III Corps

Yep, at my house it was. With IIIB you didn't have to see anything but with IIIA you had to see something at 50ft. Thus a DH or an AH.

Also, if you lost one of the 3 autopilots, you went from IIIB to IIIA. We got quizzed on that routinely.

5030N

You are correct that some CAT III approach mins are based upon the availability of rollout control, but you have to go a bit deeper than that to determine whether an AH or DH is appropriate. Think about it this way---

Most Ops Specs allow a carrier to execute a CAT III approach to mins as low as 300 RVR. To go that low, the autopilot(s) must be fail operational (LAND 3 on the 75/76) and rollout control must be available to autoland. An AH is used because there are no visual refs required. CAT IIIb approaches are typically to mins of 300 RVR or 600 RVR.

If the autopilot(s) are not fail operational, then they are fail passive (LAND 2 on the 75/76), and the lowest mins available for fail passive approaches are 600 RVR. A fail passive approach requires a DH, required visual references, and rollout control in order to autoland. Thus, you can execute a fail passive approach, using a DH, to CAT IIIb mins as low as 600 RVR.

The approach categories are not really that important, i.e., fail passive does not automatically mean you are limited to CAT IIIa, nor does te approach category determine whether an AH or DH is appropriate.

Takes a bit to get your mind around it, but once you begin thinking in terms of fail operational vs. fail passive rather than CAT IIIa, CAT IIIb, etc., it gets easier. You can take a look at AC 120-28D if you want to really muddy things up.

subicpilot

Thanks for the info, 5030N. I guess the bottom line here is every carrier/aircraft/approach combination out there has different rules and requirements. It's difficult to give a one size fits all answer.

5030N

Absolutely.

As with everything, your mileage may vary.

Cheers.

EMB120IP

I guess the point I was trying to get across was using an increased safety margin. If something goes wrong, it would be nice to have a takeoff alternate, even though you can do a cat 3 approach, that would have better mins to land in and not put all eggs in one basket. This, of course is dependent on the situation. If you have to get down NOW, then if you are capable, then yes, a CAT 3 would help. In my case, we wouldn't have this option. I'm sure a CAT 3 certified carrier has requirements for filing/or not filing a takeoff alternate with very low weather, but as to what they are, I'm not sure. Sorry for the confusion.

EMB120IP

My point exactly. Some people (at my company) are die-hard, read between the lines kinda people. They read the regs from a different point of view, and not from the point you bring up from your post. I've run across captains/fo's who have said that as long as the wx is above 1/2 mile, they don't need a T/O alternate, even with the 30 kt tailwind "because the regs don't stipulate the assumed landing runway". Stupid, I know, and trying to get this common sense across to them has been difficult. Sorry for the confusion. I don't think my post was in the right order, but now that I read it, I jumped around a bit. I type like I talk.
Joe

EMB120IP

My company too. We are allowed to use lower than standard takeoff minimums also. However, for us, if the RVR is 600, but the takeoff runway we are using allows a minimum of 1000 RVR for takeoff (per the APT diagram), the minimum for takeoff on that runway is 1000 RVR. I would of course get a runway assigned with lower allowed vis, but I don't think I made this clear. Thanks.

Twin Wasp

Since the only controlling element (in the US) is visibility, I'd say you're (legally) fine on those rare 100 and 3 days.

ChinsFive

I understand, I was just saying that where I work as long as the plate says 1 mile or less with no ceiling we can take it down to 600 RVR if we have the equipment. So if the plate says 1000 RVR but we have the lights, markings, centerline lights, and 3 RVRs (one can be out) we can take-off with 600' RVR.

I've done a 600 RVR takeoff on a runway that is 1000 RVR on the plate, and a 1000 RVR takeoff on a runway that is 1/4 mile on the plate. All perfectly legit.

Toccata

Just to add to this answer a little, fail operational does not actually always require multiple autopilots. Two HUD displays are considered fail operational, and certification, with appropriate POC demonstrations, can occur (120-28D 4.3.2) . In addition, an 'autoland' is separated into pre-touchdown/touchdown (approach, flare, touchdown), and post-touchdown (rollout). Some aircraft, such as the DC10, are technically fail-passive on rollout, but AC 120-28D permits them to now be "considered to have rollout capability equivalent to fail operational...." and land at 3/3/3 with a 100' AH.

Just thinking fail operational vs. fail passive, as did work with 120-28C, does not necessarily provide all the pieces now. Unless you always leave rollout out of the picture. As stated above, the AC is an excellent source, as 120-29A is for CAT I/II questions.