Went out to the country today to do some flying with my outdoorised cx2.

All was going well enough until the dreaded random motor cut started happening, the first time when up around 20-40m in the air. Total motor power loss, followed by plunging towards the earth.

Rinse and repeat 3 or so times till i got sick of it happening and posted here.

Why on earth does a cx2 that has been happily flying for a few minutes randomly cut out at no particular distance? The power cut is infuriating as it leads to significant damage due to no error on my part.

My heli has always done this randomly since i got it. Some times I can go for weeks without it ever happening, and then other days it does it before I can even get up a few meters distance.

You might be hitting the range limit of the transmitter. If/When that happens the 4-1 defaults back to whatever was programed when the transmitter and receiver were "bound".

This is usually neutral for Elev, Ail, Rud, and throttle @ 0.

The CX's are meant more towards indoor, then outdoor. Distance can be trouble.

how many flights do you have on the cx2? i had a similar problem with my beltcp,the bird would drop out the sky.i traced it to the battery,it was over 2 years old with many flights on it.i changed the battery,the problem, went away.

Funny thing is distance doesnt seem to play a part. Sometimes it will be 10m sometimes 200 sometimes 30 or 40m. I have tried several different sets of batteries with no discernable pattern. I check all the lipo voltages before and after use.

Would a bigger spektrum transmitter pump out a stronger signal than the stock cx2 unit? Is there any way to hook in an amplifier into the cx2 unit?

I have had it as far as 300m away on some days with absolutely zero issues, so I cant seem to work out a common factor, except maybe proximity to power lines.

I fly my chopper over, under, and around power lines all day, and I've had no issues with interference so far. Mainly I have to do this because my backyard has too many short trees to fly in. So I have a choice between power lines everywhere or tree limbs 5 foot off the ground.

I use a 72 mhz e-sky 4in1 receiver and matching model 0404 radio.

I'd have to agree with the above that it's electrical in nature. Possibly a mosfet that's overheating and stops responding. I'd also suggest carrying some masking tape, a multimeter, and an ink pen with you when you fly. When your power cuts out on you, write down the voltage on both cells of the battery and your flight time on the tape then stick it on the battery you were using. That way maybe you'll find some type of pattern.

Take your heli and cycle the servos for a good few minutes and see if you can get it to reboot. Be sure not to give it any throttle.
If/when it does this, either remove one servo from the RX or just not move that servo with the sticks and repeat (to try and find the offending servo).
I have twice had this problem and both times it was a binding/cooked/bad servo.

Well I worked out what causes it... just not why.

If I move the servos a lot, I can see the green led intensity varies. If I do this for long (~5-10 sec of moving both servos) I can produce a reset repeatedly.

Any ideas how to fix this? Which area of the circuit board is associated with the BEC/etc?

What model receiver are you using? Is it the cx2 with the 6100 receiver? I was doing some research last night on 2.4 ghz receivers and found a lot of forum posts on "brownouts" involving that receiver.

Edit: Upon watching the movie it looks like you've got one of the older 72 mhz 4in1's. I would say replace the 4in1 unit, but it's possible a bad servo is causing the issue. Have you got another set of servos to test out on the chopper?

Just did some more testing, seems like one of the two servos causes the issue on any of the channels, so its not a 4in1 problem

Its a 2.4 4in1, and its definitely thhe servo, thankfully!

If it has the E-flite S60 servos,then that is what is causing it.Just replace the bad servo,it'll be fine.It seems the S60 servos can have extreme current draw after some use,I have had to replace several.The first one was replaced no charge,by Horizon Hobby,but after that I never bothered to contact them,because it was getting to well used.I think they should have stuck with the S75 servos.