ILS Landing Minimums

M

meritflyer

Well-Known Member
Could someone briefly explain the differences between Cat I, II, and III landing mins (which category allows the lowest mins?).

Do they only occur on ILS approaches?

Thanks.
 
meritflyer said:
Could someone briefly explain the differences between Cat I, II, and III landing mins (which category allows the lowest mins?).

Thanks.
Click to expand...

I dont know the specifics, but i believe CAT III B is the lowest mins.
 
CAT I ILS approaches generally have a 200' AGL decision height

CAT II ILS approaches generally have a 100' AGL decision height

CAT III ILS approaches are split into A, B, and C. They offer lower minimums (CAT III C offers a 0/0 ability). Keep in mind the aircrews and aircraft must be certified, trained, and have specific equipment aboard).

Lots of good info on Google!
 
after your instrument rating you can just do CAT I - 200' and 1/2 mi vis. CAT II and III require special crew training/certification and airplane certification.
 
Well, technically speaking a Cat I approach can be a Cat IIIb approach, it all depends on how quickly the NFP, says "MINIMUMS". :-)
 
Just remember, you can actually descend below DH on an ILS. Once you reach DH, you have to make a decision to land or go missed. So you'll descend slightly below DH while you make the decision (and possibly the go-around). He who makes the slowest decision gets in.
 
Continuing below minimums without the appropriate stuff in sight could be a really, really, really poor career move, know what I mean?

:)
 
meritflyer said:
Could someone briefly explain the differences between Cat I, II, and III landing mins (which category allows the lowest mins?).

Do they only occur on ILS approaches?

Thanks.
Click to expand...

Visibility required for the approach. Cat I highest Cat III lowest. Cat III is further subdivided into A, B, C again just a matter of visibility required. Note I said visibility not ceiling as the former is the determining factor. NOTE: following from memory. Visibility for Cat I was 1200 RVR and CAT IIIC zero. That's the range, as I remember it. Cat IIIB was the lowest I remember at Delta Air Lines, and I think it was 300 or 400 feet.

That's the basics...then it gets really complicated when you put in the operator's (i.e. specific airline) operating specifications, airplane equipment required, pilot requirements ("X" time in that airplane) and even ground equipment.

When you get down to the lower minimums generally speaking an autoland is required. I say generally as my experience was with Delta Air Lines and the 757/767, MD88/90. In all cases for Cat III an autoland was mandatory.

As for "pilot requirements" again, goes to the operator's specifications. And it can get pretty complicated. For instance there were "high minimums" i.e. for a newly minted Captain, until you had a certain amount of time in the airplane the minimums were higher....except if you flew an autoland approach, then those "high minimums" could be waived.

The key is knowing just what your minimums were as "big brother" aka the FAA is the one you will ultimately have to answer to if you violate them.

To give you a perspective on a really low visibility Cat IIIB approach. When the "alert light" comes on at 50' AGL, you are way past the threshold, the power is coming to idle, the airplane will be on the ground in a second or two and you still have seen NOTHING outside the cockpit! When the nose wheel comes down to the runway (all this is done on autopilot with autothrottles) you can see at best TWO of the centerline painted segments, maybe four or five green centerline lights, and if you look real hard a couple of the runway side marker lights. Oh yeah, you're still rolling along the runway at over 100 MPH, the autopilot still tracking the localizer, auto brakes slowing the airplane!

Eventually you get stopped and NOW the real job begins. Trying to get off the runway and taxiing to the gate!
 
The only simulated Cat II approach I did was on my CFII ride where the inspector had me shoot an ILS at 120kts and go below minimums to about 50' AGL before taking the hood off. It was a 9000' runway, so there was plenty of room to bleed off the speed. It is really cool to see how accurate the glideslope and localizer can really be that low... it takes you right down to the centerline.
 
ROFCIBC said:
When you get down to the lower minimums generally speaking an autoland is required. I say generally as my experience was with Delta Air Lines and the 757/767, MD88/90. In all cases for Cat III an autoland was mandatory.
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The 737-800 with the HGS is certified to handflown CAT III A. In that case you aren't even allowed to use the autothrottles. You could dispatch to CAT III weather with no autopilot/autothrottles and captain's flight director inop.

Went out one day and did touch and goes with the captain's windshield covered. Worked like a charm.
 
ROFCIBC said:
To give you a perspective on a really low visibility Cat IIIB approach. When the "alert light" comes on at 50' AGL, you are way past the threshold, the power is coming to idle, the airplane will be on the ground in a second or two and you still have seen NOTHING outside the cockpit! When the nose wheel comes down to the runway (all this is done on autopilot with autothrottles) you can see at best TWO of the centerline painted segments, maybe four or five green centerline lights, and if you look real hard a couple of the runway side marker lights. Oh yeah, you're still rolling along the runway at over 100 MPH, the autopilot still tracking the localizer, auto brakes slowing the airplane!

Eventually you get stopped and NOW the real job begins. Trying to get off the runway and taxiing to the gate!
Click to expand...

Thanks for that ROFCIBC. I had some knowledge of what a CATIII approach was. As human beings it seems it would take a lot of fortitude to actually complete one of those approaches.
 
flyover said:
The 737-800 with the HGS is certified to handflown CAT III A. In that case you aren't even allowed to use the autothrottles. You could dispatch to CAT III weather with no autopilot/autothrottles and captain's flight director inop.

Went out one day and did touch and goes with the captain's windshield covered. Worked like a charm.
Click to expand...

Is the FD system on the HGS (I'm assuming this is the headsup display) seperate from the PFD FD? I was under the impression the headsup display is just mirroring data (literally) from the PFDs. How would you get a flare cue if you didn't have any vertical pitch guidance? Either way, that is pretty neat that you could handfly a 0-0.
 
BobDDuck said:
Is the FD system on the HGS (I'm assuming this is the headsup display) seperate from the PFD FD? I was under the impression the headsup display is just mirroring data (literally) from the PFDs. How would you get a flare cue if you didn't have any vertical pitch guidance? Either way, that is pretty neat that you could handfly a 0-0.
Click to expand...

When you use the HGS in the autoland mode it is completely separate from the FD. It generates it's own data. The FO's FD must be working to help him monitor the approach.

Can't go 0-0 though. On the handflown approach you must have touchdown zone lights visible at 50'. I would have felt no apprehension in hand flying a CAT III. It is a great system.
 
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