Cat III

[Brian Abraham]

Could you tell this rororhead the logic for the following from the 737 manual

"Maximum allowable wind speeds, when conducting a dual channel Cat II or Cat III landing predicated on autoland operations, are:
• Headwind 25 knots"
Would have thought a headwind would not be an issue.

Also asked before but no answer - what is the typical alternate minima for a Cat IIIb
Thanks to all. :8 :8

[Pilot Pete]

I suspect the 25kt headwind is due to the demonstrated limitations of the autopilot system and autothrottle. One of our resident experts will, I am sure be able to expand (or even deny!) my theory.

what is the typical alternate minima for a Cat IIIb Not sure I understand your question. Do you mean what planning minima does one need for our alternate airport if destination is operating in LVOs? If this is what you are asking, then at the planning stage any Alternate, be it Destination or En-route (under JAR-OPS) which has CatII or III approach capability, must meet CAT1 planning minima. Once you have dispatched, the planning minima is now irrelevant and it is the commanders decision as to where he wants to use as an alternate.

All the 737s we have are only CatIIIa, but I believe that some later versions are catIIIb capable.

PP

[Brian Abraham]

Thanks Pete re the minima. :ok:

[NW3]

On the 757 you always set vref30+5 for an autoland, no matter what the headwind - maybe it's connected: it might be that the autopilot flare mode gets things a bit wrong if the airspeed or groundspeed is different to the conditions it was designed for.

Total guess!

NW3

[Pilot Pete]

You do the same on the 737, but I am not sure that is what he was asking. He wanted to know why the 25kt headwind limit for autoland, which I recall was the same on the 757? I don't think groundspeed comes into it (just a guess), I am assuming it has something to do with the autopilot and Flare Mode, setting a pre-determined nose-up attitude that has been tested in winds up to 25kts head. I guess this is also why we have to factor our landing distance calculations (by adding extra distance) for autolands, as the pre-determined flare attitude will only guarantee a touchdown within a certain 'range' on the runway as opposed to a certain point on the runway. Perhaps if the thread was moved to Tech Log forum it would get a better response?

PP

[Cough]

Maybe its the turbulence that a >25kt headwind generates that they are worried about... Turb + a few gusts may take the A/C outside the design limits for the autoland.

[Rainboe]

I think something to do with it is: with a h/w of >25kts, why autoland? The turbulence at some places may overwhelm the autopilot close to the ground, and there is no need to autoland at that windspeed because you should not get significant fog. Having said that, I know there are places that give high speed fog, but it is not worth factoring that into the design and causing extra expense for every operator.

[MarkMcC]

FWIW I think that the autopilot certification limits are the reason for this. The aircraft will be certified for a max demonstrated headwind and crosswind - the regulations specify what these are, and while they vary from country to country (well...JAR to FAR to CAR...etc.) they are largely aligned (specifically I'm aware of the 25 kt headwind). Once the max value required is demonstrated it becomes part of the limitations in the aircraft type certificate, and therefore the flight manual. As with many things it's not because the aircraft will fall out of the sky at 28kts headwind, it's simply that it's not approved because it wasn't demonstrated because the certifying agency didn't require it and the customer didn't want to pay for it.

Frankly, if there was a great demand for Cat III approaches with greater than 25 kt headwinds (or 15 kt crosswinds) then somebody would have gone to the expense of demonstrating and certifying it. These are also expensive conditions to find during a flight terst campaign :)