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Steering The Main Landing Gear  
User currently offlineTupolev160 From , joined Dec 1969, posts, RR:
Posted (2 years 2 months 4 days 12 hours ago) and read 9992 times:

I was surprised to notice that the main wheels on the AN-124 are actually capable of steering which is the first time to hear anything similar about a wide-body aircraft (or an aircraft in general). Has anyone among you got some other examples of aircrafts capable of steering the main landing gear in order to avoid excessive tire-friction? Is that an issue on the 777 for example which has a quite long landing gear as well? Besides that, do you know how are those wheels operated, automatically together with the front-ones or via a separate mechanism? If so, who is operating them, the flight engineer or also the pilots themselves? Furthermore, are they all capable of steering or only the 2 first and last rows? Thanks.

http://www.airliners.net/photo/Volga...d=4d58602287926fe8623abcefcec523b8

37 replies: All unread, showing first 25:
 
User currently offlineZANL188 From United States of America, joined Oct 2006, 3566 posts, RR: 0
Reply 1, posted (2 years 2 months 4 days 12 hours ago) and read 9989 times:
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747 body gears steer & I believe one axle on each 777 main gear steers as well.


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User currently offlineStarlionblue From Greenland, joined Feb 2004, 17118 posts, RR: 66
Reply 2, posted (2 years 2 months 4 days 11 hours ago) and read 9957 times:

Quoting ZANL188 (Reply 1):
747 body gears steer & I believe one axle on each 777 main gear steers as well.

Indeed. The rearmost axle on the 777 mains steers.

Quoting Tupolev160 (Thread starter):
Besides that, do you know how are those wheels operated, automatically together with the front-ones or via a separate mechanism? If so, who is operating them, the flight engineer or also the pilots themselves?

It's automatic, as well as speed and deflection dependent. Transparent to the pilot. Besides on the 744 and the 777 there is no flight engineer.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlinevikkyvik From United States of America, joined Jul 2003, 10259 posts, RR: 26
Reply 3, posted (2 years 2 months 4 days 8 hours ago) and read 9859 times:
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C-5 rear main gear bogeys are steerable. AN-225's rear 3 axles (I think) are steerable.

Quoting Starlionblue (Reply 2):
It's automatic, as well as speed and deflection dependent.

Don't they get locked out for takeoff or something like that?



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User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2572 posts, RR: 25
Reply 4, posted (2 years 2 months 4 days 8 hours ago) and read 9856 times:

Quoting vikkyvik (Reply 3):
Don't they get locked out for takeoff or something like that?

On the 747 the body gear is normally locked straight ahead. It will only unlock at low ground speed and with a large tiller input, so by default they are locked during takeoff and landing. In the 747-400 this is entirely automatic. The 747 Classic has a body gear steering disarm switch which is guarded (disarm position) and only set to arm during taxi. On the Classic ground speed isn't an input to the BGS control system, hence the need for manual disarm.



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User currently offlineCitationJet From United States of America, joined Mar 2003, 2459 posts, RR: 3
Reply 5, posted (2 years 2 months 4 days 7 hours ago) and read 9835 times:

Quoting Tupolev160 (Thread starter):
Has anyone among you got some other examples of aircrafts capable of steering the main landing gear in order to avoid excessive tire-friction?

B-52, although not a wide body.
A notable feature of the landing gear was the ability to pivot the main landing gear up to 20° from the aircraft centerline to increase safety during crosswind landings.
http://en.wikipedia.org/wiki/B-52_Stratofortress



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User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 6, posted (2 years 2 months 4 days 2 hours ago) and read 9709 times:

Quoting vikkyvik (Reply 3):
Quoting Starlionblue (Reply 2):
It's automatic, as well as speed and deflection dependent.

Don't they get locked out for takeoff or something like that?

On the 777, you have to taxi straight ahead at low speed for the rear axles to straighten and lock. If you try to advance the throttles without the MLG steering locked you'll get a takeoff warning.

Tom.


