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A380s Main Landing Gear?  
User currently offline747400sp From United States of America, joined Aug 2003, 3620 posts, RR: 2
Posted (3 years 2 weeks 6 days 22 hours ago) and read 5919 times:

Why do an A380 main landing gear front wheels lean down, where on a 747, 777, A340 and A330, the back wheels of the main landing gear, lean down? The 767 has a simalar lay out to the A380, which made for harder landing on 767, so why put this system on the world largest airliner?

30 replies: All unread, showing first 25:
 
User currently offlineMax Q From United States of America, joined May 2001, 4521 posts, RR: 18
Reply 1, posted (3 years 2 weeks 6 days 21 hours ago) and read 5894 times:

That is a good question, the forward tilt on the 767 is an unforgiving design with the result that anything less than a perfect touchdown feels very hard.



From what I understand the A380 is the same way !



The best contribution to safety is a competent Pilot.
User currently offlinefr8mech From United States of America, joined Sep 2005, 5451 posts, RR: 14
Reply 2, posted (3 years 2 weeks 6 days 20 hours ago) and read 5863 times:

Quoting 747400sp (Thread starter):
The 767 has a simalar lay out to the A380

This question has been asked and answered many times for the B767. The gear is tilted forward in order to fit into the main wheel well. I'd imagine that's the reason for the A380's forward tilt.



When seconds count...the police are minutes away.
User currently offlinerwessel From United States of America, joined Jan 2007, 2352 posts, RR: 2
Reply 3, posted (3 years 2 weeks 6 days 13 hours ago) and read 5680 times:
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I doubt the gear tilt makes much difference in terms of landing firmness on most aircraft - the tilt is simply not "held" with any real force (relative to the weight of the aircraft), and absent that, the truck will simply rotate to match the ground as soon as any pressure is put on it, no matter what it's initial angle.

The longer 777s locked tilt feature is a notable exception - it holds the truck at a fixed angle so that the aircraft can rotate around the last wheel's axle, to reduce the chances of a tail strike on rotation.


User currently offlinebrenintw From Taiwan, joined Jul 2006, 1647 posts, RR: 1
Reply 4, posted (3 years 2 weeks 6 days 5 hours ago) and read 5525 times:

Quoting rwessel (Reply 3):
The longer 777s locked tilt feature is a notable exception - it holds the truck at a fixed angle so that the aircraft can rotate around the last wheel's axle, to reduce the chances of a tail strike on rotation.

OK, I can understand why this is -- and I assume the truck is locked by an hydraulic system.

All the following is based on the above assumption of an hydraulic system for the truck lock:

Has that hydraulic system ever been known to fail, and keep the truck in the locked position after take off? How would that affect the landing?

I realize that at rotation, the wings are actually carrying most of the weight of the aircraft, so I would imagine there's some kind of release valve whereby if the weight on that rear axle reaches a certain point, the actuator is compressed and the hydraulic fluid is forced through the release valve and the truck settles down with all three axles on the ground.

Or am I showing my rather significant ignorance of all things related to the design of aircraft?



I'm tired of the A vs. B sniping. Neither make planes that shed wings randomly!
User currently offline474218 From United States of America, joined Oct 2005, 6340 posts, RR: 9
Reply 5, posted (3 years 2 weeks 6 days 5 hours ago) and read 5518 times:

Quoting brenintw (Reply 4):
OK, I can understand why this is -- and I assume the truck is locked by an hydraulic system.


A hydraulic cylinder (actuator) "does not lock" the truck in position but "holds" the truck in position. When the gear makes contact the hydraulic cylinder then acts as a shock absorber.


User currently offlineMax Q From United States of America, joined May 2001, 4521 posts, RR: 18
Reply 6, posted (3 years 2 weeks 6 days 2 hours ago) and read 5465 times:

Quoting rwessel (Reply 3):
I doubt the gear tilt makes much difference in terms of landing firmness on most aircraft - the tilt is simply not "held" with any real force (relative to the weight of the aircraft), and absent that, the truck will simply rotate to match the ground as soon as any pressure is put on it, no matter what it's initial angle.

