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Why do the main landing gears of aircraft like the A330, 777, 787, etc tilt up? This does not happen with the 767. Interestingly, for the 747, the inner landing gear does not tilt up while the outer landing gear (under the wing) does. Why does that happen?

enter image description here

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    $\begingroup$ To clarify, the main gear of the A330, 777 and 787 appears to be configured so that the trailing wheels of the bogie are lower than the leading wheels before landing, whereas the 767 seems to be configured to have the leading wheels lower. The plane whose gear is pictured is moving from left to right. $\endgroup$ – David Richerby May 5 '15 at 9:44
  • $\begingroup$ Yes, I wanted to understand why that is so (the landing gear being tilted). $\endgroup$ – Madhav Sudarshan May 5 '15 at 9:50
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Pulling mostly from the answers provided here it seems that

  • The torque produced by the straightening gear helps to flatten the plane out.
  • The main gear tilt on the B747 also provides air/ground sensing. All four main gear bogeys tilt when the aircraft is airborne, so on touchdown any systems relying on air/ground logic can activate
  • The gear is tilted to fit into the wheel well

You can check the link for the rest of the explanations.

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    $\begingroup$ On that thread, it is also explained why the 767 is different: Interesting note, the 767 has the unusual tendancy to pitch down quite rapidly during landing, which is why the MLG on the 67 is tilted forwards. Upon touchdown, the MLG straightens and stops the pitching forward. The downside to this, however, is that the the forward gear on the MLG touchdown first, so the 767 landing is quite a bit rougher than the 757 or the 777, both of which have up-tilted gear $\endgroup$ – ROIMaison May 5 '15 at 14:20
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    $\begingroup$ The gear is tilted to fit into the wheel well uhm, is that person sure not to be switching cause and effect? $\endgroup$ – Federico May 5 '15 at 15:08
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    $\begingroup$ @Federico, apparently not; see the other answer. The thing is that the gear needs to be in a specific position for retraction. And while it could be designed for any position, the maximum tilt (either forward or backward) is easier to achieve as the actuator only needs to act in one direction. $\endgroup$ – Jan Hudec Jun 13 '16 at 13:27
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I will use the 757 as an example, but everything is true for every multi-axle landing gear I've ever been around (except the 777).

The truck (bogie) is tilted into a specific angle so that it'll fit into the wheel well, all other functions are secondary. The truck must be angled because the landing gear strut itself is not completely perpendicular to the fuselage, due to other considerations. This is what the 757 Maintenance Manual says about the truck position actuator (aka tilt actuator): "This actuator applies the force to put the truck assembly at an angle to permit clearance with the structure."

This YouTube video shows the extension/retraction of the 757 landing gear, you can clearly see in the video that if the truck was not at a specific angle it would impact either the keel beam or the wing to body fairings.

The reason it stays tilted in the down position comes down to cost and complexity. The actuator is a simple one-way hydraulic actuator, meaning it can only be powered into the extended direction. Any time hydraulic power is supplied to the landing gear, either in the up or down position the truck positioning actuator is powered on the extended side, regardless of gear position.

Extra information, for the non-TL:DR crowd

When the aircraft touches down, the actuator attempts to keep the truck tilted, which massively increases the pressure in the actuator (normal system pressure is 3000psi). A check valve prevents the hydraulic fluid from being forced back into the hydraulic supply system, then at 4500 psi dumps the fluid into the return allowing the actuator to collapse. At takeoff when the aircraft rotates it again forces this actuator even further into the retracted position as the aft wheels get closer to the fuselage. As you can see the stroke of this actuator must be quite long. So fully extended gives you a toe up position and fully retracted gives you a toe down position.

To have the truck powered to any position other than fully extended would first require the ability to power in both directions, which adds a lot of complexity to the hydraulics system and actuator and this would only get you fully extended and fully retracted. To position the truck in any other position would require complex control valves with position feedback or multi-position actuators and also sequence valves to ensure any time the gear was not down and locked the truck was in the proper position. Since there is no need to flatten the truck for landing, this is not down.

Finally having the truck position always powered fully extended means the truck will always be in the correct position for retraction and you can use the truck tilt to detect air/ground (weight on wheels). In fact on the 757, if pressure to the truck positioning actuator fails, the aircraft will not switch to ground mode until the nose gear shows weight on wheels.

To address the nose down moment that has been mentioned, while technically this does happen the moment is very small relative to the aircraft and is over in a fraction of a second. The actuator itself is fairly small, smaller than a spoiler actuator and reacts its load on the strut very near the truck pivot pin, so the moment arm is short. Also this same force would fight against aircraft rotation at takeoff.

The 767 does nose down because its struts are at a different angle than the 757 and they have to be nose down to fit in the wheel well, this is a design decision due to a limitation at LGA.

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    $\begingroup$ "The truck (boogie) is tilted..." Pretty sure it's a bogie, not a boogie. $\endgroup$ – Sean Jun 8 '18 at 13:29
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    $\begingroup$ @Sean, thanks I fixed it, $\endgroup$ – OSUZorba Jun 10 '18 at 3:10
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    $\begingroup$ @OSUZorba Sorry -- I messed up (long story short: if you copy-paste from YouTube's "share" link, it's at youtu.be; if you copy-paste from your browser, it's from youtube.com. The two use a different URL format and I didn't notice). Now fixed. I set it to start at 4:20 but, of course, you should change that if you think there's a better time. $\endgroup$ – David Richerby Jun 11 '18 at 23:41
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    $\begingroup$ I really would LOVE to know details about such limitation at LGA, could you expand it, @David Richerby ? $\endgroup$ – user18674 Jul 27 '18 at 16:50
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    $\begingroup$ @user18674 It's OSUZorba who made the claim about LGA, not me. My edit just changed the start time of the linked YouTube video. $\endgroup$ – David Richerby Jul 27 '18 at 17:02

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