quid, jetjocky737 I refer you to my previous post; before you or the local inspector can authorise an operation the certification standard of the system or what part remains serviceable, has to be understood.
I have experience where a Cat3 HUD operator was given local authorisation for take-off in lower RVR than originally certificated. What the operator and local inspector were unaware of was the lack of an adequate HUD control law to provide guidance for an engine failure; the original low vis take-off certification had assumed that there was some external visual scene, thus an engine failure at the reduced RVR value would have been hazardous.
For 73 vs NG, check the details; Cat 3a vs Cat 3a enhanced (lower RVR?), procedures after failure, and crosswind limits; I hope that the safest common denominator is used and not the best operational capability.
For the J41 or any other system, they all require a method of cross monitoring sensor inputs and autopilot (F/D) output. In a dual autopilot the ‘second’ system or channel usually provides the monitor. Single channel Cat 2 autopilots and FDs usually have an additional monitor computer; they light the Cat 2 ‘green light’ when operative.
Therefore, it is incorrect to immediately assume that the operational capability of a system can be applied to different scenarios or that a degraded system’s approval extends to alternative approach procedures; don’t assume – check.
Cat 3 sim training; why do so many regulators / operators spend so much time checking standard words / procedures (normal operations), when the critical issues are the failure cases – continue to land after a failure or GA. For most Cat 3 operations (not withstanding my earlier post on Cat 2 problems), there is little point in checking rechecking the visual contact and decision making in the sim. The sim rarely represents the more difficult visual cases accurately, and in real operations, providing the operation is conducted within authorised limits, the crew will always see sufficient visual cues to land (90% plus combined probability of everything working – ILS, autos, and visual segment). Modern systems are very reliable; crews under stress may not be so good.