It makes all the difference in the world actually, the same touchdown in a 757 that would feel gentle feels much harder in a 767. The rear axle tends to 'slam down'



This is accentuated by any drift that has not been taken out.



I would bet you a dollar that the A380 has the same issue.



The explanation that the tilt has to be that way to 'fit in the gear wells' is not valid as the trucks can be tilted to the desired angle during extension or retraction.



The best contribution to safety is a competent Pilot.
User currently offlinegigneil From United States of America, joined Nov 2002, 16347 posts, RR: 84
Reply 7, posted (3 years 2 weeks 6 days 2 hours ago) and read 5460 times:

That's definitely the reason given for it.

NS


User currently offlinerwessel From United States of America, joined Jan 2007, 2352 posts, RR: 2
Reply 8, posted (3 years 2 weeks 5 days 23 hours ago) and read 5418 times:
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Quoting Max Q (Reply 6):
It makes all the difference in the world actually, the same touchdown in a 757 that would feel gentle feels much harder in a 767. The rear axle tends to 'slam down'

But you're talking about two different airplanes, with different aerodynamics. As we all know, some airplanes are easy to grease on, other not. And it's not generally (just) the gear, although a gear with a great deal of shock absorbing potential certainly helps (probably a big part of the reason trailing link gears are popular on bizjets). Low wing aircraft with short gear legs often are easier to grease than high wing birds with long legs (obviously a generalization) because the very sharp increase in lift due to ground effect in the last bit before landing naturally (and gently) removes a significant amount of vertical velocity.

But unless there's some big energy absorber for the gear tilt, there just isn't anything there that would absorb a lot of energy when the front (or back) wheels of the truck hit and caused the truck to tilt. And in any event, if there were, you could certainly absorb that energy from either direction of tilt (although you’d have to mount the shock in the opposite direction).

As for the rear axle slamming down - of course it would - that's the point at which you actually start loading up the strut (before that you're mostly just tilting the truck, and applying minimum load to the leg). And if the 767's gear dangled the other way, you'd probably feel the *front* axle slamming down.


User currently offlinefr8mech From United States of America, joined Sep 2005, 5451 posts, RR: 14
Reply 9, posted (3 years 2 weeks 5 days 21 hours ago) and read 5369 times:

Quoting Max Q (Reply 6):
The explanation that the tilt has to be that way to 'fit in the gear wells' is not valid as the trucks can be tilted to the desired angle during extension or retraction.


Exactly, the bogie is tilted to the desired angle, which happens to be forward wheels down. I don't recall ever seeing a number.

Now, I can't speak for the A380, but the B767 is definitely tilted forward in order to fit in the well. If you look at the geometry of the trunnions, you'll find that in order for the wheels to lay parallel to the keel, the bogie will have to tilt forward. I suspect that the A380 is the same.



When seconds count...the police are minutes away.
User currently offlineMax Q From United States of America, joined May 2001, 4521 posts, RR: 18
Reply 10, posted (3 years 2 weeks 5 days 7 hours ago) and read 5175 times:

It is not an aerodynamic issue on the 767.



Fact is, the forward trailing gear is less forgiving than a rear trailing design.



From fourteen years experience flying the B757 and B767 I can tell you that with a certainty !



As I mentioned earlier, the 767 gear is especially unforgiving if you do not remove all drift prior to touchdown.



If you think about the geometry involved it makes sense. The trailing rear bogie will relatively gently
help straighten out the Aircraft on touchdown. The forward trailing design forces a much more abrupt straightening.



This is one of the main reasons that 767 landings can feel so hard.




As I said the rationale for the forward tilt being mandated for stowage reasons is not valid as the trucks can be hydraulically tilted during extension and retraction to accommodate this (the 777 does this)

Quoting rwessel (Reply 8):




As for the rear axle slamming down - of course it would - that's the point at which you actually start loading up the strut (before that you're mostly just tilting the truck, and applying minimum load to the leg). And if the 767's gear dangled the other way, you'd probably feel the *front* axle slamming down.

