smartt1982 From United Kingdom, joined Nov 2007, 225 posts, RR: 0 Posted (3 years 10 months 3 weeks 3 days 13 hours ago) and read 4146 times:
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?
In theory this should mean that Min Eye height above threshold (MEHT) for a PAPI should be able to accommodate the must crucial aircraft ie 747,A380 (where the pilots are much higher above the main gear) and still bring the aircrafts main gear 50ft above the threshold?
zeke From Hong Kong, joined Dec 2006, 10738 posts, RR: 76
Reply 2, posted (3 years 10 months 3 weeks 3 days 6 hours ago) and read 4005 times:
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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