Autopilot disconnect with RVR < 300m
This applies to fail passive autolands only and is covered in detail in IEM to App 1 to Jar Ops 1.430. The assumption is that a go around will be made if this occurs below DH, with the proviso that a landing may be made if this is felt the safer course. Most modern stuff is fail operational, only degrading to fail passive, and therefore we would be looking at a multiple failure for this restriction to become a factor.(eg for 757/767, LAND3 = fail operational, LAND2 = fail passive)
Downgrade on Approach
The rule is that an approach can be continued if the RVR falls below minima after the OM or equivalent (JAR OPS 1.405). This is not the same thing as and aircraft downgrade. The rule was written to cater for normal variations in visibility over time - ie it doesn't work the other way around! If you did not have reversion RVR at the OM then you must fly a go-around.
[ 09 December 2001: Message edited by: Offset ]