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Drift On Landing

Old 17th Feb 2020, 21:22
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Drift On Landing

Is it normal not to get rid of the drift for touchdown?
https://news.sky.com/story/pilots-sk...J0yx6S0PM1BFKw
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Old 17th Feb 2020, 22:58
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What drift? He had a substantial crab in to prevent (or minimize) drift. With gusty crosswinds, it is near impossible to react to every gust to keep drift to 0.
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Old 18th Feb 2020, 09:12
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Drift angle being the angle between heading and track, he had plenty of drift. I can’t answer for Airbus but one of Mr Boeing’s many approved x wind landing techniques is to land with drift applied up to max authorised x wind limit.

“Three methods of performing crosswind landings are presented. They are the ​
de-crab technique (with removal of crab in flare), touchdown in a crab, and the ​
sideslip technique.“. Cut and paste from FCTM. So, no it’s not pretty but it works!
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Old 18th Feb 2020, 19:45
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Drift angle being the angle between heading and track
Yep, although I admit it could be called crabbing.

one of Mr Boeing’s many approved x wind landing techniques is to land with drift applied up to max authorised x wind limit.
Thanks. Must put a heck of a strain on the u/c and tyres. Still if it's within the limits, I guess they must be designed for it.
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Old 18th Feb 2020, 20:13
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Yes. Not my preferred technique, which is the de crab one. With those winds the sideslip method would be likely to scrape (or worse) an engine.
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Old 18th Feb 2020, 22:14
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Originally Posted by deltahotel View Post
Drift angle being the angle between heading and track, he had plenty of drift.
In context, drift is also the angular difference between track and runway centerline heading. if track is along the centerline, there is 0 drift - which is THE MOST IMPORTANT aspect of a landing in high crosswinds.
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Old 19th Feb 2020, 08:46
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The most important (as you say) is to maintain the CL (track). To achieve this in a crosswind the ac will need to point into wind (heading). The difference between the two is drift. I’m not sufficiently IT literate to draw the triangle of vectors that we all learn about on Nav 1, but Hdg/airspeed + W/V = Track/GS.

I was on the jumpseat on Saturday watching my colleague landing with 35kt crosswind and trust me - there a whole bunch of drift! Until he neatly removed it in the flare with appropriate use of rudder to align the ac with the runway.
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Old 19th Feb 2020, 10:07
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Always remember that on touchdown the aircraft is still mainly wing borne so there's not too much stress on the gear as you straighten her up. Removing drift too early in the flare can be exciting.
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Old 22nd Feb 2020, 21:51
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Until he neatly removed it in the flare with appropriate use of rudder to align the ac with the runway.
Yes, that was what I was expecting to happen.
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Old 22nd Feb 2020, 23:35
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Cool

Originally Posted by deltahotel View Post
The most important (as you say) is to maintain the CL (track). To achieve this in a crosswind the ac will need to point into wind (heading). The difference between the two is drift.
Most of us in the US recognize that as the crab angle, while drift is the difference between CL and actual track. The goal is 0 drift, using some combination of crab and slip.

Of course, terminology may be different east of the Atlantic...
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Old 23rd Feb 2020, 08:28
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Tomayto tomahto. But just out of interest, when not in the landing phase ie normal flight what do you call the difference between heading and track?
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Old 23rd Feb 2020, 18:09
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Originally Posted by deltahotel View Post
Tomayto tomahto. But just out of interest, when not in the landing phase ie normal flight what do you call the difference between heading and track?
Wind Correction Angle.
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Old 23rd Feb 2020, 20:05
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Thank you - every day’s a school day
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Old 24th Feb 2020, 00:20
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Drift Angle is the Wind Correction Angle that makes the actual track equal to the desired track (no cross-track drift)
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Old 24th Feb 2020, 02:56
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Aircraft tracking towards a runway appear to 'crab' sideways but that's not exactly what they are doing. You turn into the wind, so that the resulting drift gives you a track down the runway. If there is a change in the wind, you need to turn to fix the effect that the change will have had (it will have moved you in one direction or another before you can correct for it) and re-establish the correct track. At the crosswind limit, that drift will be around 15 degrees.

After the flare you need to use rudder to align the aircraft with the runway but you need to do that smoothly, as a secondary effect of any rudder input is roll and you don't want any roll near the ground, particularly towards the downwind side - and that's the effect that the straightening rudder will have. So, when you start to straighten the aircraft, you cancel out your wind correction, and the wind will start to take you towards the downwind side of the runway. In smaller aircraft you can correct it with a small amount of bank in the opposite direction, but that is not really an option on the airliners, as engine clearance generally becomes an issue.

You also have to allow for the gear which will be considerably on the downwind side from the cockpit. If you put yourself on the centreline as you would normally and you have 15 degrees of drift, the main gear will be displaced by about 25ft to the side for every 100ft cockpit to main gear separation so you need to move yourself upwind by an equivalent amount, to ensure the gear stays near the centreline.

Even when you're down, the wind will still be trying to push you to the side of the runway, so you may have to use quite large and abrupt control inputs to ensure the aircraft stays where you want it.

Boeings can be landed with all of the drift still intact. Airbus are limited to 5 degrees of drift at touchdown
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