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Recently I was looking over pictures where the An-225 is parked (because that's a normal thing for a normal person to spend their time doing). I noticed that the chocks are almost always placed around the third wheel from the front in the undercarriage: enter image description here (Thanks to ratchet freak for the better picture).

Presumably that's because the centre of mass (COM) is closest to that wheel? But why would the undercarriage not be centered around the COM?

Bonus question: Very occasionally, I've seen the chocks placed around the entire undercarriage, in front of the first wheel and behind the last wheel. Why the occasional difference?

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    $\begingroup$ There's a closer pic on wikipedia: en.wikipedia.org/wiki/File:Antonov-225_main_landing_gear_2.jpg (under CC BY 2.5) $\endgroup$ – ratchet freak May 19 '17 at 12:30
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    $\begingroup$ The center of mass of an empty aircraft is at a known point; once the aircraft has stuff on it, the center of mass becomes variable. During loading & unloading, the center of mass can vary quite a bit -- beyond the range which it's kept within for flight. $\endgroup$ – Ralph J Jun 4 '17 at 21:48
  • $\begingroup$ Note that you got the front wheel carrying some weight too, so your CG is probably designed to be a bit aft of the central wheel of the main undercarriage. $\endgroup$ – yo' Jun 5 '17 at 15:06
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    $\begingroup$ +1 for "that's a normal thing for a normal person to spend their time doing" $\endgroup$ – Peter Kämpf Jun 7 '17 at 20:08
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It's a matter of procedure. Chocking a center wheel provides the greatest defence in the unlikely case the chock is jumped. Other wheels in the truck must also contend. If an outside wheel is chocked and that chock is jumped away, all is lost.

No matter tipping or rocking the aircraft in extraordinary winds or loading, the center of a wheel truck is the recommended place to chock. A chock should be snug against both sides of a single tire to prevent any sway.

Most chocks are paired together by a rope or chain which would prohibit chocking around a large truck of wheels as described in your bonus question. What's more, chocking at either end of a truck allows more elasticity and movement between the chocks than a single wheel chocked snug.

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  • $\begingroup$ With chocks in front of the lead wheel & behind the aft wheel, if the aircraft goes forward, every wheel is then dealing with the chock nearest the nose, and vice versa if rolling backwards. Small wheel chocks are typically connected with a length of cord; once the chocks are large enough for wheels like this, carrying two at once becomes cumbersome & they aren't necessarily connected. Not sure I'm persuaded here... $\endgroup$ – Ralph J Jun 4 '17 at 21:43
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    $\begingroup$ Then why not chock the center wheel instead of the third wheel? $\endgroup$ – Sneftel Jun 4 '17 at 21:43
  • $\begingroup$ The chocks weigh a lot. So long the truck is chocked toward the middle and not the end, there's no reason except exercise to carry them three feet further. $\endgroup$ – STWilson Jun 4 '17 at 21:48
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    $\begingroup$ It's also possible (now I'm just making this in my mind, so I got no reference for it) that the requirement is not to chock the first or last two wheels. Then, if I were the mechanics guy carrying the chocks, I would soon develop the habit of chocking the 3rd wheel. $\endgroup$ – yo' Jun 5 '17 at 15:04
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    $\begingroup$ @Sneftel, the 4th to 7th wheel (axle, rather) are steerable on An-225 (or possibly free-castor; I've seen skid marks from them after pushing the aircraft back - in fact, you can see them on the photo). It makes a little more sense to chock a 'rigid', non-steerable wheel. In reality, I think, it's either a checklist requirement or a habit. $\endgroup$ – Zeus Jun 8 '17 at 5:33

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