PilotjohnS
Well Known Member
I found a single point failure in ignition systems that use a switch having a BOTH position.
I had a ACS FAA-PMA ignition switch in my RV9A short out during takeoff and ground out BOTH mags, engine went dead. Plane had 10 hours on the Hobbs.
During run up, at 1700 RPM all is normal.
But during takeoff at 2300 RPM both mags would cut out momentarily, This was experience as a stutter in the engine.
After convincing myself it couldn't be the switch, I changed the switched to a Cessna 152 part and all is well.
Theory: turns out the P-leads from the magnetos have high voltage on them when the engine is operating. I read something like 300 V at 1700 RPM and 500 Volts at cruise RPM. if the BOTH position develops a high voltage short, then it will shutdown BOTH mags. They are not independent redundant systems with a common switch.
I am changing to two locking toggle switches with a Key start soon.
I would like to find a high voltage tester to see where the breakdown is occurring.
I had a ACS FAA-PMA ignition switch in my RV9A short out during takeoff and ground out BOTH mags, engine went dead. Plane had 10 hours on the Hobbs.
During run up, at 1700 RPM all is normal.
But during takeoff at 2300 RPM both mags would cut out momentarily, This was experience as a stutter in the engine.
After convincing myself it couldn't be the switch, I changed the switched to a Cessna 152 part and all is well.
Theory: turns out the P-leads from the magnetos have high voltage on them when the engine is operating. I read something like 300 V at 1700 RPM and 500 Volts at cruise RPM. if the BOTH position develops a high voltage short, then it will shutdown BOTH mags. They are not independent redundant systems with a common switch.
I am changing to two locking toggle switches with a Key start soon.
I would like to find a high voltage tester to see where the breakdown is occurring.