The weather: Fair, visibility 13nm, wind Easterly force 1 to 2, pressure 1024mb,
temperature 11C.
The Flight: Duration 5 minutes, maximum altitude: 90 metres agl, average 65 metres agl,
min 45 metres agl. Power consumption, 30 amps in flight which is approximately 690 Watts. Battery
discharged to 55%, terminal voltage 23.2V.
The UHF telemetry link: Signal to noise ration seen by the Tx/Rx
on the UAV 10dB. (Signal -77dBm S8) Signal to noise ratio seen by the ground
station 11dB (Signal -90dBm S6)
Take-off was in flight mode LOITER and the landing in flight mode RTL (Return to Launch).
The log shows a sudden and rapid loss of hight from about 90 metres down to 50 metres which
is a bit worrying.
The following video is from the GoPro camera on board the UAV. Please refresh the page in your
browser if the video appears to be unavailable.
The APMPlanner log files provided an explanation for the loss of altitude and the unexpected behaviour of the UAV. I had intended to select flight mode CIRCLE once the UAV had reached 80 metres agl. However the log shows that the mode changed to flight mode AUTO. In flight mode AUTO the UAV will follow a predefined route or 'mission' which has been previously loaded into the UAV's memory. I did have a 'mission' memorised and it appears that the UAV started to follow that 'mission', resulting in a decent to about 50 metres and a change of course. From the ground it was obvious that flight mode CIRCLE was not working as expected and so I switched back to LOITER via STABILISE. The brief period in STABILISE resulted in a rapid assent to 70 metres agl. Once in flight mode LOITER the assent was abruptly halted and in the video you can hear the motors momentarily stop. The altitude remained unchanged while in LOITER. With about 2 minutes remaining on the 5 minute countdown timer I selected flight mode RTL. The UAV returned above the launch point without any loss in altitude before descending to land. It's worth noting that, from the ground, it looked like the UAV was going to come down on our heads but this turned out to be an illusion. I need to reprogram the way the flight modes are selected. I shall also remove STABILISE mode, because it is rather dangerous. And in future I need to make a point of checking the telemetry from the UAV to see the confirmation that a command to change flight mode has been successfully processed by the UAV. If in doubt, always return to LOITER.