Of course, most of the above posts are also correct as I didn't specify my example applied to EFIS/FMS (Incl ex AA MD11) and certain manufacturers' GPWS systems.
For example, the Honeywell system GPWS/EGPWS as fitted on AA F-100's uses the radar transmitter/antenna for terrain mapping, along with the ppos database. If the wx radar isn't switched on and terrain selected ONLY the standard samplings i.e. VSI, rad alt etc. will provide warnings.
Other flight envelope samplings also occur depending on the aircraft's configuration.
For EGPWS on AA F-100;
turn Wx radar ON and push terrain button ON which, hopefully, turns a pleasant shade of blue. This is where the "signal" attenuation is sometimes mis-read and misinterpreted in heavy rain.
Radar alt on F-100 isn't "supposed" to work above 2500' agl.
However, along with the spurious GPWS warnings at 11,000' the rad alt can also read 1000' (in a nice green hue) in heavy rain. Honeywell is working on a fix at this time.
Cheers,
Last edited by dallas dude; 10th Jun 2002 at 01:27.