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50Ft Screen Height?  
User currently offlinesmartt1982 From United Kingdom, joined Nov 2007, 225 posts, RR: 0
Posted (2 years 3 months 1 week 2 days 7 hours ago) and read 2930 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?

3 replies: All unread, jump to last
 
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17044 posts, RR: 67
Reply 1, posted (2 years 3 months 1 week 2 days 1 hour ago) and read 2819 times:

I would think it is 50ft on the altimeter. I think the altimeter is calibrated as equivalent to the height of fuselage somewhere but not sure.


"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlinezeke From Hong Kong, joined Dec 2006, 9109 posts, RR: 75
Reply 2, posted (2 years 3 months 1 week 1 day 23 hours ago) and read 2789 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.



We are addicted to our thoughts. We cannot change anything if we cannot change our thinking – Santosh Kalwar
User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 3, posted (2 years 3 months 1 week 1 day 13 hours ago) and read 2654 times:

Quoting Starlionblue (Reply 1):
I would think it is 50ft on the altimeter. I think the altimeter is calibrated as equivalent to the height of fuselage somewhere but not sure.

The radio altimeter is generally calibrated to be pretty close to wheel height above the ground at typical touchdown attitude...in other words, RA=0 should be when the wheels hit in a normal landing.

Tom.


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