Your contention is not accurate, the B707, 747, 757, 777 all have trailing edge bogie tilt (787 too) that make for a much gentler touchdown as the front axle does not 'slam down'



The best contribution to safety is a competent Pilot.
User currently offlinefr8mech From United States of America, joined Sep 2005, 5451 posts, RR: 14
Reply 11, posted (3 years 2 weeks 5 days 6 hours ago) and read 5159 times:

Quoting Max Q (Reply 10):
As I said the rationale for the forward tilt being mandated for stowage reasons is not valid as the trucks can be hydraulically tilted during extension and retraction to accommodate this (the 777 does this)

But, not without added complexity. The designers of the B777 apparently felt the multiple tilt angle feature of the gear was worth the complexity, cost and weight. Apparently, the designers of the B767 did not feel the same way.


From the B767 AMM:

Truck Positioner (Fig. 9) (1) The truck positioner is a hydraulic actuator which tilts the truck to fit into the wheel well during gear retraction. It is mounted on the aft side of the shock strut between the truck and shock strut. The positioner retracts to tilt the gear during retraction; during landing an internal relief valve allows the positioner to be extended by the gear.

1) Gear Extension (a) To extend the main gear, the landing gear control lever should be moved to DN. Quadrants and cables then move the main gear selector valve to direct pressure from the center hydraulic system to the left and right main gears. (b) The truck positioner receives pressure from the selector valve to tilt the truck. This allows the gear to extend without the truck interfering with the wheel well.

By the way, I found this from a post I made back in July 2004.



When seconds count...the police are minutes away.
User currently offline474218 From United States of America, joined Oct 2005, 6340 posts, RR: 9
Reply 12, posted (3 years 2 weeks 5 days 6 hours ago) and read 5158 times:

Quoting Max Q (Reply 10):
Your contention is not accurate, the B707,



?


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User currently offlinerwessel From United States of America, joined Jan 2007, 2352 posts, RR: 2
Reply 13, posted (3 years 2 weeks 5 days 6 hours ago) and read 5143 times:
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Quoting Max Q (Reply 10):
It is not an aerodynamic issue on the 767.

Fact is, the forward trailing gear is less forgiving than a rear trailing design.

What mechanism do you propose that allows the rear-tilting gear to dissipate energy more effective than a forward tiling one? It only moves in one dimension. And there can’t be a large amount of force pushing the front (or back) wheel down, as that reduces the load on the pother wheels after touchdown, which messes up the distribution of weight on the ground, and reduces braking effectiveness on the “other” wheels. For example, if there was force equivalent to 10% of the total load on the front wheels, that would mean that after the truck is fully down, the front wheels would be carrying 60% of the weight of the aircraft, and the back wheels only 40%.

Quoting Mlax Q (Reply 10):
As I mentioned earlier, the 767 gear is especially unforgiving if you do not remove all drift prior to touchdown.

If you think about the geometry involved it makes sense. The trailing rear bogie will relatively gently
help straighten out the Aircraft on touchdown. The forward trailing design forces a much more abrupt straightening.

This is one of the main reasons that 767 landings can feel so hard.

Hitting with the front wheels first in a crab can have two effects. First, it'll move the point at which the side force is applied to the aircraft further forward. As that will be closer to the CG, it will, in fact, reduce the "straightening" effect that can happen before the other wheels touch down, leaving more to occur after that point, making that more abrupt. That is, however minimized by the limited amount of force the gear applies on the first wheel down, and the very brief time between first axle and last axle down.

A second effect is that hitting with the front wheels first will cause a twisting moment on the gear in the wrong direction, which will effectively turning the gear a bit away from the direction of travel. That increases the side force from the wheel, which would also have the effect of decreasing the "early" straightening effect a bit more. And probably adds some snap back as the gear untorqued. Again that's limited by the amount of pressure on the front tires and the very brief time between the front and back wheels touching down, the truck is also not all that long. If that's a significant problem the gear should have been designed somewhat stiffer.

And none of that means anything if you touch down with limited sideways motion.


I'm not saying the 767 isn't unforgiving on touchdown, but the tilt of the truck should have, at most, a limited effect on that, at least with modest crab angles.


