1st flight results in flyaway, what happened?

Hello, my F450 build was finally ready to fly (or so I thought)… I first slowly throttled up into a hover about 2’ off ground. Landed. All was good. I then lifted off again, got to about 5’, tried to level off with the throttle stick but the copter continued to rise. I have the tx setup for two modes, stabilize and alt hold. In the log I can see where I freaked out and flipped the mode switch hoping to get it to stop lifting up, that seems to have only made it worse.
Fortunately a tree branch at about 20’ caught a prop, disarmed the motors and caused the quad to crash or there’s no telling where it would have gone. Only damage appears to be a broken prop and landing gear leg.
Can anyone look at the log and tell me what may be the issue? Maybe I missed something in setup or preflight? Thanks!

You hit your battery failsafe voltage and the quad went into RTL so went up to its RTL height of 15mtr. It only got to 10mtr when you switched to Alt Hold. If you’d left it a few more seconds it would have got to 15mtr, come home and landed.

Seem’s you also started the flight with a part drained battery (11.4v not 12.6v) and it may be an over used or under powered (too low a C rating) battery??? it doesn’t seem to hold it’s voltage too well under throttle, dropping to 9.7v at 46% throttle.

Ah yes that would be it-I’m using an old battery I thought was OK… time to get a new one. Thanks!

I would also turn up your logging for the first few flights so we have more info about what’s going on. With APM having logging turned up may slow done the processor so only for the first few flights.

Mike