User currently offlineJumboJim747 From Australia, joined Oct 2004, 2465 posts, RR: 44
Reply 7, posted (2 years 2 months 3 days 14 hours ago) and read 9519 times:

Not meaning to plug my photos but here you can clearly see the 747 mlg at an angle in a turn
http://www.airliners.net/photo/Unite...d=89f13c83d25b6115db81a395c2a77dc4



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User currently offlinevikkyvik From United States of America, joined Jul 2003, 10259 posts, RR: 26
Reply 8, posted (2 years 2 months 3 days 14 hours ago) and read 9502 times:
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Quoting tdscanuck (Reply 6):
On the 777, you have to taxi straight ahead at low speed for the rear axles to straighten and lock. If you try to advance the throttles without the MLG steering locked you'll get a takeoff warning.

So in the case where they do a sharp turn onto the runway, then commence the takeoff roll from right there, do they ignore the warning?



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User currently onlinefreeze3192 From United States of America, joined Oct 2006, 166 posts, RR: 0
Reply 9, posted (2 years 2 months 3 days 7 hours ago) and read 9369 times:

Quoting vikkyvik (Reply 8):
So in the case where they do a sharp turn onto the runway, then commence the takeoff roll from right there, do they ignore the warning?

Absolutely not. Takeoff warnings are not to be taken lightly. If the crew gets a takeoff warning light/horn, the automatic response is to abort the takeoff and figure it out later.



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User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 10, posted (2 years 2 months 3 days 5 hours ago) and read 9323 times:

Quoting vikkyvik (Reply 8):
Quoting tdscanuck (Reply 6):
On the 777, you have to taxi straight ahead at low speed for the rear axles to straighten and lock. If you try to advance the throttles without the MLG steering locked you'll get a takeoff warning.

So in the case where they do a sharp turn onto the runway, then commence the takeoff roll from right there, do they ignore the warning?

No, you have to fix it. But it will come up as soon as you advance the throttles so you'll catch it before you gain any speed. Abort the takeoff (super low speed RTO), let the gear lock, takeoff.

Tom.


User currently offlinevikkyvik From United States of America, joined Jul 2003, 10259 posts, RR: 26
Reply 11, posted (2 years 2 months 3 days 2 hours ago) and read 9262 times:
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Quoting freeze3192 (Reply 9):
Absolutely not. Takeoff warnings are not to be taken lightly. If the crew gets a takeoff warning light/horn, the automatic response is to abort the takeoff and figure it out later.

I'm not saying they are to be taken lightly. But having never been in an airliner cockpit during takeoff, I have no idea if there are different levels of warnings or whatever.

Quoting tdscanuck (Reply 10):
No, you have to fix it. But it will come up as soon as you advance the throttles so you'll catch it before you gain any speed. Abort the takeoff (super low speed RTO), let the gear lock, takeoff.

Does that mean they allow the gear to lock prior to entering the runway? Just curious, as there are plenty of places with relatively sharp 90-degree turns onto the runway....

Thanks.



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User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 12, posted (2 years 2 months 3 days 1 hour ago) and read 9247 times:

Quoting vikkyvik (Reply 11):
I'm not saying they are to be taken lightly. But having never been in an airliner cockpit during takeoff, I have no idea if there are different levels of warnings or whatever.

There are different levels, but "warning" is a defined level:
Warning: Immediate crew action required
Caution: Immediate crew notification required, prompt (but not immediate) action may be required
Advisory: Crew notification required, crew action may be required as resources allow
Status: Crew notification not required after dispatch
Memo: Informational only (e.g. "APU", signifying that the APU is running)

A Warning (at least on a Boeing) means an aural, illumination of a red warning light directly in the pilots' field of vision, and a red message on EICAS. It may be accompanied by other red illuminations relevant to the situation (e.g. the fire handles will light up red for a fire warning).

Quoting vikkyvik (Reply 11):
Quoting tdscanuck (Reply 10):
No, you have to fix it. But it will come up as soon as you advance the throttles so you'll catch it before you gain any speed. Abort the takeoff (super low speed RTO), let the gear lock, takeoff.

Does that mean they allow the gear to lock prior to entering the runway?

Yes. The gear locks automatically when it centers and unlocks when steering angle exceeds some preset value. As soon as you line up on the runway it should lock.