User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 79
Reply 14, posted (3 years 2 weeks 5 days 1 hour ago) and read 5102 times:

Quoting Max Q (Reply 6):
The explanation that the tilt has to be that way to 'fit in the gear wells' is not valid as the trucks can be tilted to the desired angle during extension or retraction.

You're correct that it can be altered during the extension/retraction cycle, but there is a significant complexity cost for doing so because, on aircraft with a toe-down geometry when stowed, you'd have to add extra sequencing valves to flip the tilt direction around part way through the cycle. Provided you can do safe and compliant landings, there isn't a very compelling reason to add more systems (which, if failed, prevent gear retraction) just to soften the touchdown.

Quoting Max Q (Reply 10):
Fact is, the forward trailing gear is less forgiving than a rear trailing design.

From fourteen years experience flying the B757 and B767 I can tell you that with a certainty !

I don't think anyone disputes the handling qualities but there is more at work than just the truck position. The airplanes are aerodynamically different too, which changes how they enter ground effect and flare. That has a bigger influence on landing performance than truck position (e.g. 727).

Tom.


User currently offlineMax Q From United States of America, joined May 2001, 4521 posts, RR: 18
Reply 15, posted (3 years 2 weeks 5 days 1 hour ago) and read 5102 times:

Quoting rwessel (Reply 13):

A second effect is that hitting with the front wheels first will cause a twisting moment on the gear in the wrong direction, which will effectively turning the gear a bit away from the direction of travel. That increases the side force from the wheel, which would also have the effect of decreasing the "early" straightening effect a bit more. And probably adds some snap back as the gear untorqued.

Now you are starting to understand !



The best contribution to safety is a competent Pilot.
User currently offlinerwessel From United States of America, joined Jan 2007, 2352 posts, RR: 2
Reply 16, posted (3 years 2 weeks 5 days 1 hour ago) and read 5089 times:
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Quoting Max Q (Reply 15):
Now you are starting to understand !

So now you're starting to understand? In a landing with no sideways motion, the direction of truck tilt is largely irrelevant?

And since your selective quoting omitted: "Again that's limited by the amount of pressure on the front tires and the very brief time between the front and back wheels touching down, the truck is also not all that long." We can agree that except with serious amounts of sideways motion, the lack of force on the first wheel and minimal amount of time between the front and back wheels making contact can produce only a minimal effect?


User currently offlineflipdewaf From United Kingdom, joined Jul 2006, 1572 posts, RR: 0
Reply 17, posted (3 years 2 weeks 4 days 18 hours ago) and read 5012 times:
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Quoting rwessel (Reply 16):

Its basically that the movement is caused by a force infront of the pivot point on the 767 and behind on other aircraft, equivelant to trying to pull a bike from the front or push it backwards from the front.

Maybe a better analogy is if the ground forces were like the aerodynamics (and centre of lift) and the pivot like the centre of gravity then the bogies on the 767 would be aerodynamically unstable but the A330 would be stable.

Fred


User currently offlinerwessel From United States of America, joined Jan 2007, 2352 posts, RR: 2
Reply 18, posted (3 years 2 weeks 4 days 7 hours ago) and read 4867 times:
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Quoting flipdewaf (Reply 17):
Its basically that the movement is caused by a force infront of the pivot point on the 767 and behind on other aircraft, equivelant to trying to pull a bike from the front or push it backwards from the front.

Maybe a better analogy is if the ground forces were like the aerodynamics (and centre of lift) and the pivot like the centre of gravity then the bogies on the 767 would be aerodynamically unstable but the A330 would be stable.

So long as the front wheel is behind the aircraft's CG, it will turn the aircraft in the direction of the skid. Being nearer the CG, it will have less of a "straightening" effect than the rear wheel would. If it was exactly on the CG, it would have no "straightening" effect at all. I don't know enough about the 767, but it appears that the front wheel on the mains is behind the CG at the aft CG limit - in any event it cannot be more than a tiny bit ahead of the CG (this might actually occur on an aircraft with exceptionally long trucks - the Tu-134 and 777 come to mind), since the CG *must* be ahead of main strut (if not, you're flying the tail-dragger model of 767).

