I'm reposting this from another forum. The man telling the story is no amateur. He's an experienced avionics technician.


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_polic...cumentid/23159

Wow! Scary! I never knew this kind of failure could happen... I'll for sure start making sure the prop's clear before applying master power.

Thanks for sharing and I'm glad nobody got seriously hurt!

Not really that uncommon. One of the first things I was taught at A&P school is that this can and will happen...and to NEVER have anything in the prop arc when you energize the system.
John

Yep had that happen to me working on a 180 as a young A&P. Learned a lot in those few seconds.

Can also be a problem when it fails during start - engine fires up but starter is still spinning - which is trashed by the time you land. Some put in indicator light to show solenoid is engaged. May be the cheapest part causing the biggest problem...

Yup, just happened a day ago to one of our AW139's (helicopter). In a crowded hangar, plugged in the GPU and one of the engines started spooling up. Engine mode switches were off. Lucky to get it unplugged before either the main or tail blades cleared a swath.

In an earlier life I was an exotic car mechanic. Lamborghini, Ferrari, Maserati, Lotus, Aston Martin, Rolls Royce, etc, etc, etc, etc. Did that for 20 some odd years. Not once did I ever have a car try and start when turning the ignition key to the "on" position, and I worked on tens of thousands of cars. Happens in aircraft (and I guess helicopters) all the time. Thanks FAA for keeping certificated aircraft in the stone age.

I've had a car try and start when I hooked the battery up after an engine swap. Turned out I had the start wire touching the positive terminal on The solenoid.

Even more exciting I cranked an older chev 350 over with no carb on it but the fuel line pointing at the hole where the carb should be, let's just say it didn't take long for the oil pressure to come up!

I probably shouldn't admit to these things...

River rat, I have to laugh at your carbless runaway because I did exactly the same thing with a 69 Cadillac / 472 engine. I don't remember why or what possessed me to turn it over but it took off like a rocket. Of course I still had my hand on the key and shut it off immediately, and thankfully it didn't backfire because gas was everywhere.

Ya it's amazing how long it takes your brain to process what's happening and turn the key off!

That's funny, so did I.
In an earlier life I was an exotic car mechanic. Lamborghini, Ferrari, Maserati, Lotus, Aston Martin, Rolls Royce, etc, etc, etc, etc. Did that for 20 some odd years. Not once did I ever have a car try and start when turning the ignition key to the "on" position, and I worked on tens of thousands of cars. Happens in aircraft (and I guess helicopters) all the time. Thanks FAA for keeping certificated aircraft in the stone age.

However here are some things that did happen to me, I owned and rebuilt a 1968 Land Rover , one day I was out in the yard and could hear an engine cranking over, HMmm that's odd, it was coming from the Rover, the plastic on the starter relay had warped and shorted out the starter contacts, I usually use the Rover for dragging the yard, in four wheel drive and low range, thank God I left it in neutral, had it been in gear who knows where it could have ended up. Second happened to a service writer at a British car dealer that I was working at, about 1975 as I recall the customer had a new MGB with a complaint that the starter engaged when the parking brake was pulled, The car was pulled into one of the stalls and the service writer sat in the car with his legs out the door and pulled the parking brake, you guessed it, the car lunged forward smashing a mechanics tool box and the front of the new MGB.
Oh and the other that was not uncommon with the early Jag's was bumping the ignition with out engaging the starter and having the engine start.








i

I know you remember the Lucas moto; "Home before dark"

My brother came home one night to find the tractor idling in the yard (starter wires melted into a lump) lucky it was in neutral! Just imagine the carnage it could have caused with the bucket a couple feet off the ground, I think it was pointed at the house too!

Wow!! You always treat a firearm as if it were loaded and I would guess it makes sense to treat the propeller as if the mags are always hot.

For those who have driven behind electrics provided by The Prince of Darkness, check this out: http://www.mez.co.uk/lucas.html

TD