USAFHummer
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757-type Main Landing Gear Bogies...

Mon Oct 28, 2002 2:35 am

My question is...when the main gear bogies of aircraft with 4 or more wheels extend, they are obviously tilted as such in this picture...Im assuming that the reason for this is so that the gear bogie can fit in the main gear bay

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Photo © Miguel Snoep



...but what I would like to know is at what point of the approach phase does the gear bogie shift to this position at touchdown, or is the touchdown on the rear two wheels of the bogie...


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Photo © Justin Cederholm



Hope this is clear, if not Ill try to clarify...

Thanks,
Greg
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fritzi
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RE: 757-type Main Landing Gear Bogies...

Mon Oct 28, 2002 4:43 am

The 757 will touchdown on the rear two wheels of the bogies and they will tilt into the position as shown in the second picture when enough weight is put on them.

You can see in the picture below of the A332 that the bogies stay in the tilted position until the aircrafts sets its full weight on th ground during the touchdown.


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Photo © Andy Graf



/Fritzi

 
cdfmxtech
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RE: 757-type Main Landing Gear Bogies...

Mon Oct 28, 2002 8:56 am

Common question.
I'll explain the B757 for you. There is a truck positioner actuator located on the aft part of the gear. When weight is off the wheels, hydraulic pressure positions the actuator to extend and drive the aft wheels down. Now the purpose as the Aircraft manual states is to permit clearance between the gear and aircraft structure during gear retraction and extension. But I remember an instructor once telling me that they testes a B767-200 some time ago to see if it would clear the structure if not tilted. He said it did. (The B767 has the actuator on the front part of the bogie and the forward wheels go down.).

I also remember another incident at my airline a couple of years ago. And aircraft had to do an air return due to the landing gear lever lock solenoid not releasing after the aircraft transitioned to air mode. Now, I believe the crew pushed the lever lock override button and raised the gear. I mention this because when the aircraft was troubleshot, it was found that there was a hydraulic leak upstream of the truck positioner actuator and the wheels were sort of dangling, meaning that you could tilt them with your hands up and down. There was no hydraulic pressure to position the gear. Maybe the gear just happened to be in the correct position when they retracted the gear that time, because there were no problems.

So the gears are tilted to allow for clearance into the gear (official reason), and a secondary function is to provide air/gnd mode logic, since sensors depend of gear angle and wows (This applies to the B757/B767/B777).

As for you other question as to when this happens. On the B757/B67, the gears are tilted whenever weight is off the wheels. The gears are always tilted. The B777 is different. The rear wheels are tilted downward for ldg (unless there is a high approach angle. At that point the gear will actually adjust to help compensate for a steep ladg angle) and the rear wheels are tilted up during retraction. The can only fit into the wheelwell this way.

Hope this helps, I need water now!

PS...Maybe someone can shed some light on Airbus. I'm going to ASS-U-ME it works in a similar manner on the A300/10/30/40. But u know what happens when you ASS-U-ME
 
USAFHummer
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RE: 757-type Main Landing Gear Bogies...

Fri Nov 01, 2002 10:51 am

Thanks for the response, it makes sense now...

Greg
Chief A.net college football stadium self-pic guru
 
RyDawg82
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RE: 757-type Main Landing Gear Bogies...

Fri Nov 01, 2002 5:30 pm

Quick correction, and thanks for the great info so far. The 777 truck actually tilts back at takeoff and landing. When the truck is positioned for retraction, the truck tilts forward, and remains in this position until it has been lowered again. It's a rather interesting process to observe....Ryan
You can take the pup out of Alaska, but you can't take the Alaska out of the pup.
 
cdfmxtech
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RE: 757-type Main Landing Gear Bogies...

Fri Nov 01, 2002 9:27 pm

The rear wheels are tilted downward for ldg (unless there is a high approach angle. At that point the gear will actually adjust to help compensate for a steep ladg angle) and the rear wheels are tilted up during retraction.

That's what I said. Just said it a little differently
 
USAFHummer
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RE: 757-type Main Landing Gear Bogies...

Sat Nov 02, 2002 4:18 am

Just to check also, at takeoff then on aircrafts qith 4-6 wheel bogies, the front wheels will lift off first before the aft wheels? The aircraft won't pivot on the gear so that all 4 wheels get off at the same time?

Thanks again,
Greg
Chief A.net college football stadium self-pic guru
 
bio15
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RE: 757-type Main Landing Gear Bogies...

Tue Nov 05, 2002 12:35 pm

That would be the case with the 767 I believe, which tilts the bogeys foreward. On the backward tilted bogeys, the foreward 2 wheels will lift off the ground first.

-Alfredo
 
AJ
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RE: 757-type Main Landing Gear Bogies...

Tue Nov 05, 2002 1:45 pm

These are good photo's taken at the point of rotation/liftoff:
Until the wings are supporting the aircraft's weight all main wheels remain on the ground:

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Photo © Andrew Hunt
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Photo © Garry Lewis


Rearward tilting bogey's, and level bogey's, aft wheels leave the ground last:

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Photo © Rob Rindt
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Photo © Carlos Borda


Forward tilting bogey's forward wheels leave the ground last:

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Photo © Felix Sieder
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Photo © Carlos Borda


This also depends on whether the bogey tilt has hydraulic assistance to the tilted position. Notice these A310s bogey's remain at the angle at which the aircraft left the ground:

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Photo © Oliver Brunke
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Photo © Glenn Alderton

They will 'level' for retraction.
All in all it's a beautiful thing:

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Photo © Don Boyd


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