Now the front wheel will apply torque in the wrong direction to the bogie itself (and the solution to that is stiffening the gear structure, if it's a problem), but *not* to the aircraft as a whole *unless* it's ahead of the CG.

In either case, there cannot be all that much force on the first wheel touching down, thus limiting any effect. Nor does it have much impact unless there a significant amount of lateral motion.


User currently offlineredflyer From United States of America, joined Feb 2005, 4329 posts, RR: 28
Reply 19, posted (3 years 2 weeks 4 days 7 hours ago) and read 4857 times:

Quoting tdscanuck (Reply 14):
I don't think anyone disputes the handling qualities but there is more at work than just the truck position. The airplanes are aerodynamically different too, which changes how they enter ground effect and flare. That has a bigger influence on landing performance than truck position (e.g. 727).

I have a question re the landing "feeling" harder on a toe-down configuration: Assuming all things are equal (and I realize that's not possible given everything you've stated re ground effect and flare), since the front wheels are ahead of the main strut, wouldn't the energy be more easily absorbed into the strut and transferred to the airframe than if it was a toe-up configuration, resulting in the feeling that it's a "harder" landing?

The only way I can describe this is to hold a pencil between your index finger and thumb just slightly behind the pencil's CG so that it is in a toe-down position. Now, push that pencil along the carpet on the floor with the toe-down end at the front. It's going to encounter more resistance than if you were to reverse it and run it the other way mimicking a toe-up bogey configuration. I realize it's overly simplistic, but it does mimic the concept of pushing vs. pulling an object that is going to encounter resistance in the process along the friction surface.



My other home is a Piper Cherokee 180C
User currently offlinerwessel From United States of America, joined Jan 2007, 2352 posts, RR: 2
Reply 20, posted (3 years 2 weeks 4 days 6 hours ago) and read 4852 times:
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Quoting redflyer (Reply 19):
I have a question re the landing "feeling" harder on a toe-down configuration: Assuming all things are equal (and I realize that's not possible given everything you've stated re ground effect and flare), since the front wheels are ahead of the main strut, wouldn't the energy be more easily absorbed into the strut and transferred to the airframe than if it was a toe-up configuration, resulting in the feeling that it's a "harder" landing?

The only way I can describe this is to hold a pencil between your index finger and thumb just slightly behind the pencil's CG so that it is in a toe-down position. Now, push that pencil along the carpet on the floor with the toe-down end at the front. It's going to encounter more resistance than if you were to reverse it and run it the other way mimicking a toe-up bogey configuration. I realize it's overly simplistic, but it does mimic the concept of pushing vs. pulling an object that is going to encounter resistance in the process along the friction surface.

There won't be much effect from that horizontal drag, since you're rolling wheels on the runway. The "hard" landing comes from the vertical motion of the aircraft being dissipated more or less gently.


User currently offlinebonusonus From United States of America, joined Nov 2009, 403 posts, RR: 0
Reply 21, posted (3 years 2 weeks 4 days 6 hours ago) and read 4846 times:

Quoting redflyer (Reply 19):
The only way I can describe this is to hold a pencil between your index finger and thumb just slightly behind the pencil's CG so that it is in a toe-down position. Now, push that pencil along the carpet on the floor with the toe-down end at the front. It's going to encounter more resistance than if you were to reverse it and run it the other way mimicking a toe-up bogey configuration. I realize it's overly simplistic, but it does mimic the concept of pushing vs. pulling an object that is going to encounter resistance in the process along the friction surface.

I don't think this is an accurate thought experiment because we are talking about wheels. Think of a wheelbarrow with one front wheel only. The amount of friction between the wheel and the ground (rolling friction) doesn't change when you change the tilt of the wheelbarrow (nearly parallel to the ground vs 45 degree angle, etc)

In the moment in time when the front wheel of the bogie is contacting the ground (767-type configuration), it shouldnt be any different from a rear wheel contacting. The amount of force is the same regardless whether the wheel is in the front or in the back, assuming the moment caused by the force required to rotate the bogie is negligible (which I'm pretty sure it is)


User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 79
Reply 22, posted (3 years 2 weeks 4 days 5 hours ago) and read 4834 times:

Quoting redflyer (Reply 19):
since the front wheels are ahead of the main strut, wouldn't the energy be more easily absorbed into the strut and transferred to the airframe than if it was a toe-up configuration, resulting in the feeling that it's a "harder" landing?

