Balliol wrote:
It also doesn’t react like an ILS (track rather than heading).
Eh what? The only thing an ILS sees is the track, just like GPS. The heading is irrelevant, track is what matters.
I looked at GPS ILS and it would have put you in the trees of Loxley wood on Wellesbourne runway 36. It also doesn’t react like an ILS (track rather than heading).
I suspect you mean runway 18 
I looked at GPS ILS and it would have put you in the trees of Loxley wood on Wellesbourne runway 36
At what height AAL would that have happened?
The MDH which you apply is obviously the key.
Wellesbourne airfield is officially 48m, Pit Farm next to it is showing 46, Wellesbourne Wood (which I guess you call Loxley Wood) is 95m (300ft), so if a sensible MDA was applied, say 800ft…? That is 500ft AGL. That’s assuming there are no other obstacles.
I cannot see anybody believing that going under 500ft AGL in un-surveyed land is a good idea. Even VOR approaches, or most GPS/LNAV approaches, don’t go that low.
There is also the issue that if you create a virtual ILS to the runway, for every runway out there, obstacle clearance will be compromised before you reach the tarmac, on some of them. That is why a nonprecision approach has to have the level-flight portion, carried out at the MDA.
Or is there some other issue?
Ultimately it has to be an MDA (or MDH) issue, IMHO.
There is a good reason why the author of this software (whom I remember as a very smart guy) wrote this bit of information into the instructions for his app:
“Furthermore, it will blindly offer a predetermined slope to a runway touchdown point on approaches which have not been surveyed or verified. In some cases, that might include the flight path going straight into a mountain or obstacle.”
An app like skydemon gives you the vertical terrain cross-section – one could include that in the virtual ILS calculation.
Peter
GPS ILS had it as a 3 degree glideslope whereas PAPIs are set to 4 degrees to clear the woods safely, think it would be about 30ft clearance if you extrapolated a 3 degree glideslope out from threshold. Obviously if you set a high MDA you wouldn’t get that low although I hear people are using it to fly to 200ft for ‘training’
Tomjnx
The HSI presentation shows track not heading a sit is GPS derived so all the training / technique of applying drift correction is not done
Peter wrote:
That is why a nonprecision approach has to have the level-flight portion, carried out at the MDA.
NPAs certainly don’t “have” to have a level-flight portion. In fact, the recommended (mandatory for CAT) way of flying a NPA today is as a Continuous Descent Final Approach (CDFA) which does not have a level-flight portion.
If you look at a Jepp chart for a NPA (at least reasonably recent ones) you will see that they list DA/DH rather than MDA/MDH and also that they include tables with nominal altitudes at regular intervals to guide the approach.
the recommended (mandatory for CAT) way of flying a NPA today is as a Continuous Descent Final Approach (CDFA) which does not have a level-flight portion.
And the CDFA can be flown all the way to the runway?
The “recommended” way of flying it is the continuous descent but that is just a camouflage for the real geometry of the obstacle clearance to which the procedure was designed.
Peter wrote:
And the CDFA can be flown all the way to the runway?
Ideally it takes you right to the PAPIs which you can then follow until touchdown.
