The following command will connect MavLinkTest app to the Simulator and enable logging of all mavlink commands to and from the PX4.

MavLinkTest -server:127.0.0.1:14550 -logdir:d:\temp

Sometimes you will also want to log the "output" from the Simulator that is being sent to the PX4. Specifically this will capture the "hilgps" and "hilsensor" messages that are generated by the Simulator. To do that run this as well:

MavLinkTest -server:127.0.0.1:14389 -logdir:d:\temp\output

You will then see log files organized by date in d:\temp\logs, specifically *input.mavlink and *output.mavlink files.

For MavLink enabled drones, you can also use our Log Viewer to visualize the streams of data.

INFO  [logger] Opened log file: rootfs/fs/microsd/log/2017-03-27/20_02_49.ulg
If you are using Pixhawk hardware in HIL mode, then set parameter SYS_LOGGER=1 using QGroundControl. PX4 will write log file on device which you can download at later date using QGroundControl.