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Yesterday I took my new 3DR hexa outside for the first time. Weather was nice and sunny with brisk -7°c temperature. Even though everything worked well when testing, I immediately ran into problems outside. After connecting the battery all of the ESCs started beeping rapidly (two times per second) and refused to spin the motors. I took the hexa inside for a while to warm up for a while and the problem went away and everything seemed to work fine. Unfortunately as we all know, the assumption is the mother of all fuck ups.
I hovered the hexa, but each time I tried to land motors sped up suddenly and the whole thing flipped over faster than I could react. After breaking two props I started investigating further. There was a couple of helpful threads, one offering working solution to calibrate ESC at low temperature.
What I think what happened is this: Pwm value that ESC reads from APM changes a little with temperature. If we assume that lowest value in room temperature would be 1000, in -7 degrees it could be say 990. Now during calibration esc has saved value 1000 as the smallest possible value. When it reads value 990 at cold startup, it decides it’s not a valid value and refuses to start and starts to beep as a warning.
The real danger is in situation where ESC has started successfully, but the temperature goes down. At low throttle pwm value goes under the min limit and ESC turns off as a safety measure. Since not all ESCs do this at the same time, what happens during the flight is that APM tries to compensate this with other motors resulting an immediate flip.
Working solution seems to be to calibrate the ESCs at the lowest temperature you are going to fly. After doing that I had no problem flying outside. I think this would be important thing to mention in the wiki.