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C152 - Propeller goes off on Master only (with keys out)
Those familiar with C152 / Lycomings / the way the ignition starters are wired:
C152's propeller turns when the master switch is on (both halves - battery + aternator) but the ignition keys are out of the ignition lock (i.e. in none of the R, L, BOTH positions or START :-). I would suspect faulty / burnt ignition switch is the culprit. Is there any other potential reason? Does this happen often (as none of the people observing the fault have seen similar thing and they do have some experience) What would happen if the engine caught up and the ignition was firing into the running engine? (i would expect there would be heaps of out of place ingintions making the engine run uneven, am I right, wrong, is there anything else?) Thank you guys! |
Starter jam?
Mostly ignition switches fail to make contact rather than develop shorts,which are more common in wiring faults.
The starter motor pinion could be jammed in the engaged position. .On starting the starter motor solenoid firstly pushes the starter pinion into engagement with the start ring gear and at the end of the travel of the pinion a switch inside the solenoid powers the starter motor. If the motor pinion jams in engagement the solenoid switch is permanently on and thus power to the motor is supplied as soon as made available by the master switch the motor will then turn the engine and prop. The ignition switch in L R and BOTH positions only enables the selected magnetos and has no effect on spark timing. However the above is pure conjecture and if it isnt the jammed pinion you need to make electrical tests to track down the fault. |
Does the engine start?
Or just turn over? |
Originally Posted by AndyBoo
(Post 10503794)
Those familiar with C152 / Lycomings / the way the ignition starters are wired:
C152's propeller turns when the master switch is on (both halves - battery + aternator) but the ignition keys are out of the ignition lock (i.e. in none of the R, L, BOTH positions or START :-). I would suspect faulty / burnt ignition switch is the culprit. Is there any other potential reason? Does this happen often (as none of the people observing the fault have seen similar thing and they do have some experience) What would happen if the engine caught up and the ignition was firing into the running engine? (i would expect there would be heaps of out of place ingintions making the engine run uneven, am I right, wrong, is there anything else?) Thank you guys! But I would highly doubt if it would fire for the same reason - without the mags on there is no source of spark. I imagine the problem can only be some kind of short with the starter circuit. |
Self ignition?
Was this one time event or it happens regularly? I am always turning off the engine with mixture lean. Apparently if you just turn off the magnets the gasses in the cylinder can self ignite (it happened before). Otherwise there might be a problem with electric circle switch witch is stuck in off position so the magnetos are not grounded. Hope this might help 😉 |
I would say the starter solenoid is jammed/sticking closed, battery and starter solenoids are right side of engine on the firewall just inboard of the battery. The ignition switch provides a small current to the starter solenoid that closes and allows power to the starter and cranks the engine. Rotation of the key from OFF to L to R to Both de-grounds the mags making them live and allows a spark to the plugs, next turn closes the solenoid and energises the starter motor that throws the bendix into the starter ring and turns the engine over, releasing the key it springs back to Both position isolating power to the starter which allows the rotating starter ring to throw the bendix out but continues to de-ground both mags, i.e it keeps them live. (Some countries have a starter warning light, UK as an example, to show there is current flow if the starter solenoid sticks in) What is happening is the starter solenoid is sticking every now and then as it is failing, when power is applied because the solenoid is closed, power is supplied to the starter and it cranks. This can be dangerous because if you start the engine and the solenoid sticks closed you will continuously throw the starter in with the engine turning it as well and this can cause a fire. Hence the UK has a light to warn you it is still connected. https://www.pprune.org/private-flyin...ing-light.html .. |
Thank you guys for your comments / suggestions! The plane got back from inspection, I will contact the owners and ask what was the culprit but I am now almost dead sure it has to be the solenoid stuck close.
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Yes, it is the starter solenoid (as opposed to the master solenoid) welded closed, it must be replaced. This is a hazardous situation but the engine will not run with the keys out of the key switch, unless there are other failures too. None the less, a good reason to perform routine shutdown mag checks (at idle power please), and never turn on the master when someone/something is in the prop arc. If the aircraft has been found with a welded starter contactor, the starter motor should be examined, and maybe replaced. It is possible that it continued running for the entire flight following the previous start. There may be pinion and ring gear wear and damage, overheating damage, and brush wear. It would be difficult to determine that such inspection is not necessary, and it's not burdensome, so it should just be done. This is a good reason for a starter engaged light, which is required in some locales.
In the case where you see the starter engaged light prior to takeoff, shut down and investigate. For a Lycoming, you could safely taxi back to the apron to shut down. For a Continental O-300 or larger, the engine should be shut down immediately, and the aircraft towed back. |
Thank you for coming back and giving us an update, a lot don't you know and it can be frustrating. Win Win really as you have learnt from it and can share the information with those of your colleagues who haven't seen it before..
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Originally Posted by Pilot DAR
(Post 10506355)
Yes, it is the starter solenoid (as opposed to the master solenoid) welded closed, it must be replaced.
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Because they stick first and can sometimes be reset by tapping them, eventually the weld themselves, but you want to have changed it long before that as it is dangerous. The barrel has a plate with three terminals on springs that rotate with the key making and breaking contacts on the rear plate, the chances of that internally shorting is more or less none existant.. True they wear and on Cessna's the plate and terminals are renewed every 2000 hours. The surge suppressor diode on the lower image was introduced to prevent arcing as the solenoid closed to reduce the chances of burning or welding the contacts as mentioned. http://www.ercoupe.net/images/sb583.jpg http://www.ercoupe.net/ads/76_07_12.html http://tangodelta.smugmug.com/Airpla...L-DIA-no-M.jpg |
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