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Main Undercarriage Rotation/position

Sat Sep 13, 2003 5:33 am

look at this two pixs, why are the main wheels in the back rotated forwards in one and back in another?



does it bother the landing

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RE: Main Undercarriage Rotation/position

Sat Sep 13, 2003 5:42 am

Gear retraction (geometry position) requires the gear to be in like this.
At least in the 747... and other Boeing birds...
On touch down, this gives the the airplane signal it is "on the ground"...
I.e. ground spoilers deployment in the 747...
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RE: Main Undercarriage Rotation/position

Sat Sep 13, 2003 6:13 am

That's interesting...before today I simply thought that the center of gravity on the bogies was forward/aft of the pivot point, and I wondered why the 767 is the only airliner which has forward-tilting bogies. Interesting explanation!
Some questions: what happens during a crosswind landing when only one main bogey touches down? Do the speedbrakes/ground spoilers deploy? Only one side? Not at all? Also, on a normal landing, do the autobrakes engage when the nosewheel is lowered? Thanks in advance!
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RE: Main Undercarriage Rotation/position

Sat Sep 13, 2003 6:55 am

As far as I know, the 767 is far from the only airliner with forward tilting gear. Certainly a number of older Russian planes featured this - I always thought they looked so cool coming in to land. I'm sure the Tu-154 was like this, which just added to its mega-cool aura. I also think the centre bogey on the A340-5/600 does the same.

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RE: Main Undercarriage Rotation/position

Sat Sep 13, 2003 11:30 am

As to all the automatic functions at touch down, some are activated by the truck leveling, if it does not work, by wheel spin up, and often, if that does not work, nose gear touch down as backup. Many backups...
Both sides must provide a signal (crosswind question)...
As an example, wheel spin up provides for spoilers deployment... if it did not go by wheel spin, the nose wheel touch down will do...
Forward or rear tilt... I would not know the difference when landing...
Is just a function of the gear retractiom geometry by aircraft design...
Honest, an irrelevant consideration...
Hope this answers your questions...
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RE: Main Undercarriage Rotation/position

Sun Sep 14, 2003 1:21 am

Forward or rear tilt... I would not know the difference when landing...

There is usually slightly more vibration on initial contact with forward tilted gear assemblies. A bit more difficult to make "greaser" landings. Very dependent upon individual aircraft design. i.e. 767-200 is almost always a "rougher" landing than 767-300 (probably nobody but pilots would notice though). Other than that... no noticable differences. JMHO.  Wink/being sarcastic
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RE: Main Undercarriage Rotation/position

Sun Sep 14, 2003 2:35 am

I admit that as a pilot in the 747, sometimes on training or test flights, there are times when it feels that you can almost identify each pair of wheels as they come in contact with the runway.
This subject was discussed before - and the procedure(s) for "smooth kiss" landings are definitely not recommended and potentially unsafe, there again, my admission that I have played these games in the past.
In the DC8s that I once flew (ONA) we did not "arm" the spoilers for touch down to impress passengers. I described "what can be done" - same idea for the 747... and I often mentioned that landing a heavy cargo airplane, is generally much "smoother" than a light/empty passenger 747... Maybe, AAR90 it could be that the 767-300 being slightly heavier has a smoother landing.
There are airplanes that a are inherently smooth landing airplanes, definitely the 747 is... and rough, the case of the DC8... I only know the 707s and the 727s besides that... The 707 was very smooth at touch down, the 727 did require some experience to achieve a smooth touchdown.
Continue back pressure (and slow flare/rotation) prior to touch down does "increase" the vertical speed at which a set of wheels enters in contact with the runway, as the wheels are aft of the axis. What I personally do in a 747, is, just prior to touch down, when the "10 feet" radio altimeter voice call is heard, - about a second later (I assume I am 5 feet above), I reduce the "back pressure" on the controls, slightly decreasing the attitude - let us say about one degree, to achieve touch down - this decreases the "vertical speed" of the wheels that will first touch down... touch down is achieved about a second after that reduction of "flare" back pressure...
Hope my explanation is clear, pilots here, know what I mentioned...
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