Category III C Question
 

Can autoland systems of big Boeing/Airbus jets perform crabbing, de-crabbing and side-slip manuevers as responsively as a pilot would?

Are there windspeed limitations for Cat III approaches that take into account the performance capabilities of these autopilot systems?

Thanx.

:ok:

Most modern autoflight systems use a rudder channel, which enables de-crab etc. I can’t comment on the quality of Boeing / Airbus, but the Avro RJ dealt with crosswinds better than I could within its limits – and beyond!
There are usually separate wind limits for autoflight systems as the autoland certification (i.e.Cat 3) requires a different standard to the normal manual operation of the aircraft.

Yes.
The L1011 for example has the following wind limits for CATIII...

Cross, 15 knots
Headwind, 20 knots
Tailwind, 10 knots

These are the basic Lockheed limitations, some operators have more conservative numbers.

Typically 15kts max crosswind for A/L [AutoLand] B74. Some carrier SOPs are max 10kts. But CAT-III weather is usually associated with heavy fog which is not condusive to a lot of wind. [CAT-IIIc is zero-zero]. The hard part in any CAT-III condition [fog] is not about landing, but about taxiing and finding your assigned parking spot, or gate without clipping your winglets or running into something. :ooh:

The normal crosswind limit for the B767 I fly is 35kts in VMC and manually flown and it has more than enough control capability to handle that. I've done a couple of landings recently right on the crosswind limit without any problems.

For an autoland in VMC the limit is 25kts and I've seen it demonstrated on a revenue sector years ago and it worked just fine.

For an autoland in Lo Vis conditions the limit is 10kts.

The aeroplane decrabs and drops a wing if it needs to in an autoland.

The B747-400 operates the same way though I can't remember the limit winds.

Dunno about the other Boeings or Airbii.

Regards,
BH.

B747-400 limits:

25kt headwind
25kt crosswind (only 15kts in USA in CAT2/3:hmm:)
15kt tailwind
Cat 3B No Decision Height.

Above limits apply on 3 or 4 engines, and very impressive it is too.

Boeing 737NG fail operational can do CAT III operation up to a crosswind limit of 25 kts and it is quite good at it with a very nice decrab manouver and perfect centreline tracking.

De-crabbing with perfect centerline tracking into a 25 kt cross-wind?

:eek:

Wow.

Didn't realize these big Boeing/Airbus autopilot systems were that accurate and responsive with the aileron and rudder manipulations during such a critical phase.

In the US, the FAA will not approve an Operations Specification for CAT III operations (Ops Spec C060) unless a crosswind limit of 15 knots (including gusts) or less is specified. This is also true for CAT II; however, the CAT III number used to be 10 knots, and was changed to 15 in recent years. I do not know the reason for these limits, but I suspect it has to do with the "seeing condition" requirements that also drive the minimum RVR for those approaches. Neither of these numbers has anything to do with the airplane's capability; the B757 is limited to 25 knots of crosswind during an autolanding, as well as 25 knots of headwind. I suspect that the airplane limits are driven more by signal gain issues, such as the autoflight system's ability to identify tracking trends due to gusts in a timely manner. Control authority, as far as I know, is not a problem.

One point that is often overlooked is that, from the FAA's perspective, the crosswind limit for CAT II or III must be observed at the time of touchdown. It is not subject to the same type of "approach ban" criteria as minimum RVR. You have to be within limits at touchdown, no matter what. So theoretically, a tower report of RVR below mins once inside the final approach point (1000 feet for UK) would not force you to abandon the approach, whereas a tower report of crosswind in excess of limits would.

No- Autoland X-wind limit 25kts, Max demonstrated manual 38kts.

Still not bad though!!

A320 - autoland limits (IIIb)

tail - 10 kts
cross - 20 kts
head - 30 kts

Not just new(er) designs.
L1011, max demonstrated cross 35knots...and I have personally landed (automatic approach/land) several times with this wind (gusting to a bit more) in visibilities of less than 600 meters, with precise centerline tracking.
Not bad for an old analogue design, that was the gold standard at one time.

And, just whom do we have to thank for these precise autopilots today?
The pioneers...Smiths and Hawker Siddley, with the Trident, as they did it first so very long ago.

Credit where credit is due.:ok:

And, not forgetting Caravelle, with it's Lear designed autoland system, although I personally don't know much about its development.

Lots of great info.

Is there airline or FAA reference material covering these matters?

Also, is there a centerline deviation limit for Cat III operations for the said Boeing/Airbus planes at major airports?

Thanx.

:ok:

Depending on the localizer accuracy, the 'ole L1011 will track the centerline plus/minus six feet, either side.
Not too bad for an old design.

Thanks. Hard to find answers sometimes to even basic questions via a web search without specific search terms.

Still impressed to know that even L1011 autopilots could respond well enough for high centerline accuracy during even high-speed cross-wind gusts.

The airlines/avionics manufacturers don't seem to upload much basic performance data of their aircraft and systems.

Fortunatly CAT111 conditions (fog) are very seldom accomapanied by strong winds. It's unlikely you will have to do a genuine Cat111 with a wind of more than 10kts.

Not impossible though.

DH

Mig,

Autopilot usually decrab before touch down.But if the wind is over a threshold,it would land crab.

Personally, I've witnessed an autoland dead on centerline with 40kts x-wind on a CAT I ILS. That makes the B777 a beast!:ok:

737 cat3a doesn't de-crab at all, it just touches down with the offset, and lets the main gear correct the drift. 737 main gear is built with the ability to castor a few degrees either way - which can be obvious if you are taxiing behind one in a crosswind (as it taxies somewhat "sideways" :} )

Given some of the pilots I have flown with, YES - the system performs as well as a pilot! ;)