Aerad definitions
 

Hi

Could anyone provide the definitions for both MEHT and RDH. I used to have an Aerad legend booklet but have mis-placed it. Would be grateful for a quick reply.

Regards

Without the aid of the aid of the booklet and subject to a dodgy memory

MEHT Mean Eye Ht above Threshold
RDH Radio Datum Ht

Can anyone expand on the definitions of MEHT and RDH?

I think one is to do with the threshold crossing height on the ideal glidepath and the other something about minimum 'on-slope' indications. Is that correct and if so which is which?

Thanks

Not sure about RDH but MEHT is what dv8 described. It relates to glideslope and visual guidance provided such as PAPIs (Precision Approach Path Indicator) so that adequate wheel clearance is provided when over the landing threshold. Lower MEHT values normally mean smaller aircraft types. Info should be on CAA web site in document CAP168, Chapter 6 (lighting)

RDH stands for Refrence Datum Height. It is difficult to explain without any figure, but I will try:
The glide slope beam in an ILS is not an absolute straight line which is extending from GS antenna. In fact it is somehow curved in the vertical plane and this bending is more pronounced as the terrain closes particularly over threshold. That is why the extension of avarage of glideslope curve between the distances 1600 and 4500 feet from runway threshold, crosses the threshold at a slightly different height than actual Glide slope beam. Now I will put some extraction from a document mentioning about RDH:

Instrument Approach Procedure designers use the ILS Reference Datum Height to determine the obstacle clearance surfaces of the
approach in accordance with ICAO DOC81684(Pans-Ops). This is whythe reference datum height is quoted as 15m with only a positive tolerance. If the instrument approach procedure is constructed using a reference datum height of 15m, then any increase in RDH serves to increase the safety margin of the procedure.