I run the same pair of Xbees on my 'copters and Plane and have no problems getting telemetry into the mission planner from my PixHawk on an Octocopter and with earlier APM hardware.
I have recently upgraded to an APM 2.5 and loaded Arduplane 2.77.
I get no telemetry using the same hardware that works on my other APM gear.I have checked the wiring, and the COM port is correct and a loopback test works. I can see a datastream coming out of the APM with a scope and data is being transmitted from the mission planner and being received but no link is established. If I use a terminal programme (RealTerm) rather than mission planner it will not resolve the data unless I send it random characters when it will return a few pairs of hex characters.
Any suggestions as to how I can debug the problem?