lateral separation in DOC4444

[DFC]

topdrop,

how does 8nm account for slat range errors? What is the maximum level at which you can use that separation?

ferris,

You seem to say that an aircraft flying westbound along the 090 radial is parallel to an aircraft flying on the 270 radial?

How can two aircraft which are either head to head or in trail be flying parallel?

If you and topdrop stand back to back and he walks off on a track of 270 and you walk off on a track of 090 are you diverging?

If after 10 paces you turn round and walk back towards each other are you not converging towards the point you started from?

Simply put - aircraft following parallel tracks would have constant track separation. Aircraft following diverging tracks would have increasing separation and aircraft following converging tracks would have decreasing separation.

That isn't ATC it is simple geometry!

However, if you want the offical answer -

Routes are parallel when they;

Have about the same orientation i.e. the angular difference does not exceed 10 degrees;

they are not intersecting; and

traffic on each route in independent of traffic on the other route

The 270 radial from a VOR is a different route from the 090 radial. In simple terms both routes have a common point (intersection) - the VOR but that is the only point that is common to both routes. Perhaps you are mixing this up with the fact that separately, an ATS route may pass over the VOR following the 090 radial inbound and the 270 radial outbound. However if you are an approach controller with your VOR in the middle of no where you have 360 possible inbound and outbound routes!

If we look at the simple geometry of the question. the track separation between two aircraft at 15 miles fromthe VOR on radial different by 15 degrees is 3.88nm.

I have no problem with two outbound aircraft who are diverging i.e. the separation is increasing being cleared to the same level.

However, with two aircraft inbound, I would not be comfortable with vertical separation not being established until the aircraft which are converging are only 3.88nm appart.

Work that with the fact that a pilot can report established on the radial provided that they keep it within half scale deflection i.e. +/- 5 degrees and then add in the system tolerences and you can have two B747s converging and very close together!!

If on the other hand we use the more common 8nm as a target separation we end up with a figure of 30.9nm from the VOR. i.e. The latest to establish vertical separation for a theoretical 8nm separation between aircraft inbound is 30.9nm.

Sound familiar to anyone?

Simple trig - separation(nm) = distance from VOR(nm) * Sin Angle

Regards,

DFC