vortices in a few seconds yet the next aircraft sits for minutes because on a calm day the vortices may last longer.
Similarly, if there are parallel runways the vortices from one runway may interfere with aircraft on the parallel. Insisting on delays for aircraft using the same runway in that case will miss the hazard to aircraft operating on the parallel. Lots of research work has been carried out assessing these issues.

The potential cost to an airport in terms of reduced runway acceptance rates is huge. There are several laser based systems on the market that allow the vortices to be tracked and assessed so that only real hazards cause delays. Possibly the most advanced is WindTracer WindTracer® for Airports · Lockheed Martin (http://www.lockheedmartin.com/us/products/windtracer/windtracer--for-airports.html) which also assists the forecasters with mesoscale forecasts. It is surprising that the take up of these systems is so low.

(I do not work with LM )

[underfire]

It is because LIDAR system do not work very well in the field. LIDAR systems work on the light reflecting from particulates. If there are too few, or too many, it does not work. Rain, fog, snow, etc, doesnt work. Too clear like in AUS, or too much in the air like Dubai, doesnt work.

For a system to be used, it must be real time all weather, which LIDAR takes too long to take a measurement, and far to long to process. The current crosswind/headwind systems that you mention are close, but use ground winds from the AWOS at 10m. As pilots are aware, while that is good for braking, the winds are frequently different at altitude, and windshear at low levels. That is why these methodologies have not been industrialised yet.

That being said, there are systems and methodologies that do work, and several are in the planning stages. These provide the winds up to 300m and wake measurements every 5 seconds. This will enable these systems to be used for aviation.
Currently, Phase I will by using the reduced separation on DEP with winds, and time based ARR spacing with winds. Phase II will be dynamic DEP separation using winds and wake turbulence.
An added benefit to the systems, is identifying exactly when the aircraft is right over the array (which is approx 600m from the end of the runway) thus, is t=0, visual is not required, and ATC will know exactly when the ac is over the array. If there is a splay at or near this location, ATC has the exact time when the aircraft gets to the splay point, providing another important variable to the decision making process.

Some examples:

http://aviationsystemsinternational.com/wp-content/uploads/2016/03/Wind-on-DEP.jpg

http://i.imgur.com/cf17hN6.jpg

[underfire]

From FAA land:

http://i.imgur.com/THNjN7C.jpg http://i.imgur.com/ttIDzVA.jpg http://i.imgur.com/0WAszNY.jpg

[*Lancer*]

Practical: you're waiting for the right time to start rolling, so start timing from when the preceding aircraft started rolling.

[underfire]

lancer, well, what if you cant see them? ie. If you are waiting long and they are short?

Suffice to say that it looks like this is an ATC function, as a driver, you can verify (to some extent) and observe (again, to some extent), but in reality, isnt your focus on other things at this point in time?


Other questions, not directed at you, guidance shows that one should rotate before the leading ac has rotated. As far as rotating before the other ac rotated, does one rotate at a visual point, or at V1/Vr?
Hypothetically, if one sees the leading ac rotation point, and one passes that point before V1 is reached, what does one do? (ie. you knowingly set up a wake turbulence encounter?)

I see the guidance on this from the FAA and such, but in reality, it is not reality.

[JammedStab]

Did a heavey takeoff in a 747 out of Frankfurt a couple of years ago and heard the A320 behind us requesting two minutes separation. I can't remember the exact wording but he was definitely pressured by the controller into something less than what he wanted. It seemed like we were ,aybe 500 feet when he was getting his takeoff clearance.

[Feather44]

I totally agree with Lancer.

How many times, I've seen my colleague starting the chrono at lift off of the preceding ac. Then, waiting 2 min before to start rolling.