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Question For 737 Mechanics.  
User currently offlineMusang From United Kingdom, joined Apr 2001, 861 posts, RR: 7
Posted (10 years 6 months 1 week 5 days 5 hours ago) and read 5507 times:

Question for 737 engineers about the main gear pivot system, which isn't even mentioned in our (pilots') operating manuals. I might get an answer here sooner than I speak to an engineer at work.

After landing crabbed, and the gear swivels, is it supposed to self-center, or is it stuck in that position until next landing? I often notice the one ahead is taxiing crooked.

And is it true that Southwest accepted some NG 737s without the feature, and retro-fitted it when the pilots complained about how miserable it was to land in crosswinds? (Bunch of Whiners! Only kidding. I would....)

Regards - Musang

12 replies: All unread, jump to last
 
User currently offlineCdfmxtech From United States of America, joined Jul 2000, 1338 posts, RR: 27
Reply 1, posted (10 years 6 months 1 week 5 days 1 hour ago) and read 5439 times:

Musang
There is no 737 Main Gear Pivot/Swivel System.

The 737 Flight Manual says that the crab angle should be removed prior to touchdown unless landing on a wet or slippery runway. All landing gears are able to sustain the effects of normal side loading. But landing at a severe crab angle on this particular aircraft (Or most any) is not good.

Shimmy Dampers were installed post-delivery on our B737NGs, but for a different reason, but I don't work for SWA.


User currently offlineAAR90 From United States of America, joined Jan 2000, 3460 posts, RR: 47
Reply 2, posted (10 years 6 months 1 week 5 days ago) and read 5425 times:

After landing crabbed, and the gear swivels, is it supposed to self-center, or is it stuck in that position until next landing? I often notice the one ahead is taxiing crooked.

They will center slowly. If you taxi in a straight line for a relatively long distance.

And is it true that Southwest accepted some NG 737s without the feature, and retro-fitted it when the pilots complained about how miserable it was to land in crosswinds? (Bunch of Whiners! Only kidding. I would....)

At some point in the design process Boeing deleted the wheel caster system from some or all 737NG's. AA's 738's all came without wheel castering. At some point [I don't recall when] Boeing introduced a modified wheel caster system on all new build 737's and provided retrofit kits to AA for all earlier delivered 738's. All this happened prior to my flying 737s three years ago and I never bothered to find out any more details than the above.



*NO CARRIER* -- A Naval Aviator's worst nightmare!
User currently offlineCdfmxtech From United States of America, joined Jul 2000, 1338 posts, RR: 27
Reply 3, posted (10 years 6 months 1 week 5 days ago) and read 5409 times:

They will center slowly. If you taxi in a straight line for a relatively long distance.

OK, what are you guys talking about?
Is this an option?
What exactly does the system do?

I asked this very question once before to an instructor, and was told how the aircraft would handle such a high drag situation, but this system was never mentioned.
Looking at the 737 gear on the airplane, there is no "pivot" mechanism of any kind.


User currently offlineAAR90 From United States of America, joined Jan 2000, 3460 posts, RR: 47
Reply 4, posted (10 years 6 months 1 week 4 days 18 hours ago) and read 5366 times:

OK, what are you guys talking about?

"Shimmy Damper" and "wheel castering" used interchangeably. Sorry for the confusion for those more technically knowledgable. I was trying for a simplistic explanation. How about a Q&A from the AAPilot's website from not too long ago:

Date: 10/31/2003 737 Looks Bent
Q:I have noticed when we are following a 737 and it is going in a straight line the main wheels are off set to the right, off center, about 2 feet or so. It looks like the aircraft is bent. Why is this?

A:The 737 has shimmy dampers which allow up to a 3 degree twist on the main gear from the strut, depending on the model. Thus, after a turn the aircraft will appear to be taxiing "crabbed" due to the castoring effect of the dampers. This slowly corrects itself after some time.

-[name deleted]-
737 Technical Pilot

A better explanation? My understanding is the 738 shimmy dampers only allow 1.5 degree "twist" which is why the plane still doesn't taxi straight. Y'all should see it from the cockpit looking through the HUD....weird!



*NO CARRIER* -- A Naval Aviator's worst nightmare!
User currently offlineMusang From United Kingdom, joined Apr 2001, 861 posts, RR: 7
Reply 5, posted (10 years 6 months 1 week 4 days 5 hours ago) and read 5321 times:

Cdfmxtech - take a look at the horizontally mounted beer-can sized cylinder which links the scissor links on the main gears. It'll become clear.

Actually many of the pilots, even the veterans, at our outfit aren't aware of it. It obviously operates on a need-to-know basis.

Thanks AAR90. Good info.

