How does an autoland system kick-off crosswind drift at touchdown?

I understand that the localiser signal and the radio altimeter are the main data controlling the final phase of landing, but neither of these would provide information on the drift angle.

It seems unlikely to me that the autoland system would rely on FMS wind data at this critical point. But I could be totally wrong about that. I'd be most grateful to anyone who could clear this up for me.

All it has to do is make the aircraft heading the same as the ILS course...

....or even not bother at the low limits for x-wind on autoland.

Most autoland capable autopilot systems have a yaw channel which signals the rudder to remove the ‘drift’ (hdg/ILS course error) during the landing flare, via a rad alt trigger.
Aircraft without a rudder channel (early 737, BOAC?) usually have a low autoland crosswind limit.

or even not bother at the low limits for x-wind on autoland.

That might have been the case a while back but for the more modern types with a yaw channel the limits may not be that low, as an example on the 777 it's 25 knots across in cat 3 conditions (outside the States), 40 knots across in Cat 1.

A proper autoland system does not have any crosswind drift at touchdown -- the same as a proper manual landing. Depending on the airplane and crosswind, the system may use crab, slip, or a combination. It will hold those through the landing. The airplane's inertia will cause it to continue straight down the centerline.

While some people may prefer to "kick out" a crab angle just prior to touchdown, it is not necessary, and often not desirable.

All it has to do is make the aircraft heading the same as the ILS course...

I am confused now. There is no course information in an ILS localiser signal is there? It's not a VOR. I know from my own IMC training that runway heading is dialled as the course as a matter of "good practice", but has no influence on the localiser signal.

Crab and drift I consider as the same thing. It is the mismatch between aircraft and runway heading that needs to be corrected before the wheels touch. So runway heading must be a known.

Sideslip (or wing down) technique allows the aircraft and runway to be aligned for the whole approach. But again this implies the runway heading is a known.

Does the autloand take runway heading as a known value - either pilot set or from the navigation database?

Aircraft I have flown have an 'align' mode that engages at 50ft and applies relevant rudder inputs and slight wing down to counter the drift.... impressive to watch!

Runway QDM information gained from inbound course which need to be set the same value on both sides, part of the process to gain 'Autoland' status....

an example on the 777 it's 25 knots across in cat 3 conditions (outside the States), 40 knots across in Cat 1. - gulp! 25kts would be ok to put it on with no correction. I'm not sure why I would really want to autoland at 40 across in CATI .............

Retirement. :ok:

Does the autloand take runway heading as a known value - either pilot set or from the navigation database? - I assume if it is designed to align with c/l, the info would be easily input from the database.

I am confused now. There is no course information in an ILS localiser signal is there? It's not a VOR. I know from my own IMC training that runway heading is dialled as the course as a matter of "good practice", but has no influence on the localiser signal.

Unless the autopilot knows the inbound course to fly, when the localiser needle starts to come in, how will it know which way to turn? On your IMC course, you knew that landing (say) on runway 27, with an intercept heading of 300 degrees you would have to turn left to establish on the localiser. Unless its told, the aircraft doesn't know this so this is why you either have to set the inbound course, or it is stored in the database.

In the Airbus with the latest FMGC revision I fly, it not only stores the inbound course, but also the glidepath angle too, so it knows whether the glide is at 3 or 4 degrees...

- gulp! 25kts would be ok to put it on with no correction. I'm not sure why I would really want to autoland at 40 across in CATI .............

The 777 does lay off the drift. Apparently, at the end of 14 hours duty having flown overnight when the eyes are a little worse for wear it can be an appealing idea! A320 series limited to 20 across, but the way it lays the drift off with short flare (no floating down the runway) and maintains the centreline is quite amazing...

Unless its told, the aircraft doesn't know this so this is why you either have to set the inbound course, or it is stored in the database.

Okay I think I am getting it now. The inbound course is set - either manually or automatically from the database.

The autoland aligns the aircraft with the set inbound course using the yaw channel based on an RA trigger in the flare.

No dependence on any LNAV input (with its much lower safety criticality) in the critical stages of the landing.

Many thanks to you all for the input. :D

Crab and drift I consider as the same thing. It is the mismatch between aircraft and runway heading that needs to be corrected before the wheels touch. So runway heading must be a known.

Sideslip (or wing down) technique allows the aircraft and runway to be aligned for the whole approach. But again this implies the runway heading is a known.

Does the autloand take runway heading as a known value - either pilot set or from the navigation database?
Crab and drift are NOT the same thing! Drift is the [usually unwanted] deviation from the desired track due to wind or other influence. Crab is ONE of several techniques to correct for drift.

Drift DOES need to be corrected before touchdown. Crab does NOT need to be corrected before touchdown on many/most transport airplanes.

Sideslip may be insufficient to correct for crosswind on its own, especially when pod-mounted engines restrict bank angle on landing. Crab may be required in addition to eliminate drift.

LOC course is in the FMS NAV database when FMS isaavailable. Otherwise it is set manually to provide correct flight director commands and course/heading reference.

"Drift angle (aka crab angle): the angle between the longitudinal axis of an airplane and the airplane’s flight path relative to the earth"

Yes, but the drift angle is the negative of the crab angle: You will drift DOWNwind, but crab INTO the wind.

In the context of the OP's question regarding and Autoland, drift relative to the LOC course is bad, and crab to maintain that course/track is good. Eliminating drift by means of crab+slip is the goal.

B737 without rudder channel is fine for autoland with 20 kts across. I can't find the reference now (FCOM or similar) but I am pretty sure that below 50 ft RA the autopilot maintains ground track rather than tracking the LOC…. with varying results :E

but I am pretty sure that below 50 ft RA the [B737} autopilot maintains ground track rather than tracking the LOC…. with varying results

Logically ground track and LOC are the same thing. After all LOC defines the ground track of the runway extended centreline.

However in reality this is the difference between using a safety critical (and accordingly certified) ILS system and the much lower safety assurance of an LNAV input. Doing this switch at the critical moments before landing surprises me.

However in reality this is the difference between using a safety critical (and accordingly certified) ILS system and the much lower safety assurance of an LNAV input. Doing this switch at the critical moments before landing surprises me.

However I believe the track info comes from the IRS:es… Perhaps to avoid something similar to what happened in MUC (Yes I know it was a practice autoland without protection)

On the bus I fly the auto pilot aligns the aircraft during the flare with the magnetic course set on the ILS selector box. We have a sop call at 700 rad alt to confirm the correct course is indeed set. Having the wrong course may cause the aircraft to leave the runway, easily demonstrated in the sim. Once on the ground it still attempts to track the Localizer, it's the short period during the flare that it will get confused if the wrong course is set. hope that helps.

If it uses Mag info it raises an interesting question on the 73 Classic. The IRS magvar database dated from around 2002 I think, and was around 4 degrees in error in the UK when I 'ceased' in 2008. The NG was more up-to-date, of course. If it tried to line up 4 degrees off the actual QDM..............................

The magvar database can be updated each year if one is a big spender, even on the classic. "My" outfit opted to use a new one every two years instead of yearly, as the update each year would have cost around 50k $ per plane. So when we retired our classics in 2008 we had the 2007 magvar database.

However, on the 737 with rudder channel the loc is used until touchdown, however if the LOC signal is completely lost the autoland will continue, albeit with yellow VORLOC FMA. There is a noticeable drift downwind at the crosswind limit of 25kts, at least in the simulator. Not so with a valid loc signal.