Cross wind landings. U/C Stress ?

[Mike Tee]

Just been watching clips of aircraft landing in "cross wind" conditions and sometimes touchdown happens with quite a large degree of "out of line with the runway". As this obviously put a great deal of stress on the landing gear I was wondering if there is a maximum amount of "out of alignment with the runway" permitted at touchdown.
Thanks,

[N707ZS]

Must be something in one of the aircraft manuals.

[wiggy]

I think this came up in a thread last year.....

There will almost certainly be crosswind limits (for dry/runway/wet/ slippery/ runways etc etc) in the aircraft manuals measured in knots, an "out of alignment by maximum number of degrees" limitation is not something I have seen, and in any event in a strong cross wind you will be so busy you won't be measuring degrees, etc.....

Many/all aircraft are I believe flight tested to land "out of alignment" up to the maximum crosswind limit, and there can be risks associated with spending too much time messing around in the flare trying to perfectly "decrab" the aircraft ... sometimes you try your best but you have to accept you might land "non aligned" to some extent. It might not feel great but the alternatives might be worse.

[Piltdown Man]

Mike - are you talking about alignment, tracking or bank? Alignment would be when the nose of the aircraft is pointing an a different direction to the the runway, ie. landing crabbed. Tracking is the direction the aircraft is making over the ground. It should be the same as the runway's track, but it can be different. And you can (my preferred method) land banked, ie. upwind gear first. If you do it correctly, the aircraft's track and alignment will be the same as the runway's. However, my aircraft's manual says I can land crabbed or wing down or a combination of both. The former means the aircraft's gear can take hit on landing with a crosswind of 38 knots. I haven't tried it but some poor sod will have and the aircraft did not break when he did.

[yotty]

From a maintenance point of view the nearest I can get to anything related to this is in B737 6/7/8 series AMM 05-51-01 Unscheduled Maintenence Checks High Drag/Side Load Landing Paragraph C1. d). The a/c Skidded on the runway sufficiently to make you think damage occured. So it's really down to the commander to report, or an engineer to put 2 and 2 together to carry out the check. http://jwc.caac.net/2007/webs/conten/B737-6789AMM/05___019.PDF

[wiggy]

Some will have seen these flight test videos before, but to give the OP of what is involved in flight testing:

https://www.youtube.com/watch?v=_z2LtHrn9Jw

No or very little attempt made ( deliberately) to remove crab/drift by the TPs before main gear touchdown, the "aligment" is done prior to lowering the nosewheel, ..FWIW generic crosswind limit on our 777s used to be a blanket figure on a dry runway of 40 knots, nowadays it's weight/C of G dependant and can be up to 45 knots, however "zero crab" wing low landings not allowed above 28 knots due to "pod" clearance..

Anyhow the 777 video makes some of the supposedly "dangerous landing" youtube videos look quite benign...

[Mike Tee]

Thank you for all the replies. It does seem like the landing gear is designed to be strong enough to take just about all that is thrown at it. Perhaps I could have been clearer in my opening post in explaining my query. I would have thought that the gear would be designed to have an amount of built in "twist" movement available to instantly align the gear with the direction of travel at touchdown. Perhaps someone will come along from a landing gear manufacturer and offer further info.
Thanks again.

[wiggy]

Thank you for all the replies. It does seem like the landing gear is designed to be strong enough to take just about all that is thrown at it. Perhaps I could have been clearer in my opening post in explaining my query. I would have thought that the gear would be designed to have an amount of built in "twist" movement available to instantly align the gear with the direction of travel at touchdown.......

The B-52 had gear that could be-aligned by a set amount prior to touchdown to allow for crosswinds but I'm not aware of anything with free floating "Castoring" gear, I would have thought such an arrangement might make the subsequent roll out and taxiing a bit interesting to say the least, especially in a strong crosswind. Anyhow if you look close up at most main landing gear and all the assorted braces and attachments you'll see they are very very solid pieces of hardware, I think in reality some of the "twist" is to some extent absorbed by the tyres, the gear should be well capable of absorbing the rest.

[Rick777]

The B737 doesn't really have castering gear, but the do have a small ability to caster a few degrees.

[wiggy]

Ah that's interesting...any idea if it's lots of degrees or just a "tad" more than you'd enhanced "play" or "shimmy"...:ok::E

[DaveReidUK]

This PPRuNe thread includes some debate on the subject from 20(!) years ago:

737's crabbing on the ground (http://www.pprune.org/tech-log/188318-737-s-crabbing-ground.html)

Appropriately, given today's date, it includes some droll contributions. I particularly enjoyed the explanation that the aircraft crabs when taxying because of the torque reaction from the rotating anti-collision beacon, as well as the caution that due to the Coriolis Effect it was extremely dangerous to taxy across the Equator. :O

I suspect that the shimmy damper explanation is closest to the truth. It's hard to believe that gear is free to castor in the sense of completely unrestrained rotation.

Apropos of the latter, I can vouch for the fact that the Shorts commuters were able, with a bit of sporty manoeuvring on the ground, to run the nosewheel steering pinion completely off the rack and thereby get the gear to rotate 180°.

[noflynomore]

This PPRuNe thread includes some debate on the subject from 20(!) years ago:


I suspect that the shimmy damper explanation is closest to the truth. It's hard to believe that gear is free to castor in the sense of completely unrestrained rotation.



The gear will certainly not castor unrestrained but had a degree of movement to rotation that is not readily self-cancelling. I don;t recall for sure but doubt that a shimmy damper has any involvement, if there even is one.
The 737 gear will self align to some degree to a crosswind landing and will remain in that configuration until next stowed in the wheel well. Thus following a crosswind landing it is likely to taxi noticeably on the sosh - and they often do.

[wiggy]

The 737 gear will self align to some degree to a crosswind landing and will remain in that configuration until next stowed in the wheel well. Thus following a crosswind landing it is likely to taxi noticeably on the sosh - and they often do.

Interesting, first time I've heard of that - next time I see a 73 taxiing sideways I'll know why......:E:ok:

[DaveReidUK]

I don't recall for sure but doubt that a shimmy damper has any involvement, if there even is one.

There's no debate about whether the 737 gear has or hasn't a shimmy damper - here's an investigation report into an accident caused by failure to reconnect one following maintenance:

Vibrations, failure of the right main landing gear torsion link during landing roll (https://www.bea.aero/docspa/2011/g-ld110723.en/pdf/g-ld110723.en.pdf)

[Nipper1011]

This PPRuNe thread includes some debate on the subject from 20(!) years ago:

Apropos of the latter, I can vouch for the fact that the Shorts commuters were able, with a bit of sporty manoeuvring on the ground, to run the nosewheel steering pinion completely off the rack and thereby get the gear to rotate 180°.



Originally there were small lugs at either end of the rack that were meant to restrain the pinion. After Air Ecosse managed to get the pinion off the end of the rack (over the lug) during weather cocking on landing up at Wick, Shorts came up with the suggestion that the lug be broken off to allow the pinion to re-engage with the rack!