ATC IssuesA place where pilots may enter the 'lions den' that is Air Traffic Control in complete safety and find out the answers to all those obscure topics which you always wanted to know the answer to but were afraid to ask.
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.
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.
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!
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.
Thread drift here, but I thought some may find it interesting.
'At the IP another group cut in front and we experienced severe prop' wash. The last report on the intercom was the pilot saying "We're going over." Unfortunately, Satan's Sister could not take the strain and she broke in two at the radio room.....'
Bob Scalley, tail gunner, one of only four survivors from Lt. Arthur W. Carlson's crew of B-17 Satan's Sister, 388th BG, Ludwigshaven, 30 December 1943.
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.
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.
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.
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.
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.
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.
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...
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?
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.
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