Quoting vikkyvik (Reply 11):
Just curious, as there are plenty of places with relatively sharp 90-degree turns onto the runway....

Yes, the rear axles would normally be steering as you make the turn on. However, the normal lineup process should center and lock the gear. There is a gap beween "off-idle" and the throttle position to trigger takeoff configuration warnings, so the crew can start to bring the engines up as they're lining up and, as long as they don't go for takeoff thrust before they're lined up (which they're not supposed to do), it should work fine.

Tom.


User currently offline737tdi From United States of America, joined Sep 2007, 923 posts, RR: 2
Reply 13, posted (2 years 2 months 3 days 1 hour ago) and read 9243 times:
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Although not a widebody the DC8-61/71 had castoring bogies. The bogie on the inside of a sharp turn would unlock at high steering angles and would be hydraulically actuated.

User currently offlinevikkyvik From United States of America, joined Jul 2003, 10259 posts, RR: 26
Reply 14, posted (2 years 2 months 2 days 12 hours ago) and read 9092 times:
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Quoting tdscanuck (Reply 12):
There are different levels, but "warning" is a defined level:
Warning: Immediate crew action required
Caution: Immediate crew notification required, prompt (but not immediate) action may be required
Advisory: Crew notification required, crew action may be required as resources allow
Status: Crew notification not required after dispatch
Memo: Informational only (e.g. "APU", signifying that the APU is running)

A Warning (at least on a Boeing) means an aural, illumination of a red warning light directly in the pilots' field of vision, and a red message on EICAS. It may be accompanied by other red illuminations relevant to the situation (e.g. the fire handles will light up red for a fire warning).

Thanks Tom, that's good to know.

Quoting tdscanuck (Reply 12):

Yes. The gear locks automatically when it centers and unlocks when steering angle exceeds some preset value. As soon as you line up on the runway it should lock.
Quoting tdscanuck (Reply 12):

Yes, the rear axles would normally be steering as you make the turn on. However, the normal lineup process should center and lock the gear. There is a gap beween "off-idle" and the throttle position to trigger takeoff configuration warnings, so the crew can start to bring the engines up as they're lining up and, as long as they don't go for takeoff thrust before they're lined up (which they're not supposed to do), it should work fine.

Ahh, I gotcha. Makes sense. Thanks.



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User currently offline767eng From United Kingdom, joined Oct 2010, 49 posts, RR: 0
Reply 15, posted (2 years 2 months 1 day 8 hours ago) and read 8838 times:

The A380 body gear also has steering on the rear axle each side. Very similar in operation to the 777

User currently offline737tdi From United States of America, joined Sep 2007, 923 posts, RR: 2
Reply 16, posted (2 years 2 months 22 hours ago) and read 8716 times:
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Quoting 737tdi (Reply 13):
Quoting 737tdi (Reply 13):
Although not a wide body the DC8-61/71 had castoring bogies. The bogie on the inside of a sharp turn would unlock at high steering angles and would be hydraulically actuated.




Just to clarify this. The bogies are not "steerable". They are mechanically controlled through the tiller. Reach a certain steering angle they then break over and allow relief on the main landing gear. This is called castoring, it allows relief of torsional forces on the trunnion bearings.


User currently offlineJumboJim747 From Australia, joined Oct 2004, 2465 posts, RR: 44
Reply 17, posted (2 years 1 month 2 weeks 3 days 11 hours ago) and read 7709 times:

737tdi.
So the reason for the movement is more for bearing relief does it not assist with the turning of the aircraft.?



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User currently offlineThirtyEcho From United States of America, joined Dec 2001, 1656 posts, RR: 1
Reply 18, posted (2 years 1 month 2 weeks 3 days 8 hours ago) and read 7672 times:

Even the DC-3 had a lockable tailwheel that was free castoring until locked for takeoff. This was also on the B-17 and other large tailwheel airplanes.

User currently offline737tdi From United States of America, joined Sep 2007, 923 posts, RR: 2
Reply 19, posted (2 years 1 month 2 weeks 3 days 6 hours ago) and read 7623 times:
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Quoting JumboJim747 (Reply 17):
737tdi.
So the reason for the movement is more for bearing relief does it not assist with the turning of the aircraft.?




