Master engages starter

Let'sgoflying!

Let'sgoflying!

Touchdown! Greaser!
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Dave Taylor
I received this in an email tonight from the 'FAA Safety Team'.
This exact thing happened to me, and I don't think I've mentioned it to anyone because it seemed so unlikely and I'd never heard of it. Reading it made me wonder how many times it has happened; has anyone heard of it or seen it?

In my instance it was parked in the hangar and there was someone else in the hangar but not near the prop. Flipped the master on to...check some lights or something, and the prop started cranking vigorously. Key not even in the airplane. No previous problems with the ignition switch, solenoid, or any electrical component thereof.
When it happens, you immediately think; "Whaa? Did I forget how all this works?"

the article:
xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Question: How many times have you:
1. Removed the ignition key from the ignition switch
2. Placed the key on top of the instrument panel (for all to see), thereby insuring that the
engine/propeller is safe to work around?

News flash, that part about SAFE isn’t guaranteed!

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!
 
No, it really is a separate issue from testing to see if the ignition switch is grounding out the mags properly. Has to do with the 12/24V system.
 
Fearless Tower said:
Wouldn't this issue show up on a mag ground check?
Click to expand...

No. The mag switching has nothing to do with the starting contacts in the switch.

There are three things that can cause this:

A stuck starter solenoid, which keeps the starter running after the key is released from the start position and usually burns the starter out pretty quick if the pilot doesn't notice any extra noise. Some people like to have a starter indicator light for just that reason.

An incorrectly installed starter contactor diode, which shows up the first time the master is turned on after diode installation.

Or a bad ignition/starter switch. ACS switches have an AD against them to require checking and lubricating of the contacts and the installation of a shunt diode across the starter contactor coil to prevent burning of the contacts in the switch. There is conceivably some possibility of those contacts being contaminated with burnt bits of metal, shorting them together.

In the shop I always check to make sure nobody is near that prop when switching the master on.

Dan
 
Dan Thomas said:
No. The mag switching has nothing to do with the starting contacts in the switch.
Click to expand...
But if the engine dies when selecting both mags off, how can the engine then restart by only turning on the master? That is what I am trying to understand.
 
Hmmmm.....I'm not getting this failure mode.

If the solenoid were stuck or any of the above scenarios with the master switch in the on position....wouldn't the starter motor continue to run and burn up during the last engine run?

It sounds more like an intermittent short to the starter solenoid coil.
 
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Fearless Tower said:
But if the engine dies when selecting both mags off, how can the engine then restart by only turning on the master? That is what I am trying to understand.
Click to expand...

The engine didn't start. It only cranked. The starter got power. The mags were still grounded.
 
There was a recent thread where the pros and cons of disconnecting the battery before engine maintenance was tossed about.

This is yet another anecdote making the case for the pro side.
 
Checkout_my_Six said:
Hmmmm.....I'm not getting this failure mode.

If the solenoid were stuck or any of the above scenarios with the master switch in the on position....wouldn't the starter motor continue to run and burn up during the last engine run?

It sounds more like an intermittent short to the starter solenoid coil.
Click to expand...

The most common thing I've seen is a starter solenoid welded together from someone trying to start the plane with a dead battery (and not getting it started). A tap on the solenoid will usually free it.

I've had this happen to me a few times now. I am always pretty cautious when I flip the master switch on now. Especially when working on them in the shop.
 
One of my A&P's once installed the diode on the starter contactor (Cessna) backwards. He switched on the master and the prop began turning. If the diode is backwards or shorted internally that's going to happen.
 
I had that happen to me once on an aircraft checkout. As I was starting the 182rg, I noticed that after the engine fired and I released the ignition switch it didn't feel like the starter disengaged. I was right, and even as I went through the shutdown (mixture-idle/cutoff), the prop continued to turn, even with the mags off and the key pulled out. Yep, you had to turn the master off to stop the prop.

We quickly surmised that arcing in the starter relay had caused the starter to be permanently engaged as long as power was fed to the relay. A simple fix, but thankfully I noticed something didn't seem right before we would have eventually destroyed the starter.

For safety, we disconnected the battery and left a bold "danger" note in case any unsuspecting person tried to reconnect the battery and flip the master switch on.
 
