With fail active a single failure, usually below 400 feet, permits continuing the approach.
As to U.S. carriers with fail active you use an Alert Height (AH), which is usually 50 feet. You don't have to see anything at AH, just make sure there are no pertinent displayed warnings.
With fail passive there is typically a 50-foot DA, which means you have to have the required visual references by 50 feet.
The two are significant differences with RVR values near minimums.
These DA or AH authorizations and limitations are handled by Ops Specs, not by the approach chart.
Last edited by aterpster; 13th Jun 2010 at 12:59.
Reason: word error