SITL and XPlane simulation

The XPlane simulation works with HIL, however when I try to connect using the "SITL" checkbox, mission planner doesn't connect to the SITL. I've checked all the ports and IP addresses and I can connect to the SITL with MAVProxy and QGroundControl. I can even connect to the SITL with the "Connect" button and selection of "TCP" in Mission Planner, however this doesn't send the data from XPlane back to the SITL.

Reproduce the XPlane-to-SITL connection through Mission Planner using the SITL checkbox, then compare it with the working TCP connection and the MAVProxy or QGroundControl paths. Done means Mission Planner connects through SITL and forwards XPlane data back to the SITL; the issue names no files or tests to start from.

Written by the indexing model from the issue text.