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Inspired by a thread on the Tech Log forum (160kts til 4dme) regarding approach speeds into LHR, I wanted to ask controllers about some of the specifics of maintaining separations between aircraft on approach to LHR (I am guessing it will probably apply to other airports as well).
In the case where minimum interarrival separations of 2.5 NM are applied (assuming no need to apply wake vortex seps), are these separations applied prior to 4 DME (i.e. while aircraft are speed controlled at 160 kts)?
Since aircraft then decelerate after passing 4 DME, naturally the separations will deteriorate. Is this taken into account when setting separations before 4 DME? In other words, are slightly higher separations applied so that when the leading aircraft crosses the trheshold the following aircraft is not less than 2.5 NM behind?
Since aircraft then decelerate after passing 4 DME, naturally the separations will deteriorate
...at which stage 'reduced separation in the vicinity of an aerodrome' can be applied.
A whole raft of conditions applies before nats' 2.5nm spacing can be applied. The final arbiter is the Air Arrivals controller...who could well be sitting on the edge of his/her seat throughout...
To achieve 2.5nm on the localiser the separation on the base leg has to be extremely tight at about 2nm because half a mile is lost in the turn. If separation drops below the SMF parameters to 1.99nm the SMF will punch the number two director out of his/her seat and into the office, no tea, no biscuits. Is it really worth laying one's licence on the line for one's employer...
Maybe Gonzo will have something to say about 2.5nm 'separation'...
2.5nm is the radar minima. Normal practice is for 3nm spacing to 4DME to be applied, so that the compression that occurs is still legal in IMC (when the Tower cannot see you so cannot apply Reduced Separation in the Vicinity of the Aerodrome).
If we can see you out to 6.5nm, and there is a headwind, we can apply 2.5nm spacing to 4DME, the proviso that as the Tower are visual with the one crossing 4DME and slowing down, and the one at 6.5nm, the longitudinal separation can legally reduce below 2.5nm.
From what I understand, the spacing is based on a minimum measure crossing threshold...
At LHR, spacing and wake turbulence separation is applied to 4DME.
Curious how you get less than 2.5nm sep, with wake turbulence issues?
Well, of course, if there is a more constraining separation requirement, such as wake turbulence or diagonal separation due to an aircraft on approach to the parallel runway, then that applies. We only go below 3nm on pairs that are not wake constrained.
So for example, A320 followed by A320, or 737 followed by 767, etc.
As I said, we only go for 2.5nm when the conditions exist that when the 2.5nm begins to compress, i.e. when the first aircraft reaches 4DME and slows down, the Tower can apply visual separation.
That explains it perfectly. Does the same approach apply when wake vortex minima are used?
E.g. say you have a heavy with a medium following. The wake vortex sep. requirement is 5 NM.
Do you then add a 0.5 NM buffer on top to give a separation of 5.5 NM when both aircraft are on the approach prior to 4 DME, to take into account the compression when the heavy passes 4 DME and begins to slow down?
Suspect the agreement was written on the back of a fag packet about 30 years ago, since lost, and wouldn't stand too much scrutiny if re visited today. I bet HD knows the real answer to this one though
It's not ATC which makes such regulations or even asks for them and a good few controllers I worked with did not like 2.5nm spacing. ATC received no specific training - 2.5nm was introduced when I worked and that's going back many years. Major airports are commercial organisations and the airlines demand space for more and more flights to land at them. One answer is to put the aircraft closer together. Who determines finally that this shall be done I do not know (Gonzo might) but one day I arrived at work and was told that 2.5nm may be used under certain conditions so it was a case of sit down and get on with it.
I do like this principle of allowing the separation minimum to "compress"
Leaving wake turbulence alone for a little while (it's an approved procedure) the 2.5nm separation is a minimum radar separation. To reiterate again the separation is allowed to compress only when the tower controller is visual with both the aircraft concerned. Separations maintained with Reduced Separation in the Vicinity of an Aerodrome, i.e. visually, are not subject to the same minima and hence this is not cheating.
It is commercial pressure encroaching upon safety.
Two aircraft less than 2.5nm apart on a clear day from a tower when both are established on precision approaches is perfectly comfortable. Issuing safe and legal landing clearances is the challenge and we owe a debt to the professionalism of the aircrews involved to expeditiously and safely vacate the runway, almost always with a great deal of situational awareness of what is occurring behind them. Of course, if it doesn't work, then a go around results and this too is a perfectly normal and safe procedure, albeit to the detriment of what the commercial pressure was trying to achieve. Oh well, in this case safety wins, and it's the professionalism of the controllers that does not allow the pressure to warp their assessment of any given situation.
In my opinion the 2.5nm rules as they are allow the maximum safe throughput of aircraft on one piece of tarmac in good visibility, still conveniently putting wake turbulence aside since this is a further limiting factor.
I am certainly not following this rational on wake separation...Perhaps some views on the thought or approved process.
Two aircraft less than 2.5nm apart on a clear day from a tower when both are established on precision approaches is perfectly comfortable.
The ICAO wake tables are based on a minimum of MRS between many and similar aircraft wake cats. While these distances are based on ATC and radar control, I dont follow how that can be extrapolated to visual separation, while you can 'see' the aircraft..you cannot 'see' the turbulence....
How far does one take 'similar' aircraft in this scenario...an A380 2nm behind an A380?
Aircraft still make a vortex, and the mechanics of vortex creation are not fully understood nor developed. As an example, a lightly loaded 737-800, on final may only need flaps 30, while a heavily loaded 737-800 will need flaps 40. These flap settings create a completely different type, strength, and advection rate of the vortex pair.
Crosswinds do not only affect advection rates, it also influences creation of the vortex
I would note that it is well documented that the wake turbulence is stronger and lasts longer on a clear day.
Adding this later..the FAA RECAT program does not allow for visual sep between any wake cats, even same...
"Familiarity with RECAT wake separation standards is particularly important during visual approach operations as pilots assume responsibility for avoiding wake turbulence when cleared to visually follow preceding traffic."
"I am certainly not following this rational on wake separation...Perhaps some views on the thought or approved process."
There are two separate separation standards being discussed here - the 2.5nm (then Reduced Separation in the Vicinity of the Aerodrome) is completely separate to the wake vortex issue. Subject to certain conditions, the closest that UK radar controllers were permitted to get aircraft was 3nm - whether they were following, passing, crossing etc. Any closer than 3 miles was a loss of separation. At some point it was determined that, as discussed by others more knowledgeable above, 3 miles could be reduced to 2.5 miles for aircraft on final approach to LHR. As an aerodrome controller, you can easily get aircraft much closer than 2.5nm anyway - imagine two departures rapidly turning onto different tracks, or an inbound following something visually etc. The judgement falls to the aerodrome controller to visually assess if the separation is safe and appropriate, and as hangten says, if we can safely issue a landing clearance at a useful stage to the second aircraft.
Wake turbulence is a completely separate issue and overrides any other reduced separations.
The final approach spacing will be a combination of the above, plus any adjustments required either by the tower (eg availability of exits, braking action, departures to go in between arrivals), weather (eg tailwind, icing conditions requiring a/c engine anti-ice hence higher approach speeds) etc etc.