"Tim" wrote in message
om...
Hi,

Could anyone give me a good explanation for OCA/OCH. I know it has
something to do with the minimums of ILS-approaches and it should
always be lower than or equal to the DA/DH. I assume it provides you
with a certain obstacle clearance at DA/DH. But how is it
determined????

[ICAO version, YMMV, particularly if you use different sized miles.]
Obstacle clearance altitude OCH is the height on an IAP with the minimum
permitted clearance above obstacles on the final approach. It does not take
account the limitations associated with the navaid (system minimum). Thus
the DH or MDH is the higher of the OCH or the system minimum. The OCA is
obviously the altitude corresponding to the OCH.

For non-precision approaches the OCH is the height of the highest obstacle
on the final approach segment plus a speed-dependent margin. Precision
approach OCHs are more complex, I think. Most Cat I ILSs are limited by the
system minimum of 200 ft rather than the OCH.

Julian Scarfe