Edges of control zones

[18greens]

Does anyone know how the edges of control zones are decided?

Do they try to make them co-incide with land features or is it just wherever they fall based on distance from the airfield IAP requirements etc. Does the designer slap his zone down then say aha, if i move this bit 200m west it won't detract from separation but will give those VFR chappies a nice big ground feature to route around, thus reducing zone busts.

EG the south west of the Heathrow zone is Bagshot mast.
South east is a prison which used to be marked with a heli no fly zone (but that has been removed from the chart.)
East limit of gatwick has a big lake running through it. etc.

Are these coincidences or have I got it completely wrong.

(BTW I think it would be simpler if they painted big yellow lines on the ground where the CTZs are then there would be no problem. It would also provide a useful outlet for the people who otherwise spaypaint trains and viaducts in their spare time.)

[IO540]

I would expect them to enclose the SIDs/STARs, with margins calculated according to some rule.

[Kolibear]

A very quick measurement on a 1/2mil chart, of Gatwick, Stansted, Luton, Birmingham & Bournemouth zones show them to be based on a 30mm rad. circle, which is 8 Nm in real life, centred on the airfield.

Two lines are then drawn, parallel to the main runway and these lines create the edges of the zone. Each zone is a slightly different width, but approximately 9-10 Nm seems to be the norm.

The final shape appears to be negotiable, Gatwicks northern edge is cut away on a east-west axis, convieniently following the railway line whereas Luton's northwest corner has also been chopped away. Calais has a marked assymetric zone

[FlyingForFun]

My understanding is that the zones are based on the requirements of the instrument traffic within the zone, and not the visual traffic outside the zone.

The only exception to this in the UK which I know of is the Manchester low-level route, which has little corners cut out of the edge of both the bottom of the Manchester zone and the top of the Liverpool zone, to allow entry/exit to the zone by following the railway line from Crew or the M6 from the north. (However, all the relevant documents are quick to point out that the corridor itself is not aligned with any specific line feature.)

As Koli says, many zones have straight lines parallel with the runways to mark their edges. Compare this to America, where most zones seem to be circular - presumably enabling the use of DME for finding the edges of the zones? (And in fact, a quick look at a French chart shows that at least the lowest levels of their controlled airspace - what we would call the CTR - are circular.)

Good subject!

FFF
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[Whopity]

The zone is there to protect the traffic within it and is performance based upon the traffic likely to use it; that may well take into account the performance of a Group A aeroplane following failure of the critical power unit. That will account for the large size of some zones.