Originally Posted by
Pierre Argh
Radalt has its uses, but picture the scene: An airfield sits atop a hill, at 400ftamsl. Your minima is 200ft. But, in the approach to the runway is a valley where the land drops to 200ftamsl, rising steeply towards the threshold. You're on an instrument approach, and looking at the radalt (which gives you height above terrain NOT Height above Airfield). Now please tell me exactly what indication your radalt gives you to tell you when you've reached the MAP or Decision Altitude?
On a Cat II approach there has to be so many seconds of steady radalt descent during the last stages of the approach. I can't remember what the figures are now.
Luton is a case in point, 526ft up on top of a hill with valleys either side. We got Cat III approval before Cat II when we managed to prove there was the required no. of seconds steady radalt descent. Even so, this warning is on the approach plates:
Due to the sloping terrain in the approach area
of ILS CAT II operations, the rate of radio
altimeter height reduction prior to the THR will
be approximately double the normal rate.
At the heights, and therefore range, of DA/DH on a Cat II the terrain is almost certainly to be within the aerodrome boundary so the airport will (should
) have control over who digs holes where. Outside the boundary, within the UK, there is the safeguarding process which ensures the airport is notified in advance of any proposed development that might have a bearing on operations.