Originally Posted by
ce650
A while back at Flightsafety for recurrent, there was an ILS into MIA (dont remember which one) that has the "no autopilot below whatever altitude". Anyways, the instructor told us it was because there is a large parking lot on short final and all the cars hose the radar altimiter.
I dont know if he was full of s*&t or not?! But that was the reason he gave us.
That could very well be. There should in theory be no electromagnetic interference on the NAVAID frequencies (FCC laws) but the government can't control objects or terrain on private property that might affect a RAD ALT. Also I don't think RAD ALT freqs are protected, they might be used by a variety of sensors on the ground (door openers, etc).
Next time you fly in there, stay on GS to 960' and then see what is right in front of you on the ground. Actually it might simply rapidly rising or dropping terrain.
Autopilots doing a CAT-I should not of course even use the RAD ALT but on GPWS equipped airplane they might disconnect if the RAD ALT sensed rapidly rising terrain.