Approaching airport security with enough metal in your leg to build a small jungle gym without speaking the local language can lead to some difficult discussions on the other side of the metal detector. Nervously reaching down to your ankle to expose the leg after the fact in front of a high-strung guard may lead to a flying tackle while someone yells "GUN"!
This leads me to my first tip: zip-away convertible pants are your friend, especially when you don't speak the local language. Just drop the pant-leg from the knee and let the prosthesis speak for itself. Wearing shorts is also an option, but unless you have another pair of pants to throw on after security it can get cold on long overnight international flights.
Security screening from this point forward is always awkward and you can expect the full treatment, but I've never been asked to remove my leg. Don't be nervous, normally it's just a quick pass over with the wand and a sympathetic "have a nice day" smile.
Flying domestically in the USA was perhaps my most inconvenient experience. On leaving Nashville I was taken to a small private room where the screener did a full physical pat down, swabbed my leg and hands with three different pads for testing then spent about 5 minutes with the metal detecting wand. Lucky I wasn't in a hurry.
Seating assignments on the aircraft are perhaps the most important part of being comfortable. Because my prosthesis is on my left leg, I prefer to book a window seat on the left side of the plane in any row but the first one. Window seats have a small gap between the wall of the aircraft and the seat in front; a perfect spot to rest my leg. A window seat also means not having to get up for any small-bladdered fliers once my leg is off.
The prosthetic leg itself can make a really comfortable leg rest if you flip it upside down. The rubber on the bottom of the shoe is more comfortable than the sharp edges of the socket -- sort of your own mini travel ottoman. My right leg actually seems to get jealous of the fully stretched knee of my left side.
I've been told by many prosthetists not to take my leg off in flight. To that I say "yeah right". Clearly these people have never tried to wear one, especially not for that long in a stationary position. The socket gets mighty uncomfortable when sitting for 15 hours straight without getting to bend or flex the knee.
The medical concern is that the leg will swell up and it will be impossible to get the socket to fit after. This can easily be solved by wearing a shrinker sock or the liner on its own, although I prefer the shrinker as it lets the skin breath. I usually add a second sock for warmth over top then cover the whole thing with a blanket.
This brings me to my "seat kit". On the outside, it looks like a standard, run-of-the-mill leather portfolio for the regular business traveller. The difference comes on the inside. The following is my list of essential items and still leaves room for a magazine:
The real advantage of this kit is its slim profile. I can slip this down against the wall of the aircraft, wedging it against the prosthesis. This eliminates the need for keeping a larger, more foot-room-restricting bag under the seat in front of me. You might laugh here that I'm still trying to maximize space for my one remaining leg, however, the extra space allows me sleep fully turned on my side during long flights. That's right, neener-neener two-leggers!
One last point: many people pack extra socks, among other things, for long flights. I'm no exception to this with one small difference: no need to pack extra socks in the carry-on. The one on my left side stays fresh and clean, waiting to be swapped over just before landing!