ORIGINAL: flyer59

This post is to inform other Futaba 9C owners of an undocumented quirk in the Tx programming that nearly caused me to crash several times before I learned what was happening.

The problem is that when a mix is ON the trim levers may change the mix OFFSET instead of trimming the plane. This can happen without entering programming mode. If this happens when the plane is in flight the pilot may keep trying to crank in more trim because the plane isn't responding. If and when the affected mix is later used the pilot will be in for a big surprise because the OFFSETs will be way off.

My problem was with the Butterfly mix on a full-house sailplane. I normally fly with the Butterfly switch enabled so I can engage it without flipping a switch first. In my case applying down elevator trim did not change the trim setting, but the plane dived sharply when I engaged Butterfly mode. A later check of the Butterfly elevator OFFSET showed that it had changed from -10% to -90%.

I kept sending the radio in for service because I didn't understand why the trim wasn't working and the offsets were always changing. The guys at the Service Center wrote back the last time I sent it in and told me of the connection between the two issues.

I like the 9C, but personally I'd prefer not to have my program parameters to change unless I am in program mode to change them. Maybe there's some reason why the 9C is designed this way, but I can't imagine why and I don't see anything in the manual about it.

Be sure to adjust trim in flight only when all mixes are turned OFF to avoid changing the program offsets.