Radio Altimeter vs. Barometric Altimeter

[svcta]

All of the more regulatory stuff above is absolutely correct, but from a practical standpoint using RA on a CAT I ILS is no bueno.

Like the guy said, watch the RA move around underneath you on any approach outside the flatest of flatlands. There's just no way that it can be reliably used. Sometimes it's a little scary when you're IMC....Charlie West seems to stick out in my mind, and a few others in Colorado.

 

[VVJM265]

Read the pubs...

MINIMUM DESCENT ALTITUDE, DECISION ALTITUDE,
AND DECISION HEIGHT​

MDA and DA are referenced to MSL and measured
with a barometric altimeter. CAT II and III
approach DHs are referenced to AGL and measured
with a radio altimeter.
The height above touchdown (HAT) for a CAT I
precision approach is normally 200 feet above
touchdown zone elevation (TDZE). When a HAT of
250 feet or higher is published, it may be the result
of the signal-in-space coverage, or there may be
penetrations of either the final or missed approach
obstacle clearance surfaces (OCSs). If there are
OCS penetrations, the pilot will have no indication
on the approach chart where the obstacles are
located. It is important for pilots to brief the MDA,
DA, or DH so that there is no ambiguity as to what
minimums are being used. These altitudes can be
restricted by many factors. Approach category,
inoperative equipment in the aircraft or on the
ground, crew qualifications, and company authorizations
are all examples of issues that may limit or change the​

height of a published MDA, DA, or DH.

http://www.faa.gov/library/manuals/aviation/instrument_procedures_handbook/media/CH-05.pdf

See pg 5-18

 

[dapilot]

thnx for all the feedback guys ! really clears things up now