Apodino From United States of America, joined Apr 2005, 3635 posts, RR: 6 Reply 1, posted (7 years 4 months 4 weeks 1 day 23 hours ago) and read 2887 times:
I will try to answer the question as best I can.
There are three different categories of the ILS approach, Category 1, which is by far the most common which allows you to fly to a decision height of 200 ft and allows you to land in as low as 1/2 mile of visibility. Even though those minimums are sufficient for most operations, in some of the larger airports where the airlines operate, they have category 2 and 3 approaches to allow the airline to use even lower minimums if needed. Category 2 allows you to fly to about a 100 ft Decision height with as low as 1/4 mile of visibility. Then there is the category 3 approach, which is an autoland approach. What that means is the pilots will select the plane to approach mode and use all three autopilots, and the plane will autoland and slow down to a point where the captain will disengage the autopilot and then taxi the airplane from there. This will get you down to about RVR 700' for a minimum. Naturally, because the category 2 and 3 approaches have lower minimums, they require additional equipment, mainly a more stringent localizer and glideslope on the ground, plus the airplane needs additional equipment on board, and on top of that, the crews have to be certified to fly the approach. I believe that each aircraft has to do one autoland a month to keep its certification.
TheSorcerer From United Kingdom, joined Oct 2005, 1047 posts, RR: 0 Reply 2, posted (7 years 4 months 4 weeks 1 day 23 hours ago) and read 2874 times:
Apodino, what exactly is decision height? Is it the height at which the pilot has to be able to see the runway? IE if the decision height was 500ft and the pilot couldn't see the runway at 500ft he'd have to go around?
ALITALIA,All Landings In Torino, All Luggage In Athens ;)
Apodino From United States of America, joined Apr 2005, 3635 posts, RR: 6 Reply 4, posted (7 years 4 months 4 weeks 1 day 23 hours ago) and read 2852 times:
Thats basically it. The Decision Height is the height where you decide whether to continue the approach and land, or go around. If you can't see the runway environment at Decision Height, you should go around immediately.
Tozairport From United States of America, joined Oct 2005, 670 posts, RR: 1 Reply 5, posted (7 years 4 months 4 weeks 1 day 23 hours ago) and read 2850 times:
Just to add a bit to Apodino's post...
Cat 3 approaches have 3 categories too, CAT 3a (700 RVR), CAT3b (300 RVR), and CAT 3c (0 RVR). For CAT 3 approaches, visibility is the sole controlling minimum, and there are only alert height, no decision heights. As long as you have the minimum RVR at alert height, then you can land (with some restrictions). The aircraft, pilots, and runway all have to be certified to the appropriate category. I have done a 300 RVR landing at OAK and a 767-200 and the system worked great. The hardest part was finding the gate after landing!
The biggest difference between 3b and 3c is getting off the runway.
I remember BEA tried 3c at LHR with upgraded runway and taxiway lighting but it proved impractible and 3b is now the standard. The RVR is down to being able to see the lights to get off the runway. The crew will have no visibility of the runway until after touchdown.
Grbld From Netherlands, joined Dec 2005, 353 posts, RR: 3 Reply 7, posted (7 years 4 months 4 weeks 1 day 22 hours ago) and read 2788 times:
Quoting Tozairport (Reply 5): For CAT 3 approaches, visibility is the sole controlling minimum, and there are only alert height, no decision heights.
Tozairport, not entirely correct. A CAT 3a approach has a Decision Height, usually 50ft radar altitude. CAT 3b/3c approaches have no DH.
Alert Height is NOT the same as DH! Alert height is a different altitude (usually 200ft) at which you have another decision point during the approach if you have a fail operational autopilot.
Most 737s only have a fail passive autopilot, which means that if a significant degradation in the system occurs (eg. one autopilot fails), you have to make a go-around at all times, as long as you do not have the runway in sight.
Aircraft that have a fail operational autopilot can still do an autoland when a significant degradation occurs, but there's an extra decision point before which you should make a go around and investigate the problem. After that point, you go ahead and do the autoland anyway. That's the Alert Height.