The only other time I have seen an 'uncommanded' prop crank was during the careless use of metallic tools behind the panel, in the vicinity of the ignition/master switches - however this usually results in partial, incomplete cranking only....but surely enough to whack someone bad.
 
Let'sgoflying! said:
In my instance it was parked in the hangar and there was someone else in the hangar but not near the prop. Flipped the master on to...check some lights or something, and the prop started cranking vigorously. Key not even in the airplane. No previous problems with the ignition switch, solenoid, or any electrical component thereof.
When it happens, you immediately think; "Whaa? Did I forget how all this works?"
Click to expand...

In this case your starter solenoid is stuck. it would not be the ignition switch. If it was the switch, you would have discovered the problem during shut down.
 
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Wow. Something I've never really thought about. Can't count the number of times I've hopped in and hit the master to check a radio, light, etc, without even a *thought* of clearing the prop area. Learn something new everyday....
even if it's a million to one case, I'll glance around first from now on....
 
Another good reason, when stacking a hangar full of airplanes, not to place wings over propellers or tails in the prop arc of another airplane.

I remember one flight school that didn't have that policy. A student was preflighting in the hangar, and pulled a prop blade down (backwards, I hope) to get a look into the cowling. When he let go, the compression pushed the blade up and it axed into the wing above it.

Gets expensive.

Dan
 
Norman said:
One of my A&P's once installed the diode on the starter contactor (Cessna) backwards. He switched on the master and the prop began turning. If the diode is backwards or shorted internally that's going to happen.
Click to expand...

The only possible way for that to happen would be if the diode were completely installed wrong - from the hot main terminal (battery side) to the coil post which is where it is not supposed to be in the first place. When properly installed from the coil post to ground (to suppress kickback spike) there is no possibility of it energizing the relay as the only source for voltage is through the starter switch.

The diode is a pretty elementary electrical component but is probably among the most misunderstood devices in electronics. It seems that whenever anything goes wrong for an unknown reason the first thing to get blamed is a diode, even when it's not even a part of the circuit in question.
 
You are most likely right. It was about fifteen years ago, so the memories are dim. Whatever he did, the starter cranked when he turned on the master. He was white as a sheet.
 
Graueradler said:
The engine didn't start. It only cranked. The starter got power. The mags were still grounded.
Click to expand...

Wouldn't you still notice this during the shutdown process?

Pull mixture, kills the combustion. But the master (and the ignition, probably) is still on, so wouldn't the battery keep cranking while the pilot sits there dumbfounded wondering why the prop won't stop turning?
 
This is interesting and the second time I am hearing it. Back when I was taking my PPL check ride the examiner hit me with a question about run up and grounding the mags then he threw out this side question stating it was not part of the exam but asked something about if the engine would crank over without the ignition key in it.

After saying no, he told me a story about piloting a Baron for a charter company that started turning the right engine over when he switched the master on and upset the guy fueling the plane.

He shook his finger and said make sure to visually clear the prop areas before touching ANY switches in including the master cause you never know what is going to happen. :nono:

I had actually forgot about that until this thread, so thanks for the reminder!!!:rolleyes:
 
The story, as written, is a yarn or at the very least is missing a paragraph where they forgot to mention that they had just installed a new starter switch or relay and wired the darn thing up wrong or that Billy Bob's half inch SnapOn open end was missing and was laying across the starter solenoid posts. There is no feasible failure scenario that could cause an untouched and unpowered airplane to behave like this the next time the master switch is turned on having exhibited no anomalies prior to being shut down edit: (other than a seriously messed up switch.)
 
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Norman said:
You are most likely right. It was about fifteen years ago, so the memories are dim. Whatever he did, the starter cranked when he turned on the master. He was white as a sheet.
Click to expand...

His undies weren't! That would scare the hell out of me even if no one were in the hangar but me!
 
Norman said:
One of my A&P's once installed the diode on the starter contactor (Cessna) backwards. He switched on the master and the prop began turning. If the diode is backwards or shorted internally that's going to happen.
Click to expand...


Yep. That diode has scared/injured people.

There's also a reverse path when using the ground power plug on some Cessnas that can crank the engine by plugging into a ground power cart without the master even being on, if one of the diodes has failed. It's related to how Cessna chose to isolate the battery system completely from the ground power plug.

Since you're about a foot from the back of the prop when plugging those in, it's highly recommended they not be live when connecting. Back off and then turn on the juice from the cart.