Oh, it most definitely improves the turning radius. I don't have access to my old DC8 spec. manuals any longer but the turning radius was better on the 61/71 then the 63/73 (which does not have castoring bogies). I can remember (many moons ago) being asked if I could do a 180 on the runway at SAT, I was taxing a 71. I decided I could and put the tiller hard over and powered only the engines on the outside of the turn and she came around very nicely, I don't think I would have tried it in a 73.


User currently offlinevikkyvik From United States of America, joined Jul 2003, 10259 posts, RR: 26
Reply 20, posted (2 years 1 month 2 weeks 3 days 6 hours ago) and read 7620 times:
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Quoting JumboJim747 (Reply 17):
So the reason for the movement is more for bearing relief does it not assist with the turning of the aircraft.?

Well, as soon as you steer the main gear wheels (or allow them to turn), you are by default increasing your turning radius. But a major factor is to reduce tire scrubbing, and therefore, forces on the gear.

Quoting ThirtyEcho (Reply 18):
Even the DC-3 had a lockable tailwheel that was free castoring until locked for takeoff. This was also on the B-17 and other large tailwheel airplanes.

While that's true, the tail wheel was the steerable wheel on those airplanes (or was free castoring, as you said). It wasn't an additional steerable/castoring bogey.



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User currently offline737tdi From United States of America, joined Sep 2007, 923 posts, RR: 2
Reply 21, posted (2 years 1 month 2 weeks 3 days 2 hours ago) and read 7554 times:
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Quoting vikkyvik (Reply 20):
Well, as soon as you steer the main gear wheels (or allow them to turn), you are by default increasing your turning radius. But a major factor is to reduce tire scrubbing, and therefore, forces on the gear.




To start, I don't know of any bogies that truly steer. There are aircraft that the whole bogey will crab during landing (747/C5). With most aircraft that have "steerable bogies" only the trailing two wheels will alter their track. They are not steerable per se. The trailing wheels (the rear two of six on the 777 or the rear two of four on the DC8) simply breakover to appx. 45 degrees to reduce stress on the gear but also improve turning radius.

You have it backwards if I read your response correctly. The turning radius is reduced once the bogey unlocks. By the way, only the bogey on the inside of the turn unlocks and castors, the other bogey remains locked. I don't recall exactly when the DC8 bogey unlocks, I think it was around 15 degrees of nose wheel input. Don't hold me to that, its been quite some years since I worked on one.


User currently offlinevikkyvik From United States of America, joined Jul 2003, 10259 posts, RR: 26
Reply 22, posted (2 years 1 month 2 weeks 2 days 20 hours ago) and read 7489 times:
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Quoting 737tdi (Reply 21):
With most aircraft that have "steerable bogies" only the trailing two wheels will alter their track.

The 747 and the C-5 both have bogies that fully turn. The 747's are steerable; I think the C-5's are too, but I'm not positive about that.

Quoting 737tdi (Reply 21):
The trailing wheels (the rear two of six on the 777 or the rear two of four on the DC8) simply breakover to appx. 45 degrees to reduce stress on the gear but also improve turning radius.

Can't comment on the DC-8, but on the 747 and 777, they don't just suddenly turn to one angle. Even if they did, it would be WAY less than 45 degrees. You'd scrub the crap out of the steered tires if you turned them that sharply. They're also not freely castoring; they're steered.

And I quote:

The aft axle steering system is hydraulically actuated and programmed to provide steering ratios proportionate to the nose gear steering angles.

http://www.boeing.com/commercial/airports/acaps/777rsec4.pdf

Same quote for the 747:

This body gear steering system is hydraulically actuated and is programmed electrically to provide steering ratios proportionate to the nose gear steering angles.

http://www.boeing.com/commercial/airports/acaps/7474sec4.pdf

The 747 document also has a nice diagram on page 95 (page 3 of the PDF) that shows how the gears turn. The body gear is limited to 13 degrees max, at a nose gear angle of 70 degrees.

