How I took my iMac on a flight.
Just thought I'd share my experience of taking my 21.5 inch iMac on a couple of international flights. I carried it as a hand bag, taking it in the cabin with me. I knew size might be a problem, so I wrapped it up to keep it as compact and as safe as possible.
Briefly, I first covered the screen with the foam/cloth thing the iMac came in. Then I wrapped some cling film all around it, then bubble wrap.
Then I cut up and stitched together some shoulder straps of some old backpacks and duffel bags I had lying around, and wrapped it around the edges of the iMac (1). I took care to place the padded portion of the strap at the bottom edge of the iMac (2), as that would be where the iMac would be pressured most when I carried it around.
I wrapped and taped a piece of cardboard on the bottom part of the stand (3), for extra protection for when I put it down on the floor while at the airport.
I then cut off the edges of a duffel bag I had (4), and placed the iMac in it, making it real easy to carry. I also wrapped a couple of belts around (5), for holding the bag and everything else in place. I used plenty of tape as well.
In the planes, I placed the iMac in the overhead compartment, lying on its back, making sure no one placed anything on top of it. I padded it as best I could with my jacket, and some of the cushions and blankets you get in planes, as I didn't want it clattering around in case of turbulence. In one of the planes, a Boeing 777, the iMac actually didn't fit in the overhead compartments over the centre seats - but the compartments on the sides of the plane were bigger thankfully.
The weight and dimensions could have been a problem - the latest 21.5 inch iMacs weigh under 6 kgs, but mine is a mid-2011 model, weighing over 9 kgs, which is higher than the allowance of 8 kgs. (It also made it a slight pain to lug around). Also, its height makes it an inch or 2 bigger than the permissible dimensions of cabin baggage. But luckily no one objected, and I got away with it.
All in all, with a little planning and preparation, my journey with the iMac went very smoothly. The trickiest part was preventing other passengers from dumping their bags on top of the iMac, as not everyone is as considerate or understanding as you'd like them to be.
iMac, OS X Mountain Lion (10.8.5), Mid 2011, 21.5 inch, i5 2.5GHz, 8GB