I had an issue with some of the servos yesterday before running the engine.
I put the recommended Spektrum A606 servos in the fuselage and didn't have enough for the wings so I used Hitec 5485 there.
I set the radio up using a 4.8 pack as the 6 volt NiMh was charging. I then put the 6 volt pack in the plane and checked it before heading out to run the engine. The A6060's went nuts, chattering like crazy. Not the typical jitter some servos give when the voltage is a bit high but severe chatter. I put the 4.8  back on and they were fine. I plugged the wings in and put the 6 volt back in and the A6060 chattered but the 5485's were fine. I checked the resting voltage and it was 6.7 volts..not quite fully charged yet. I had a regulator on the bench so I  put that in, set it to 5.9 volts and the A6060 servos settled down. I experimented and raised the voltage to 6.0v  and then 6.1 volts. Anything over 6.0 volts and the  6060's start to chatter.
The specs state 4.8v or 6 v compatible but they obviously don't like anything above 6 volts.
I put the regulator on and set out to run the engine and the A6060 on the throttle stopped working. It didn't get hot but just stopped.
I spoke with Horizon today and was told these servos will not function on any voltage over 6 volts and a regulator needs to be used. Should have had that printed on the box !
So I pulled the 6060's and will be putting in the 5485's and removing the regulator.
I imagine these 6060's would be good for any electric set-up using a bec  ,but for glow or gas you will need an ibec/regulator  when using a 6 volt pack for them to operate properly.