Cap 56 life is not that easy; “Basically Boeing is not responsible to set the limits. Boeing has to prove under what conditions the aircraft is fail passive or fail operational. The certifying authority then approves the capability.”
The manufacturer has to determine the accuracy and reliability of the auto flight system to enable low vis operations. In addition the autopilot minimum use height has to be assessed (often a function of nose down failures, hence one use of trim up); in some aircraft the min use ht does not enable the lowest DH if the 80% rule is used, which for JAA operators only applies to Cat 2 and not Cat 1. (Appendix 1 to JAR-OPS 1.430). The autopilot min use ht should be published in the AFM and for FAA based AFMs there is usually a diagram showing the failure case recovery trajectory – check the assumptions made about this for your aircraft and for the recovery maneuver (1.3 g pull up?). Thus the Manufacturers limit may supercede the regulatory limit, and with a higher DH the RVR minima are also higher.
FunctionedSatis there is enormous scope for confusion between certification requirements and operating requirements; the generally accepted ICAO definitions were replaced some time ago by muddled thinking, mainly by the operational regulators (JAR-OPS).
JAR-AWO (equipment certification) is one of the better regulatory documents, it divides the categories by DH (100 / 50 / <50 ft), or no DH, and any rollout guidance requirements.
The RVR minima (visual cues) are operational limits; these are in a messy section (E) of JAR-OPS. However, with improving technology the boundaries of the minima are being eroded. i.e A ‘Super Fail Passive’ Cat 3 autopilot (still a fail passive system) may operate in 150 m RVR, which is considered Cat 3b visibility territory (Avro RJ), but the DH remains at 50 ft. Whereas Cat 3a 50 ft / 200-300 RVR allows a manual landing following failure, operations in 150 RVR mandate a GA after the failure.
The best advice is to read the manual, check the MMEL, and don’t assume anything.