Master ON preflight engine attempted to crank

woodchucker

woodchucker

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Hello all, new pilot here, on a timeshare program flying 172s. During pre-flight when I turned on the master switch the engine attempted to crank. The prop actually made a partial turn. Then a clicking sound followed. No electrical indicators popped up on the panel (fuel gauges, etc). This is a 1969 C172. I'm just curious what might be the cause. Is that the sign of an ungrounded P-lead? Or something else?
 
woodchucker said:
Hello all, new pilot here, on a timeshare program flying 172s. During pre-flight when I turned on the master switch the engine attempted to crank. The prop actually made a partial turn. Then a clicking sound followed. No electrical indicators popped up on the panel (fuel gauges, etc). This is a 1969 C172. I'm just curious what might be the cause. Is that the sign of an ungrounded P-lead? Or something else?
Click to expand...

First post...................

Welcome to POA...:cheers:..


And to answer your question ( Or something else)....

Yup... It is something else for sure/.....
 
Not the p leads, something electrical in the start circuit.

One time I was getting gas at a Podunk airport at night and an airplane on the ramp just started cranking. Nobody was inside and it wasnt tied down. I tried to get inside but the doors were locked. I scrambled to chock all the wheels and tie it down while it was cranking. Eventually either the starter burned up or the battery **** out. I left a note for the owner who was probably incredibly confused by my note. I suspect moisture.

Didn't last long, but felt like forever, creepy as hell to watch.
 
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Seems like it would have to be a failed starter solenoid, starter/ignition switch or something (an errant tool or bit of safety wire?) shorted the contacts.

Or, as usual, something else!
 
...and definitey something to be examined by a mechanic before you hit that master switch again. In fact, I think I'd disconnect the battery just to make sure.
 
Once, One of my A&P's reversed the diode on the starter contactor. When he turned on the master the prop began turning. Scared the hell out of him.
 
N801BH said:
First post...................

Welcome to POA...:cheers:..


And to answer your question ( Or something else)....

Yup... It is something else for sure/.....
Click to expand...

Hey, thank you! :cheers:
 
jesse said:
Not the p leads, something electrical in the start circuit.

One time I was getting gas at a Podunk airport at night and an airplane on the ramp just started cranking.

Didn't last long, but felt like forever, creepy as hell to watch.
Click to expand...

Owww, that would have been a strange sight to see!

Thanks for the replies all.
 
Would have been interesting if the plane started and took off. I can see the headlines now: plane stolen and found destroyed 2 miles from airport - no sign of thief's body....
 
Norman said:
Once, One of my A&P's reversed the diode on the starter contactor. When he turned on the master the prop began turning. Scared the hell out of him.
Click to expand...

I've seen that. It is scary. That OP's airplane might have a shorted diode. The diode is only present if the ignition switch is an ACS or Gerdes model. In 1969 the 172s used a Bendix switch but an AD against those switches often meant they got changed out to the ACS version, and the diode added as a result of another AD against the ACS switches. The diode shunts the voltage spike that the solenoid's coil generates when the starter is released; that spike burns the contacts in the ignition switch.

Dan
 
jesse said:
One time I was getting gas at a Podunk airport at night and an airplane on the ramp just started cranking. Nobody was inside and it wasnt tied down. I tried to get inside but the doors were locked. I scrambled to chock all the wheels and tie it down while it was cranking. Eventually either the starter burned up or the battery **** out. I left a note for the owner who was probably incredibly confused by my note. I suspect moisture.

Didn't last long, but felt like forever, creepy as hell to watch.
Click to expand...

That creeps me out just thinking about it.
 
Dan Thomas said:
I've seen that. It is scary. That OP's airplane might have a shorted diode. The diode is only present if the ignition switch is an ACS or Gerdes model. In 1969 the 172s used a Bendix switch but an AD against those switches often meant they got changed out to the ACS version, and the diode added as a result of another AD against the ACS switches. The diode shunts the voltage spike that the solenoid's coil generates when the starter is released; that spike burns the contacts in the ignition switch.

Dan
Click to expand...

Some planes have a pair of diodes connecting the coil to both sides of the relay contacts so the coil can be energized when the bus is powered and the battery is dead.
 
jesse said:
Not the p leads, something electrical in the start circuit.

One time I was getting gas at a Podunk airport at night and an airplane on the ramp just started cranking. Nobody was inside and it wasnt tied down. I tried to get inside but the doors were locked. I scrambled to chock all the wheels and tie it down while it was cranking. Eventually either the starter burned up or the battery **** out. I left a note for the owner who was probably incredibly confused by my note. I suspect moisture.

Didn't last long, but felt like forever, creepy as hell to watch.
Click to expand...

I always avoid the prop arc by a few feet when refueling and towing even if there's no pilot in the plane because of a fear of this happening. However remote of a chance that is.
 
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