Regards - Musang


User currently offlineAAR90 From United States of America, joined Jan 2000, 3460 posts, RR: 47
Reply 6, posted (10 years 6 months 1 week 4 days 3 hours ago) and read 5302 times:

Actually many of the pilots, even the veterans, at our outfit aren't aware of it. It obviously operates on a need-to-know basis.

I suppose so. All reference to main gear shimmy dampers have been removed from my AA operating manual. A result of AA lawyers "protecting" AA from lawsuits --or so they think.  Angry



*NO CARRIER* -- A Naval Aviator's worst nightmare!
User currently offlineCdfmxtech From United States of America, joined Jul 2000, 1338 posts, RR: 27
Reply 7, posted (10 years 6 months 1 week 4 days 1 hour ago) and read 5282 times:

Yes, I am already aware of the Shimmy Dampers that are on the Torque Links. But your question, as I understood was whether or not the 737s Main Gear Pivot system would automatically center itself after a crabbed landing. Well, the 737 doesn't have a Main Gear Pivot system that is designed to compensate for a crabbed landing. The 737 has a Shimmy Damper system that prevents unwanted vibration in the main landing gears and damage to the airplane

Here is the rest of an abstract on Shimmy Dampers:

Nine operators have reported 60 incidents of unwanted vibration of main landing gear during landing. In some cases, damage to the main landing gear, the passenger cabin, and high lift system of the airplane have occurred.

Data from the operators and Boeing shows that the vibration can occur on landings with a small rate of descent, when the tires touch the runway with the shock struts fully extended for a long time. The vibration occurs when the inner cylinder of the main landing gear turns to the left and right in relation to the outer cylinder.

The shimmy damper is installed on the torsion links of the main landing gear and increases the rate at which the turning movement of the inner cylinder stops. Shimmy damper will prevent unwanted vibration in the main landing gears and damage to the airplane.


User currently offlineAvioniker From United States of America, joined Dec 2001, 1109 posts, RR: 11
Reply 8, posted (10 years 5 months 4 weeks 1 day 2 hours ago) and read 5091 times:

My four-year-old books state the purpose of the shimmy dampers is as stated above but also that "they will allow an aircraft to approach and land on a runway in a crab of up to 5 degrees (three degrees will be castered out by the damper and the tires will allow slippage to compensate for the additional two degrees of crab)".

I know that there were BIG problems with the first edition. In fact three of our aircraft were found to have cracks in the web of the aft pressure bulkheads which were attributed to excessive shimmy on landing.

After the initial redesign the flight manuals were ammended to state that the dampers allowed up to 1.5 degrees of caster.

Now we're in the third iteration of the system and all mention of castering has been removed from every current manual I can get my hands on.

Sounds like Boeing doesn't want to be credited with any system that can cause the type of problems that cause cracked pressure bulkheads and broken legs.



One may educate the ignorance from the unknowing but stupid is forever. Boswell; ca: 1533
User currently offlineCdfmxtech From United States of America, joined Jul 2000, 1338 posts, RR: 27
Reply 9, posted (10 years 5 months 4 weeks 1 day 2 hours ago) and read 5077 times:

Avioniker....
Was that info on Shimmy Dampers for the 737NG or older versions??

I couldn't find anything even referencing castering.


User currently offlineAvioniker From United States of America, joined Dec 2001, 1109 posts, RR: 11
Reply 10, posted (10 years 5 months 4 weeks 1 day 2 hours ago) and read 5075 times:

It was for the NG.
Apparently an engineer in Renton had a "great" idea that was, perhaps, a little less great than "grate" (heh, heh, heh  Smile/happy/getting dizzy ).



One may educate the ignorance from the unknowing but stupid is forever. Boswell; ca: 1533
User currently offlineBarney Captain From United States of America, joined Nov 2001, 919 posts, RR: 13
Reply 11, posted (10 years 5 months 3 weeks 6 days 23 hours ago) and read 5002 times:

Lousy. That's about all I can say about the earlier NG's on landing. Without the shimmy dampeners, you could roll it on, only to have the whole AC shake like a wet dog. One "dog shake" was so bad (after a nice landing), that the Capt. cut her head getting thrown in to the HUD. Since we were the launch customer for the NG, Boeing wasn't at first believing our story. We then showed them the video tape that was shot of the main landing gear at touch down during HUD certification; twisting so violently it looked as is they were going to break in two. The mod was available a short time later.


...from the Banana Republic....
User currently offline737doctor From United States of America, joined Mar 2001, 1332 posts, RR: 39
Reply 12, posted (10 years 5 months 3 weeks 6 days 21 hours ago) and read 4986 times:

I got into our manuals here at SWA and there is no reference to castering. The main gear shimmy dampers are described exactly as Cdfmxtech mentioned above.


Patrick Bateman is my hero.
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