The 777 document also has diagrams in the first few pages; these show that even at max nosewheel steering, the rear axle of the main gear would be nowhere near 45 degrees.



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User currently offlineDavid L From United Kingdom, joined May 1999, 9537 posts, RR: 42
Reply 23, posted (2 years 1 month 2 weeks 2 days 17 hours ago) and read 7448 times:

A couple of crossed wires, I think...

Quoting vikkyvik (Reply 20):
as soon as you steer the main gear wheels (or allow them to turn), you are by default increasing your turning radius.
Quoting 737tdi (Reply 21):
You have it backwards if I read your response correctly. The turning radius is reduced once the bogey unlocks.

I suspect that might have been a bit of a whoopsie and Vikkyvik meant that the tightness of the turn can be increased (with the radius being decreased). I keep making a similar mistake with cabin altitude versus cabin pressure.

Quoting vikkyvik (Reply 22):
Quoting 737tdi (Reply 21):The trailing wheels (the rear two of six on the 777 or the rear two of four on the DC8) simply breakover to appx. 45 degrees to reduce stress on the gear but also improve turning radius.
Can't comment on the DC-8, but on the 747 and 777, they don't just suddenly turn to one angle.

Can I suggest the following amendment?

Quoting 737tdi (Reply 21):
The trailing wheels (the rear two of six on the 777 or the rear two of four on the DC8) simply breakover to up to appx. 45 degrees

Just a thought...  


User currently offline737tdi From United States of America, joined Sep 2007, 923 posts, RR: 2
Reply 24, posted (2 years 1 month 2 weeks 2 days 13 hours ago) and read 7394 times:
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Quoting David L (Reply 23):
Can I suggest the following amendment?

Quoting 737tdi (Reply 21):
The trailing wheels (the rear two of six on the 777 or the rear two of four on the DC8) simply breakover to up to appx. 45 degrees

Just a thought...





Yes!

I have never worked on a 777 or 747 so I can't tell you/answer/comment on how they steer on the ground. I just don't know.

I do know that the 777 does have castoring bogies. I did not know that they would vary their angle with steering angle input. A little more research on my part. Thanks.


25 CCA : The 777 does NOT have castoring bogies, castoring wheels are undriven, the 777 has a steering actuator to move the axle in accordance with the nose s
26 vikkyvik : Ahhhh, you are correct, thanks. I always do that. No, because I truly doubt they get anywhere near 45 degrees. There's just no possible scenario in w
27 David L : I left the "45 degrees" as is because I don't know how far they can turn. However, I didn't think 737tdi was saying that they toggle between zero and
28 vikkyvik : Fair enough! That's certainly true - they steer proportionally based on nosewheel angle, at least on the 747 and 777. I did a search, and found one a
29 vikkyvik : I just realized that this: ...was a terrible way to state what I was trying to state. The gear aren't tangent to the SAME circle. But they're all tang
30 tdscanuck : For a 777, the aft MLG axle is locked until the captain's steering tiller goes beyond 13 degrees. Then the steering law kicks in and moves the aft ax
31 737tdi : Wow, what an interesting conversation. Guys, I am not guessing here on the DC8 gear. Once you do a full turn with a DC8/71 the inside bogie will acuta
32 Post contains images vikkyvik : I've truly found it so. Relax - no one is out to get you, least of all me. I gave references for the 777 and 747, and Tom added the info I couldn't.
33 jetpilot : All operators that I know of did away with the castoring bogey mechanism on the DC-8 and just dealt with the tire scrubbing and reduced tire life. I f
34 tdscanuck : The steering input unlocks the swivel. It then tracks to wherever the actuator pressure and tire sideforce balance. We agree that it's not steerable.
35 737tdi : There was no requirement for checking the amount of breakover, only that it did breakover so yes, it was a informed guess. The only way to ops check
36 tdscanuck : That's kind of a key detail that you didn't bring up until now. On jacks, the tire has no friction. Of course it's going to spring over to the side (
37 Post contains images David L : I think it was a reasonable assumption that the rest of us were discussing the gear behaviour while taxying/rolling.
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