How do you Time Wake Turb. Separations on Departure
 

After a recent event we have been trying to get an idea of how to correctly time the wake turbulance separations applied to departing aircraft.

For a Medium aircraft Departing behind a Heavy 2 min Wake Turb. separation is needed. From what point do you time this 2 min and how do you apply it?

We all know Wake Vortex is only generated from rotation so when do you start timing the 2 minutes?

Do you start timing from when the heavy rotates? And then at what point to you issue take off clearance to the medium behind? If you time from rotation and then wait the full 2 min then issue the 2nd acft takeoff Clearance by the time this 2nd acft rolls and rotates surely about 2min 30 sec has passed now? This obviously starts eating into an airport's efficiency.

If you time from the time you issue a take off clearance and a heavy may "road run" and only rotate towards the end of the runway however the 2nd acft may rotate at the RWY midpoint thereby having rotated 20 sec prior to the heavy. In this case however the medium should be above the heavy's wake vortex however this may not remain so if ATC impose climb restrictions on the 2nd acft. the just here is that the full 2 min may have been timed from take off roll to take off roll however between rotation points there may be less than 2 min.

Do any of your units/ANSPs have prescribed (written) procedures in your local Procedures on how to time the separation etc?

Any thoughts on how you apply the Wake Turb time separations will be valued as I would like to find out how other ATCs/ANSPs comply with this.

Most pilots, concerned with wake, will hit the stop watch as soon as the preceeding aircraft starts rolling and won't roll themselves until the 2 minutes is up.

Works for me in work also.

Working on an estimate that each aircraft reaches Vr after about 35 seconds, I start the watch as it begins it's take-off. It works out to be two minutes from previous and my rotation.

You note time when the heavy gets airborne. i.e 45:00

add two mins= 47:00

take 30 secs off that= 46:30...this is the time you clear the medium for take off knowing that by the time they rotate, the 2 mins will clearly have passed.

Your initial assessment is correct, to wait two mins THEN clear for take off would cut into an airports efficiency a hell of a lot, and i know nobody that does this.

very often at schiphol you can hear clearance like this: caution wake turb of departed ...when ready cleared for t-o.
which in my point of view as atco not that bad,because controller passed information to pilot and basically transfered responsibility to the crew as well...:}

I agree on DAL208, but would also consider the company, wind, aircraft time... and other factors that could easily increase that 30 second run, and thus subtract a few on my count.
About schipol´s way, I wouldn´t feel confortable leaving the pilots, with all their pre take off checks, to take a decision we are being paid for.

At my academy (EPN, Sweden) we use the time preceding starts to roll.

What works for me is to write the EARLIEST time nr. 2 can depart on the strip of nr. 2 in field 10.

Example:
NR. 1 departs time 37:35.
I write on strip of NR. 2: 39:35.

/TH

Issue T/O clearence 2 min after preceding started his roll, usually spot on when airborne. Never had any problems/complaints about it.

Time from the first one rotating. Allow sufficient roll time for the second and clear it for takeoff. eg 747 rotates at 10:00:00, the next is a DH8C which only takes 15-20 seconds from brakes off to rotate, so you might say "cleared for takeoff" at 10:01:40.
A 757 might take 25-30 seconds from brakes off, but if it's positioning and light then it might only be 15-20 seconds. You get a feel for this after a while.

Although you can mess about with a few seconds here and there for slots / MDIs etc, you absolutely can't with vortex as it is such a safety issue. Therefore it's better to be a few seconds over the required time, but the closer you can be to the exact 2 minutes, the more expeditious and efficient you are.

The UK's good book says that the times are 'Minimum spacing at time aircraft are airborne'.

I remember being taught in college that vortices are generated from the time the nosewheel lifts, and that's where I time from.

Yep, this is it:

Ref: From MATS 1, Sec1, Ch. 9
I time from when the nosewheel detaches from the ground, on that basis.

In Oz you must also provide a radar wake turb standard for the departures controller.
This is 5 miles so sometimes your 2 minutes wont be enough.

I have never heard of someone being stood down for 1 min 58 seconds but I've seen plenty of standdowns for 4.9 miles.

AA

amberale,
you're right, 1:58 when you should have 2:00 is pretty unlikely to cause a problem.....

