Key Ignition vs. Toggle Switches

[jtpitkin06]

My local airport is KGVT, Greenville, Texas. It is home to the defense contractor L3 – Mission Integration Systems. L3 employs thousands of engineers. I am fortunate to rub shoulders with many of these engineers both active and retired.

I spent much of the day with a pilot/electrical engineer who has over 35 years designing and specifying aircraft electrical systems. We discussed the magneto switch when used with the Rotax Ducati ignition system. He agreed the notion of a magneto switch causing a failure of the solid state module with some type of "feedback" was poppycock. He also agreed the AC voltage on the Ducati kill circuit was less damaging to a switch than the P lead circuit on a magneto ignition.

Let’s look at the facts, how the system operates, and possible failure modes.

The Rotax Ducati ignition uses a shorting wire to “kill” the ignition. It is only grounded on engine shutdown and during ignition checks. The shorting wire is grounded through the “mag” switch. If the shorting wire is disconnected or breaks, the engine continues to run. The only electrical things that could damage the ignition module are overvoltage and improper or shorted wiring.

The Rotax engine dynamo supplies power to the modules. Voltage is limited by engine rpm. Current is physically limited by the size of the dynamo coils. It is impossible for the dynamo to supply excessive voltage or current to the modules. The vast majority of Ducati ignition module failures are wires breaking inside the insulation close to the module due to vibration.

The premise that AC voltage present on the grounding circuit is too high for a magneto switch is unfounded. The voltage involved when grounding a Ducati module is similar to the voltage present on the P lead of most magnetos. The internal arcing with AC voltage is less than what would be present with a pulsed DC signal from a magneto. There is no reason to assume one or more toggle switches will last any longer than a standard magneto switch.

While we have no doubt some standard type magneto switches have failed while in service on the Rotax engines, we do not believe they are the cause of ignition module failures. If the contacts on the magneto switch were to corrode or burn to the point where they no longer make continuity, the respective module would simply not shut down with the switch in the off position. I don’t think LEAF, Lockwood or anyone else can explain how an open kill circuit due to a bad switch is the cause of ignition module failure.

Advising aircraft owners to change the magneto switch to toggle switches is questionable. The magneto switches are aircraft quality devices with large contacts capable of handling high voltage and current loads either AC or DC. Magneto switches are used successfully on many Rotax 912 equipped aircraft without incident. The replacement toggle switches are largely unspecified. A 250 volt toggle switch can be a totally inadequate miniature device capable of only 0.5 amps. with quality or origin unknown.

In summary, the Rotax Ducati ignition module kill circuit is not likely to damage a magneto switch or a replacement toggle switch. Nor is any switch, magneto or toggle likely to cause a failure of the ignition module.

John Pitkin








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[cap01]

Thanks John for clearing that up , I was having a hard time understanding how the open contacts of a mag switch could cause problems with the module . The rotax installation manual does give a 250 volt .5 amp spec for the mag switches . Granted there are quality issues out there with so much crap from china . Thankfully there is some mil spec stuff still available.
There are advantages to having both the toggle switches and the keyed switch . For the times that require motering the engine without starting it ,such a purging the oil system also no chance of bumping the starter while doing a mag check using the toggle switches