Luckily perhaps, very few people ever feel the need to use that stupid ground power connection. And rightly so. If the battery is that dead, you need to remove it and charge it on the bench or replace it and fix whatever killed it. Plugging into ground power to get started is silly when you still have a totally dead battery perhaps not willing nor able to take a charge after that deep of a discharge.

I won't use the stupid plug. It's in a dangerous location and there's no point in starting the airplane from anything other than a known working battery. About all that plug is good for is powering up avionics to train something on the ground. Even then, you'd better have an avionics fan because there won't be any air coming through the blast tubes.
 
As I said earlier, if the diode is installed correctly in accordance with ACS SB92-01 there is absolutely no way a failed or reversed diode will cause the solenoid to engage and subsequently power the starter motor when the master switch is turned on. The only way a diode can cause this to happen is if it is installed completely wrong such as from hot side battery post to coil post anode to cathode direction.

THAT IS NOT WHERE IT'S SUPPOSED TO BE INSTALLED
 
Silvaire said:
As I said earlier, if the diode is installed correctly in accordance with ACS SB92-01 there is absolutely no way a failed or reversed diode will cause the solenoid to engage and subsequently power the starter motor when the master switch is turned on. The only way a diode can cause this to happen is if it is installed completely wrong such as from hot side battery post to coil post anode to cathode direction.

THAT IS NOT WHERE IT'S SUPPOSED TO BE INSTALLED
Click to expand...

If your aircraft is subject to this AD the best thing you can do is upgrade to a new switch.
 
C206p
1965, io520
I have that problem also, replaced master and starter solonoids, have the shunt and the shop still hasnt found the issue, its quite disturbing, thanks in advance
 
Bert A. Crane said:
C206p
1965, io520
I have that problem also, replaced master and starter solonoids, have the shunt and the shop still hasnt found the issue, its quite disturbing, thanks in advance
Click to expand...
Your airplane cranks as soon as the master turns on? Why would they replace the master solenoid? Time for some diagnostics with a meter and not just shotguning random, unrelated, parts.
 
Bert A. Crane said:
C206p
1965, io520
I have that problem also, replaced master and starter solonoids, have the shunt and the shop still hasnt found the issue, its quite disturbing, thanks in advance
Click to expand...
Ummm....what about the key switch? and or the starter solenoid coil power wire?
 
This couldn't happen on the old Continental starters with the T-handle cable actuated starter contactors. Don't need no stinking key starter.
 
Graueradler said:
The engine didn't start. It only cranked. The starter got power. The mags were still grounded.
Click to expand...
When and how did the starter contractor close with out power?
I do not see any possibility of this happening.
If it were maintenance induced it would have occurred during system checks upon completion of the job.
 
Stan Cooper said:
This couldn't happen on the old Continental starters with the T-handle cable actuated starter contactors. Don't need no stinking key starter.
Click to expand...
I was thinking "what difference would it make whether or not the key was in the switch?" Then I realized there are a few airplanes that use the key to engage the starter.

Been a while, I guess. ;)
 
MauleSkinner said:
I was thinking "what difference would it make whether or not the key was in the switch?" Then I realized there are a few airplanes that use the key to engage the starter.

Been a while, I guess. ;)
Click to expand...
It does have a key,
 
There are some starter contractors that are wired hot, and seeking a ground thru the starter button. If these were to become shorted to ground for some reason, The starter would spin when power was applied.
The one I know of is an after market starter conversion. not your normal P or C
 
Stan Cooper said:
This couldn't happen on the old Continental starters with the T-handle cable actuated starter contactors. Don't need no stinking key starter.
Click to expand...
But when they are converted to light weight after market starters and key start, you can configure them to be wired hot and seeking ground thru the start switch.
 
dukeblue219 said:
Wouldn't you still notice this during the shutdown process?

Pull mixture, kills the combustion. But the master (and the ignition, probably) is still on, so wouldn't the battery keep cranking while the pilot sits there dumbfounded wondering why the prop won't stop turning?
Click to expand...
You would think so. I taxied to the ramp and shut down (79 182), picked up passenger and hit the master and the prop started spinning. Stuck starter contactor. I assume I turned off the key just as I thought the prop was done spinning. Now I consciously wait before turning the switch off.

Jim
 
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