USB connection causes motor 1 to run on APM1

I just wanted to check with the community as to whether I have stumbled on a serious safety bug, or I just have some bad glitch in my setup.

APM1.4 with new ArduPPM and AC3.01 installed.
Separate uBEC, HK ESC’s connected to board with red wire removed.
Spectrum 6ch Rx, HK GPS NEO-6M

This is a minimal setup so I have not installed radio comms, just using USB to setup and program.

On the APM1 no power is fed to the Rx so to do radio calibration I needed to connect the battery, then I connected the USB, which reset the board and motor 1 immediately spun up to full rpm.
Luckily I have no props on the frame.
I have repeated this several times make sure it was not a fluke occurrence.

Do I have something amiss?
Have I not read some instructions somewhere warning against USB connections while powered up?
If this is not the case then I guess I should report it as a bug.

How are you powering your APM1.4 and your ESC/Motors?
I recommend splitting the APM R/C input power bus from the APM ESC output power bus like here: … power-hack
You really should not have the USB hooked up at the same time that your are powering the output bus of the APM with the ESCs/battery unless you have isolated the APM input power bus from the output power bus.

Thanks TCIII,

That link is what I needed to know.

I power the board from a uBEC, as I always do, and the 5V BEC supply from the ESC’s I always disconnect, so only sig/gnd to ESC’s.

On all the 2.5’s I have doing the initial radio calibrations is easy as the Rx is powered from the USB, then everything else is usually done via radio once the copter is off the bench.

I am putting together a simple ‘crash test dummy’ hex with one of a few 1.4 boards I have lying around and was going to put minimal equipment on it, but maybe I will put a radio on for simplicity and safety.