|Quoting smartt1982 (Thread starter):|
Can I just confirm, in relation to a 50FT screen height, this relates to the height of the main wheels above the threshold and not a 50ft eye height above the threshold?
Actually it is neither, and a very messy complicated subject. It is actually known as the ILS reference datum for Facility Performance Categories II
and III ILSs. For an aircraft, this datum is in reference to the receiver antenna. ICAO make the assumption that the difference between path of the aircraft glide path antenna and the path of the lowest part of the wheels at the threshold is assumed to be a maximum of of 5.8 m (19ft).
It gets messy as contrary to popular belief, glideslopes are not straight, they are normally slightly curved, and the ILS reference datum is based upon a obstacle clearance assessment of this curved glideslope after flight testing. It is the glideslode shape, and terrain from around 6000-1000 from from the runway that will determine the TCH.
A lot of work goes into this, as for a CAT 2 approach, the DH
might be 50ft, and the aircraft needs to have the correct obstacle clearance.
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