Outer Marker substitute UK?

[Boss du Manche]

On our KLM-approachplates, on all of our UK-destinations, there is a dotted line always at 4 NM from the threshold.

Most of my collegues say this is an outermarker substitute. I don't thinks so, because I cannot recall on any ILS that I have flown in the UK, there is an outermarker, so a substitute is not possible. Also if this is the OM-substitute, it would be the point to continue the approach, if the visibility is at or above the minima's, but in the UK there is always an approachban at 1000' HAT.

So my question to all of the UK-pilots is:

1. Are there any ILS-es in the UK with markerbeacons.
2. Why aren't there any markerbeacons in the UK (if answer 1 is no).
3. What is the purpose of this 4 nm before the threshold.

Personally I think this 4 nm has to do with the borderline for obstacleclearance in the final approach segment. It is a changeoverpoint for the intermediate segment to the final approach segment.

[bookworm]

1. Are there any ILS-es in the UK with markerbeacons?

Yes. Exeter EGTE is one that I can think of. But they're dying out.

2. Why aren't there any markerbeacons in the UK (if answer 1 is no)?

Because most of them have been replaced by terminal DME equipment, which removes the need for having a point on the localiser whose range is 'marked'. The DME has the advantage of giving a continuous readout of range, and groundspeed, both of which probably aid a safer approach.

In the old days, having 4 marker beacons and 2 locator NDBs off-airport was the sensible way of providing the facility. Today, it's more cost-effective, I presume, to stick a single DME transponder on the airport (easier to maintain) and radar vector the aircraft for the approaches. If that's supplemented by an on-airport NDB, it also allows for lower minima if the LOC is unavailable.

3. What is the purpose of this 4 nm before the threshold.

I don't know KLM plates. If your ILS and LOC plates are overlaid, is it either the FAF or a step-down fix for the non-precision approach? I can see such step-down fixes at Luton, Stansted and Gatwick.

[AffirmBrest]

1) Yes
2) See 1)
3) More to do with the point at which you must have the minimum viz/RVR for continuation of the approach. UK Air Law/JAR-OPS says that you must have the required minimum visibility for the approach by the 'Outer marker or equivalent position' (ie. about 4d)

For approaches with neither Marker or DME the 'cut-off' is 1000'aal.

If the vis drops below min RVR for the approach once beyond the MKR/4d/1000'aal, however, then you can carry on to the DA/MDA and land if you have the required visual references once there.

Also used as a glideslope/range/altitude cross-check but I don't know if that is a primary purpose for it being there.

Anyone any other ideas?


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