master switch

[navybipe]

I'm thinking I have a new attitude towards the master switch.... One day a fellow technician and I were working on a very nice Mooney M20. While he finished re-cowling, I decided to check a squawk on the eyebrow lighting in the instrument panel. I located the “Master Switch” and moved it to the “On” position. That was a really stupid thing to do. When the “Master Switch” moved to the “On” position the engine/propeller came to life! It took at least three propeller blades before I could reach back across the aircraft and turn the “Master Switch” to “Off”. Luckily my partner had completed working around the prop. He had just stepped through the propeller arc when it began moving. The propeller just barely missed his head, but it didn’t miss him completely. There was a long cut on his right arm. I’ll never forget the look on his face as he asked, “Why Rick – Why did you do that?”

The starter solenoid had failed on the last engine run. The failed solenoid would engage the starter any time electrical power was applied to the buss. It’s rare but, it happens. We work around aircraft that have power applied to them all the time in the hanger. We get away with it – MOST OF THE TIME, but not always.

The last sentence of FAA, AC 91-42D, Chapter 7 Aircraft Service Personnel, paragraph (f) says, “Faulty diodes in aircraft electrical systems have caused starters to engage when external power was applied regardless of the switch position.” And, I can verify this is true!

Be vigilant, be careful and Blue Skies.

Want to learn more? Download AC 91-42D
http://www.faa.gov/regulations_poli....cfm/go/document.information/documentid/23159
Rickey D. Hestilow
2015 National Avionics Technician of the Year
[email protected]

 

[Larry Lyons]

 

[TFF1]

 

[skyboltone]

 

[will moffitt]