Originally Posted by
DOUBLE BOGEY
John, reap what you sow! I have never advocated using DH to achieve the minima on a QNH approach! It is you who needs to read posts more carefully.
Ther is no ILS in the world with a state minima of 200 feet height above the threshold that would put you below that height, anywhere on the approach provided you are on the glideslope. That is made clear in pans-ops.
However, you are permitted some glideslope error at DA, accepting this at DA you may well be below 200 feet height above the threshold.
In modern helicopters fitted with a reasonable AP, coupled to the G/S such errors are eradicated.
All I advocate is that if the helicopter is fitted with a RADALT it is poor airmanship not to use it as the safety device it is clearly intended to be. To effect this it needs to be set below the listed DH minima for the approach, giving a margin to reduce the possibilities of the aural warning before the DA minima is reached. This is the design concept behind the AIRBUS bug system in the EC225/EC175 and it works well in practice.
The ILS systems in Oz do not have significant obstacles in the final approach. If they did the state minima for that approach would be greater than 200 feet DH CAT 1.
Round and round we go. Obviously you are seeing something in my posts which I am not.
I will try again. Using (as I believe that you advocated) the radalt bug set at 200ft to indicate that you are at or below the DA of 200ft can be erroneous. Many ILS approaches that I am familiar with come in over low ground (eg valleys) such that the glideslope height
AGL is significantly greater than 200ft, even though the aircraft is on glideslope and is at 200ft above the airfield reference (be it threshold height or airfield datum).
Therefore setting 200ft (as you advocate) when the actual height AGL may be more like 250 - 300ft
at that point serves no purpose. Since most ILS (and all NDA) precision approaches are designed to cross the threshold at 50ft, the ideal use for a radalt setting would be more beneficial if using a known reference point rather than a spurious and inaccurate one. My posts have been trying to make the point that the surrounding land is lower than the threshold, I see you are more concerned with land being higher: you are quite correct that this would create a higher DH, but that is not the point I am trying to raise.
If all the approaches that you are familiar with have surrounding surfaces level with the airfield, then this concept may be a difficult one to understand. But making silly remarks about Australian procedures is not a good way of discussing the issue, and I note that you have gone quiet on the PEC concept?