We just returned from my 50th HS reunion in Fairbanks. We took this opportunity to test traveling by air with our tandem as checked baggage. I built a box of 1/8" mahogany plywood stiffened with clear fir cleats and glued together with epoxy. The box measures 71" X 31" X 10" outside. I put 2" swivel casters on the bottom, which are removable. The box top hinges with some scraps of piano hinge I happened to have. The hinge is screwed to the top cleat of the box and is epoxy-bonded to the box top, no fasteners there. The box top seals down with duct tape. TSA just cuts the tape and retapes.
Our medium/small Co-Motion Speedster tandem fits in there perfectly with wheels, rack, and rack bag. As you can see, I removed the wheels, captain's bar, stoker's bar, and stoker's saddle to get it to fit. Less obvious, I removed the RD, partly to protect it and partly to make the box shorter. I also removed the drum brake cover and cassette to make the rear wheel fit more friendly-like beside the frame. The box measures 114-7/8" with the casters on and 112" with the casters off. Some American airlines have either a 112" or 115" length+height+width total measurement limit for any oversize baggage, so this box can be taken almost anywhere. British Airways, though, has a 75in x 29.5in x 25.5in limit, so be sure to check with the particular airlines you will be flying.
If one had a larger frame, I think it could still be done within this size limit by also removing the stoker's saddle and making the box both longer and lower. Our box with bike, wheels, rack, and bag filled with bike tools, ready to fly, weighed a hair under 70 lbs, the usual European overweight checked baggage limit being 70.5 lbs. or 32 kilos. We could have cut the checked baggage weight a little by putting the heaviest tools in the other checked bag with our panniers, thus a heavier tandem also could make the weight limit.
The box and bike flew perfectly, no hint of damage. It was very convenient to be able to wheel the box around and the box made it very easy to slide the bike into various vehicles. It was all a great success. It would take many flights to save the money spent on a coupled tandem. We paid $50 each way on Alaska Airlines. It took about 2 hours to disassemble or assemble the bike at each end.
Next month, we fly Condor Flugdienst to Frankfurt and then Lufthansa to Prague for a 3-week tour. We will store the box with our other luggage inside at our arrival and departure hotel.
We used TG's system of 3/4" PVC tube with QRs to protect the drop-outs at each end.