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Autopilots, Crosswinds and Air Masses

Peter, Achim: You were right.

(What I regret the most now was that I wrote that "i write abou this stuff for 20 years now as a journalist". Thank you for not letting me know what you think of that ;-)) Sorry for that ignorance.

But I think this topic deserves some more explanation (maybe for others who make the same mistake), and maybe there's more to it than I realize(d).

Scenario 1: You are on a final approach with a crosswind. To stay on the ground track to the threshold you (first) use the crabbing method. You keep the wings level with the ailerons and prevent the airplane from drifting away from the centerline by using constant rudder pressure into the wind.

Scenario 2: You are on a GPSS flight plan and the airplane is steered by a 2-axis autopilot. Since the AP has no control of the rudder it steers the airplane to a heading calculated by the GPSS computer to keep the airplane drifing away from track. Once the new heading is established the plane just flies straight and the correct groiud track is only the result of the flown wind correction angle.

If we had an autopilot that controlled the ruder ONLY (like in older model airplanes) the autopilot would have to use constant rudder pressure into the wind for the aircraft to stay on the ground track.

Correct so far?

PS. What's funny is that i wrote several articles about the myths of the "downwind turn and stall" and all those other legends. No further comment.

Alexis, you don't need the rudder at all when you fly a WCA (wind correction angle) into the wind. You need it over the runway when you start to flare - you keep the wings level and decrab with the rudder, i.e. align the aircraft nose with the runway centerline.

If you use the low wing method for crosswind correction, you let the windward wing hang into the wind and apply constant rudder to keep the aircraft aligned with the runway (crossed controls).

EDxx, Germany

nobbi, that's not how it is taught usually.

The best procedure is to fly a crabbing aproach first with wings level and change to the low wing method shortly before touchdown.

To fly the whole final with crossed controls is pretty inefficient. Watch the airliners, they ALWAYS use the crab until very close to the ground, then use low wing method for touchdown.

Only if a engine jet lands in a very strong crosswind it might be necessary to land with some crab because one of the outboard engines could touch the ground if the wing is lowered too much.

Yes, you are right ( and I did it for almost 38 years )

I just tried to correct you when you say

" prevent the airplane from drifting away from the centerline by using constant rudder pressure into the wind "

EDxx, Germany

If we had an autopilot that controlled the ruder ONLY (like in older model airplanes) the autopilot would have to use constant rudder pressure into the wind for the aircraft to stay on the ground track.

No. Once the nose of the aircraft is turned into wind, the rudder can be centered again. The aircraft will fly in a state of equilibrium with no rudder pressure required until either the speed of the aircraft or the windspeed changes.

Only if a engine jet lands in a very strong crosswind it might be necessary to land with some crab because one of the outboard engines could touch the ground if the wing is lowered too much.

For that reason, some large airlines (e.g. the A380) are approved for landing with substantial crab angles. The landing gear bogies are steerable and turn against the crab angle so that the weels will be lined up with the runway even if the aircraft isn't. Watch the videos on YouTube of the A380 doing extreme crosswind landings during certification!

EDDS - Stuttgart

what next, but on a final approach with wings level you DO need constant rudder pressure to stay on the centerline. I know that one for sure :-) But what's the difference?

Correct for the A380 ldg bogies... I saw those beeing installed in Toulouse, very impressive parts

nobbi, ok, ;-)

The "wing low" method is by far the easiest method. It can't be done in an airliner or larger GA aircraft a) because it would risk a wing scrape, and b) because the unbalanced flight would be uncomfortable for the passengers...but the advantage is that there is no last minute requirement to "kick out" the crab angle late in the flare with just the right amount of rudder and aileron....too early and you will start to drift sideways, too late and you land crabbed....personally in PA28s and Cessnas I crab to about the last 100 feet or so then change to wing low....

This also suits coupled approaches as the autopilot will always ensure a crabbed approach until disconnection for the actual landing

EGPD / OMDW / YPJT, United Kingdom

I know that one for sure :-)

You only need rudder pressure if you transit from crab angle to wings low, because the crossed controls "work against each other" and therefore both require constant pressure. Maintaining a constant crab angle with wings level down to the runway requires no control force at all, believe me. When I take over from the autopilot at the usual 200ft I will get a fully trimmed aircraft with all controls very close to their neutral position, esp. the rudder (which is only used by the yaw damper part of the autopilot in our aircraft). If the wind would stay constant all the way down, I could fly free-handed till touchdown, only needing to "kick off" the drift (or crab angle) in the last possible moment.

EDDS - Stuttgart

I did the free handed approach many times... but I needed the rudder to stay on the centerline. Let me think about this a little more (and realize what I really do, because I've been flying intuitively for so long now ...)

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