View Full Version : Why do B737's taxi skew?

21st Sep 2008, 17:58
I was following an SAS 737 the other day, close behind, possibly number 11 for departure, and the B737 had his nose wheel on the yellow line, and yet the aircraft was extremely skew all along the lengh of taxi. By measure of the distance between each of the main gear and yellow line, it was close to one third and two thirds difference between them, meaning the aircraft must be taxying as much as 10 degrees-12 degrees off centerline heading. Taxi way heading was 272 degrees (opposite of line up runway) and I would say this guy had a heading of 285, whilst tracking 272. Very clearly I thought the right main gear was not tracking straight, but rather tracking inboard. I have observed many other B737's and they all seem to taxi skew, however, on this occasion is was very obvious. Any idea's why they built the B737's like this?

21st Sep 2008, 18:13
I was told that because the 737 autopilot does not have a rudder channel the main pintle bearing has certain amount of "play" to allow any drift to be absorbed during an autoland. This is then set until the next cycle. (or something like that)

21st Sep 2008, 18:24
the angular displacement of the vertical axis is there for certification purposes. It allows the aircraft to land in an angle that is not fully aligned with the runway axis (crab/de crab technique during cross wind landings).
It has been discussed in this forum before. So for more info try the search function.
Happy landings.

21st Sep 2008, 19:29
How wacky is that?

I've often noticed this phenomenom myself when behind a 737, but I never expected it to be related to the autoland.

How weird!

21st Sep 2008, 20:10
Its my understanding that the NG'S don't taxi skew ways but all previous mdoels did, due to the nose wheel shimmy damper or something to that effect.
Could be complete bull all the same.

22nd Sep 2008, 06:27
I believe its the Main Gear Shimmy Dampers causing this....

22nd Sep 2008, 09:51
I beleive Boeing designed it for lazy or pilots who could not land In a x/wind. Many years ago I had to fly with a capt who could not land in x/wind on the type we were flying he had come of the 7373 fleet Frightning It was

22nd Sep 2008, 10:05
t211, I believe you refer to landing in crab.

Boeing requires you NOT to decrab all the way above a certain crosswind value (I believe it is something around 17kts but someone should confirm this?) otherwise too much wing down will cause a CFM56 scraping the runway.

The CFM56 engine is simply slightly too big for the low wing, for that they needed to allow a slight crab during landing.

22nd Sep 2008, 10:29
I beleive Boeing designed it for lazy or pilots who could not land In a x/wind


I can't believe you wrote that!

Oh yes, I can just imagine the chief design engineer on the 737 many years ago sitting down with his fellow designers when that eureka moment struck them....

" Guys, how about this, they are a bunch of lazy gitz those pilots, lets design an aircraft that they can land in a x/wind without having to do anything!":ugh:

22nd Sep 2008, 12:41
737-200 Does not Autoland
737-200 Does not have same clearance issues as CFM56
737-200 also crabs on taxi

Go Figure PROS!

Oh, those lazy pilots fly other planes too!

22nd Sep 2008, 14:22
Possibly something else that will accentuate what you can see is any slope (left or right) on the taxiway. Of course you will track the line with the nosewheel whilst the aircraft weight (mostly on the mains) will try to tend downhill one way or the other.

22nd Sep 2008, 14:49
I thought it was so any ex "Crabair" pilots did not get homesick !

22nd Sep 2008, 20:19
The main gear can shimmy 7 degrees. Max autoland x-wind limit is 15kts I think.

Did an autoland yesterday but forgot already! Thank god for handy dandy's eh!

NG's taxi skew as well and more so when there's a cross wind pushing on the tail and winglets!


23rd Sep 2008, 02:37
Not this one again............................

Seems to surface every 1 to 2 years.

23rd Sep 2008, 09:47
Time to put it to bed then...

The main gear are fitted with hydraulic shimmy dampers. They need to be bled every now and again. A give away is the crabbing and lots of shoulder wear on the main gear wheels. The thing is that when you bleed them you have to select the gear up and down to cycle the shuttle valve. This allows fluid to flow through the damper. Not popular for those with a weak heart. :\

I think.... it's been a while since I worked a real aeroplane. It's all button pushing and resets these days. :*

Tommy Tipee
23rd Sep 2008, 10:07
Electricdeathjet you need to check your "facts".
The B 737-236, operated by BA and GB Airways, was certified for CAT 3A autoland.

Swedish Steve
23rd Sep 2008, 10:10
Try pushing a B737 back into a hangar tail dock. You get it going straight down the yellow line, then it swerves off to one side. Can be very infuriating as it goes in and out a few times

23rd Sep 2008, 10:16
because most of them are cut 'n' shuts!!!!!!!!!!!!!!!

10th Nov 2008, 09:31
The MLG Shimmy dampers piston on the MLGs can move sideways,hence the effect.

Human Factor
10th Nov 2008, 09:36
737-200 Does not Autoland

Now you tell me! I must have done a couple of dozen in the late nineties..... :eek:

10th Nov 2008, 09:59
Now I know why the autolands were so heavy!

Because autoland was activated on the BA 737-236s, they did something to the ailerons to increase response as I understand the authority was deemed inadequate by the CAA. The result was the darn thing was always wing rocking on final approach, and it was not pleasant doing a manual appoach as the nose was always wandering off-course. You could tell the BA ones coming in on aproach with constantly waggling wings. So there you have it, crab taxi > -200 autolands > aileron modifications!