From: firstname.lastname@example.org () Organization: Rockwell Avionics - Collins, Cedar Rapids, IA Date: 04 Dec 95 01:14:50 References: 1
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The L-1011 has an automatic landing system certified to Weather minimum Category IIIB which is 0 ceiling and 300 ft (or perhaps a little more) runway visual range. Category IIIB requires rudder control because the autopilot must fly the airplane for some time while it is on the ground and before it slows sufficiently for pilot control at low RVR. Category IIIA has a 50 ft decision height meaning that the crew must visually see the runway at that point or are required to Go Around by procedure. It also has a longer RVR, perhaps 1400 ft. The Autoland system monitors itself and announces in advance if it has sufficient redundancy to procede into Category IIIA or B conditions. There may have been some failures indicated such that only Category IIIA or even Category II was only allowed (100 foot DH). The crew may have elected to attempt an approach hoping to sight the runway at or before DH. There may also be crew certification issues. I'm not sure on this but I think crews are required to perform a certain number of automatic landings (even in good weather conditions) to maintain proficiency and certification to perform them in actual weather conditions. Perhaps the crew was only certified for Cat II or Cat IIIA. If the conditions were marginal, they might have had a reasonable probability that conditions would permit a landing. So they can make an attempt hoping not to divert but if the Decision Height criteria are not met, they must abandon the approach at the DH.