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As a pilot, this is the part of the ignition switch you’re used to seeing… the part where you put the key in when you start the engine.  It’s also the part you use to do your mag checks.  Beyond that, it’s “Out of sight, out of mind,” when it comes to ignition switches.

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But what exactly is behind that panel where the ignition switch is mounted?

That’s a good question, a valid question, and a question worth digging into.

Most small airplane ignition switches are Bendix / TCM switches, or ACS / Gerdes switches.

Both of these types are affected by a recurring Airworthiness Directive.

The first is AD 76-07-12 – This one applies to certain Bendix ignition switches.

You can read it here if you like:

http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgAD.nsf/AOCADSearch/C14960A415D956BD86256E520053A53E?OpenDocument

The other main AD for ignition switches is 93-05-06, (for ACS / Gerdes switches,) and is available here:

http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgAD.nsf/AOCADSearch/B8ABD56539B4684886256A3E00759DBF?OpenDocument

Interestingly, both of these AD’s only apply to switches that have a “start” function.  However, in my opinion, any rotating type ignition switch should be checked regularly to make sure it is functioning properly, and not causing a hot mag when it’s in the “OFF” position.

Now back to those AD’s, there is a reason why the switches with a start position have a special tendency for having problems.

It’s because of what can happen when the starter switch is released.

Listen to this week’s episode and find out what happens, that over a long period of time, can cause this inside your ignition switch:

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Notice the black deposits and severely worn contacts, as a result of electrical arcing.

If you want to get geaky and read a great article about ignition switches, here is a great article from the April 2011 issue of Light Plane Maintenance:

http://hhh.gavilan.edu/hspenner/iLearnInfo/AMT111/ElectricalSystems/IgnitionSwitch.pdf

Very educational, I must say!

Thanks for listening, and if you like the show, please leave a rating and review on iTunes… I would appreciate it!