It's interesting to read the comments above alongside the NOTAM for Gloucester's newly commissioned ILS, which states that auto-coupled approaches should not be used further than 6NM away from the threshold.
RWY 27 UNDULATION OF ILS GLIDE PATH MAY OCCUR BEYOND 6NM. AUTO COUPLED APPROACHES SHOULD NOT BE CARRIED OUT BEFORE 6NM.
I'm guessing this may be due to the ridge a few miles east of the airport.
So given the choice, does this and the other points in this thread mean an LNAV/VNAV GPS auto-coupled approach would always be better than the ILS?
Am slightly puzzled by what the effects of the use of the ILS auto-coupled vs manually would be at that range - I guess the autopilot might track the ILS much more responsively than a pilot would, making it more of a rough ride.
I'd be happy to try the ILS there and see what it actually does.
However, looking at the Jepp plate for EGBJ, the ILS platform is at 2000ft and the GS starts at only 5D, so one would not be on the GS at 6D anyway.
There is an alternative 8D DME arc procedure, at 2300ft, and that should pick up the GS at 5.75D.
So this precaution seems to apply only to people intercepting the GA from a higher level than published, which nobody should be doing anyway.
In the US, the standard GS service volume is 10 NM and it is not uncommon to intercept the GS at a higher altitude than the one that is charted. Often, rather than descending from the IF to the GS intercept altitude, it is easier (or lazier) to let the autopilot do it as long as one can intercept from below the GS.
There may be ground reflection interference that causes the GS to be less than smooth over the ridges. In the US, this would show up during flight tests and get the note added to the approach chart. Normally, I have seen this note to not use the autopilot below a given altitude.
Of course a LPV would be unaffected.
I may be wrong but I don't think Gloucester EGBJ has any radar capability, nor do they have a deal with a radar unit (e.g. Brize EGVN) to offer such.
So the approach must be procedural, which means it has to be flown as published.
I may be wrong but I don't think Gloucester EGBJ has any radar capability, nor do they have a deal with a radar unit (e.g. Brize EGVN) to offer such.
So the approach must be procedural, which means it has to be flown as published.
Doesn't the (R) after the Gloster Approach frequency indicate that radar service is available?
Regardless, my understanding is that after the beginning of the intermediate leg at D8.0, one should descend from 2300 to 2000 to intercept the GS at or before D5.0. I understand the altitudes to be minimums. Forgetting the autopilot aspect of a rough GS outside of D6.0, I would have no issue with intercepting the GS by remaining at 2300 when past D8.0. The GS at D8.0 is nominally at 3166 feet and the aircraft would intercept from below the GS very easily at 2300 feet and at a point nominally between D5.8 and D5.7. In other words, following the GS inside its service volume is one way to get to the intercept altitude.
The Gloucester ILS procedure was designed to be the same as the existing NDB/DME format to 'standardise' our suite of procedures.
The glidepath undulation issues are caused by the high ground east of the airfield. The signal goes just outside the permitted ICAO Annexe 10 tolerances within the 'standard' 10NM coverage, hence the restriction. Operationally, you're unlikely to use it beyond 6NM anyway because the published profiles aim for a 5NM descent. The CAA insisted that we promulgate the restriction because, technically, the GP was non-compliant, even though it is perfectly adequate for the published procedures.
We do have primary radar although it is not always available due to staff availability - not all our controllers are radar qualified. Even when vectoring, though, we'd always descend you to intercept the glidepath from below.
When we do the LPV upgrade, the profiles are likely to be the same but, obviously, there won't be any restrictions.
