At this ILS CAT 2 approach, the DA is at 1570 ft, the DH is at 100 ft (because of CAT 2). But why is it.......

Question

... written that the RA is at 107? (what is RA by the way)? Should I enter the DH (100) or the RA (107) in the radio altimeter?

http://i213.photobucket.com/albums/cc115/PilotFerdi/akdf2008.jpg

Answer 1

DA is referenced to Mean Sea Level.

DH is the height sometimes (ICAO) above the Runway Landing Threshold, and sometimes (US) above the Runway Touchdown Zone Elevation.  --Depends on the charting authority.  Some countries' charts (South Korea) do it one way or the other.

The charted RA is the glidepath's elevation there above the surface below; the aircraft's radio altimeter measures absolute altitude.

Answer 2

The reason for the discrepancy between the 107' RA and the 100' DA is simply that the reference point (ground) is not perfectly flat.

A DA is measured above the touchdown zone elevation.  When you are you' above that, you won't be directly above the touchdown.  You will probably be somewhere short of the runway.  Some places there are things like drainage ditches or whatever short of the runways.  So the 107' is simply what your Radio Altimeter will be reading when yuo are 100' above the TDZE.

Make sure you enter the RA into your altimeter, that is the controlling number.

Answer 3

RA is the radio altimeter minimum - the reading of the radio altimeter when the airplane is at the DA of 1570 feet and centered on the glideslope.  The small difference between DH and the RA reading is because DA/DH is measured from the touchdown zone elevation (you can confirm this by comparing the DA with the Runway Elevation in the next box); RA is measured straight down from where the plane reaches DA on the glideslope just before the beginning of the runway.  Terrain features/slope before the runway account for the difference.

Set 107' in the RA to use that minimum.

Answer 4

DH 100ft