However.....if the following aircraft were, heaven forbid, to crash, and lawyers acting for whoever were to determine from SMR replays etc that you had failed to provide the required minimum, even by a few seconds.....:confused:

I know the fatal on approach to Mexico City a few months back was more extreme than just a couple of seconds, but it's a sobering reminder of the dangers of vortex.

Quoted from the UK book apparently: ".....continues until the nosewheel touches
down on landing."

Incorrect. If you've ever watched, say, a B737 approach leaving visible vortices, the vortices stop the instant the main wheels touchdown (and lift is dumped). It has nothing to do with the nosewheel touching down. IMHO and from observation.

I'd like that clarified as well. I was always under the impression (and taught others) that it was when the main wheels touched down that lift was dumped. On departure, isn't the nosewheel lifting off purely a function (reaction) of the elevator pitching upwards, and not lift? I thought that on departure, the 'lifting' occurs (I guess, technically) a little prior to the main wheels separating from the tarmac. That is, as weight 'begins' to be taken off the wheels. Nothing to do with the nosewheel on/off the runway. Pilot/Physics gurus?

There's a bit of ambiguity about this subject that I'd like a clear interpretation of, too.
I've always used the method described above by thpaulsen, the moment a/c 1 starts to accelerate down the runway, note the time, and that becomes the reference for following departures. (As a technique, personally I make a mental note of the time, and/or write it to the nearest 10s on the strip, but some use an electronic timer.)

Thing is, though, it does state "at the time the a/c is airborne.." (or similar) so unless you know exactly how long a/c 2 is going to take to get airborne, it becomes a guess. Which means it's a guess. (As several posters to this thread have alluded, and use, and it's probably a more than adequate guess.)

I would think the lawyers would argue that since the only reference you can be sure of is the time you observe a/c 1 lift off (or rotate) that timing should commence from that moment, and a T/O clearance not issued to the following until the required time interval is up. That would (perhaps) be a purely legal, pedantic view.

In practice, that would certainly do nothing beneficial for efficiency, of course. Unless all ADC started actually doing it in an effort to get the ATS OPS folk to re-write the rule, or provide a clearer interpretation.

So in practice, all my timings commence at the start of the takeoff roll, and 99.?% of the time that will be adequate.
(Where it may not be is the situation where a very slow medium or heavy is departing in front of a real zippy light. Think, maybe, Argosy departing ahead of Learjet. #1 = 50s~ to accelerate to Vr, #2 maybe 25s.)

Re when the vortices start/stop, for landing, it seems to me that once the mainwheels have touched, the AoA is reduced by about 3 degrees, which will immediately reduce but not eliminate the vortex. A second or two later, though, the spoilers (when fitted) will pop, and the vortices will immediately be reduced by a very significant amount. The nosewheel then comes down, and in theory, no more vortex.
For takeoff, the spoilers should not be up and the moment the aircraft starts to rotate, vortices are being produced. I would think their intensity would be at its greatest at or just prior to liftoff, unless fuselage length precludes the best rotate angle for lift until clear of the deck.

It cannot really make a difference whether the stopwatch is started at 'Take-off clearance' or at 'start of roll', provided the same trigger point is used for both (all) aircraft. A crew's reaction time after receiving clearance is (slightly) variable of course - though with a couple of minutes notice everyone should be ready 'immediate' - the only one that is pretty fixed (+/- 5 seconds) is 'start of roll'. I was told years ago that ATC use 'Take-off clearance' for timing because they cannot stop 'doing' and wait to observe the start of every take-off roll. If ATC and crew use the same trigger, clearance for #2 is going to be issued in the last 15 - 10 seconds of the 2 minute period; if the crew observe that it is issued at about 1m30s then ATC are probably using 'clearance' (it doesn't matter, If you timed from start of his roll then you start to roll at 2 minutes).

Rotate point is a variable along the runway, even for the same type of aircraft, and a LIGHT will usually be airborne before the preceding MEDIUM/HEAVY so no issue there.

On the 'Rotate' issue, the wing generates lift as soon as it presents a positive angle of attack to the airflow, this lift force increases progressively until (at the appropriate speed and AoA) it generates sufficient lift to carry the aircraft mass. So lift is generated very shortly after the nosewheel unsticks - but not much, initially.

When nose weels lift off?
 

What if in low visibility take off? We don’t know when the nosewheels lift off!

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I would expect at most, if not all, airports operating LVPs, that the increased taxi time from the Cat 3 holds would more than compensate for the elapsed time required. If in doubt, request an airborne report from the pilot & use that.