The pencil example is all about torsional stability. No matter how good the pilot is, the wheel will never be 100% perfectly aligned with the direction of travel at runway contact. Although the force to tilt the truck is pretty negligible (relative to the weight of the aircraft) the impulse to spin up the wheels is fairly big and, on a toe-down configuration, tends to want to twist the gear *more* away from the line of travel. Similar to pushing vs. pulling a bike.

I think the part of a hard landing that can be attributed to toe-down is really the torsional "snap" associated with the wheel spin up and alignment of the gear with the direction of travel. It's not an vertical issue. There is just no vertical resistance in the truck itself that would account for a noticeably different landing feel. Vertical feel all comes from the oleo strut and that doesn't start doing anything until all wheels are down anyway, making touchdown orientation of the truck moot.

Tom.


User currently offlineflipdewaf From United Kingdom, joined Jul 2006, 1572 posts, RR: 0
Reply 23, posted (3 years 2 weeks 3 days 21 hours ago) and read 4738 times:
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Quoting rwessel (Reply 18):

I mean the pivot on the truck is analogous to the CoG of the aircraft, the actual CoG of the aircraft has nothing to do with it.

Fred


User currently offlinerwessel From United States of America, joined Jan 2007, 2352 posts, RR: 2
Reply 24, posted (3 years 2 weeks 3 days 12 hours ago) and read 4651 times:
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Quoting tdscanuck (Reply 22):
I think the part of a hard landing that can be attributed to toe-down is really the torsional "snap" associated with the wheel spin up and alignment of the gear with the direction of travel. It's not an vertical issue.

While I agree that the alignment of the gear with the direction of travel causes different torques on the gear depending on which wheels hit first, that effect is still limited by the actual amount of pressure on that first wheel.

But wheel spin up is more complicated. Yes, the drag on the forward tilting truck will "dig in" the front wheel and increase the pressure on it, making it spin up faster. Whereas with a rear tilting truck the drag from the wheel spin up will tend to lessen pressure on the back wheel, slowing down the spin up. In either case if the spin up doesn't complete by the time the other wheels on the truck touch down, they *will* finish at that point.

So the spin up happens, with the same total energy, in either case, and in a very brief time - just a bit briefer to be sure in the front-tilt case. Interestingly, that added drag in the front tilting case would create additional upforce on the main strut early in the landing cycle, which would tend to slow the rate of energy dissipation (IOW, contribute to smoother landings). But it's hard to compare the magnitudes.

Then there's the other set of wheels. When the other wheels on the bogey hit the ground they will be spun up with even more intensity than the front wheels on the forward tilting truck, simply because there's now real pressure on those tires. So wouldn't there be a big kick from those?

Quoting flipdewaf (Reply 23):
I mean the pivot on the truck is analogous to the CoG of the aircraft, the actual CoG of the aircraft has nothing to do with it.

I'm sorry, I don't follow.


25 Post contains links Phen : That is fascinating and finding little jewels of information like that is the reason I love A.net so much. Thank you for posting that. Just to illust
26 Post contains links western727 : Fascinating discussion. Now, what about the 787's MLG bogeys? They're obviously rear-slung but upon retraction they become 767-like forward-slung (pre
27 Post contains images Speedbird2263 : I certainly don't want to stray from the subject at hand, however I've researched the net in general and A.net and haven't been able to find a conclus
28 tdscanuck : I do not know the answer for sure, but I strongly suspect it's related to how the 787 MLG are sequenced. The 787 doesn't use sequence valves (it's ba
29 Post contains images CALTECH : From the 767-400 A.M.M. It is merely to fit into the wheel well. L. Truck Positioner ( Fig. Fig. 11 ) (1) The truck positioner is a hydraulic actuato
30 CALTECH : From the 767-200 A.M.M.
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