I'd like to preface this post with this statement: I love my (lab's) Udrones Hexa C, but I feel that this simple fix would save a lot of grief for Udrones flyers.
So! I have noticed that the Udrones Hexa C that I use has a nasty habit of vibrating its motor mounting screws loose. Twice before, I have landed my hexa only to find that one of the motors was dangling by one screw, because the other had vibrated loose. Well this time it finally happened! During the landing sequence from 80m, the rear left motor came unscrewed and hit the arm, flying off.
Here's the sequence of events:
18:25:45: Hexa is returning to launch. I notice the it sounds rattle-y. I've heard that sound before, it means that a motor has lost a screw. Since it was already about to land, I let it continue. Landing sequence commences at 80m above home.
18:26:06: Motor comes off at 55 meters. At this point, the copter stays airborn but cannot stay in one spot. Missing one motor, it yaws at a good rate, making manual flight very very difficult. I can't compensate for the yaw, because it makes the copter tip over.
18:26:18: At this point the height reading in the tlog says that the copter very nearly touched the ground. I can assure you it did not, I was trying to get control of it in the air, no lower than 20m at the very least. At this point I was deathly afraid of the copter landing on a roof, or much worse: the pool. I attempted to stear it towards the safest place I knews, the forest.
18:26:37: Not quite reaching the forest, the copter is flying over the tennis courts at a frightening clip. The telemetry says it was underground at this point, but I assure you it was about 20-30m up.
18:26:46: At this point I am still trying to hit the forest but barely miss it. At this point I am running through the tennis courts. I see that the copter is going towards a medium sized road. Under no circumstance do I want the copter to crash anywhere near moving cars, it would be dangerous to everyone involved. So I lowered the throttle until the copter entered a descent.
18:27:01: From my point of view, the copter vanishes behind a stand of trees, still descending. I give it two seconds, and then cut the throttle completely.
18:27:04: Impact. From that point, I dash over and am immensely relieved to see that the copter landed upright on a grassy field with nothing broken beyond the missing motor and a chipped prop. I went back to the launch site and recovered the top half of the motor, complete with propeller still attached. The bottom half of the motor was still dangling from the copter, its mount cracked.
The moral of the story is: apply threadlock to your motor mounts, and check them before every flight! Otherwise the screws will vibrate loose and possibly detach your motor. I urge Udrones to add threadlock to the motor mounting screws of their ARF copters.