Definitely !
Van's Air Force
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Ignition Switch Fatality
- Thread starter DanH
- Start date
Definitely !
Okay, posts 48 and 50 are in direct contradiction. Now what, flip a coin? Post 48 does offer what seems like experience why not to use the shield to ground the mag. What are the pro’s for post 50?
Either will work. The point here is don't allow a p-lead to flop around suspended only from its terminal crimps.
As I mentioned, in my other post, this is poor practice in my opinion.
It relies on the integrity of the shield portion of the P lead to ensure that the mag is disabled when the switch is off
All of the repairs I have ever done on P leads has been because of a failed shield connection.
Yes, I agree there are ways to mitigate that from happening, such as a strain, relief clamp at the magneto and, but it still can happen, and I see no point in adding extra risk.
If the SOP is to test the continuity of the shield at shutdown by briefly gounding the mags and confirming the engine dies, then at least at time of shutdown (and assuming the leads--hidden behind a cowling or under a panel--aren't bumped or broken afterwards) the continuity should be good and the mags safe.
If the installer just connected one leg of the switch to the P-Lead and the other leg to the braid, then a failed braid connector at the mag would leave the mag hot.
Agree with you Dan -
All the p- lead connections I’ve done use the shield without any issues however the reason for this is I use heat shrink that is far superior than a lot ( my observation) of installers use - it is 4:1 glue lined clear heat shrink, I also use that on mic/phone jacks. They never fail !
The shield pigtail at the magneto end of the P lead.
They may have all been instances where no strain relief clamp was installed on that end of the P lead……. it’s been too long since I’ve dealt with one to be able to remember, but it doesn’t matter.
Since either installation method works, and there Hass to be a ground wire run to the vicinity of the ignition switch(s) to provide a ground for the starter, activation circuit, it makes more sense to me to just use that ground wire, and avoid the possibility of an open grounding circuit for the mags. Even if the chance of that is remote, if it’s all installed properly.
When I do pre-purchase inspections or certification inspections, it is rather rare that I see pleads that are locally restrained at the magneto.
That's the key point. It doesn't really matter which end provides the ground. I can be perfectly happy grounding the primary at the switch end. The issue isn't schematics. It is workmanship.
Installations like this...
or like this...
...are, in my opinion, both unairworthy.
This is the minimum, p-lead fully fixated so the wires are not flopping around while hanging on a crimp.
If exposed braid isn't desired, use a solder sleeve pigtail.
https://www.steinair.com/product/18-solder-sleeve-wpigtail/
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Idle cut-off and oddball Magnetos
There are a number of posts in this thread that triggered thoughts in me.
First - Idle cut-off. Yes we typically shutdown by shutting off the fuel supply in the carburettor. I’ve always understood this is done for two reasons;
1) Safety - Remove the fuel from the intake so there is nothing to burn. More on this in a sec
2) Create a lean condition where any possible hot-spots in a cylinder can’t ignite the remaining air/fuel after shutdown leading to the engine running on even when the magnetos/ignition are off (dieseling).
Now as for no.1 above. I grew up around vintage aircraft. When I was a kid I was thoroughly educated by my father about the danger of props. It was probably about the time that golfer walked into a prop, when I remember Dad demonstrating how you could never trust the engine not to start. He shutdown the Franklin engine on his Stinson with the idle cut-off. He then explained what that meant and why we did it. He then left the mixture at idle-cutoff, turned the Mags on and cranked the engine. It fired up and ran a few blades. - Certainly enough to drive home the point he was making.
Secondly; Another one of teachable moments came not long after I got my first aeroplane flying, A British built 1946 Auster J4 with an inverted inline Cirrus Minor, with no electric starter, and British BTH Magnetos. I’d checked, gapped the points and replaced the points cap and it’s P-lead. When I started the engine next, by hand-propping, while ‘sucking in’, (priming the cylinders after flooding the intake manifold for a cold start), The engine ‘popped’ or fired as it went through compression and the impulse let go. It didn’t run just popped enough to really get my attention. I checked the mag toggles were off, checked the wiring and earthing all were good and not faulty. I then went on to start the engine normally with the impulse Mag on (the right Mag on this aeroplane). I then double checked each mag shutdown the engine. No problems were obvious.
A little while later it dawned on me. The British BTH SG-4 mags earth the points through the mag cap with a carbon brush connected to the P-lead that rubs on the centre shaft of the points cam. The body of the points are earthed and the centre shaft is insulated and you earth the centre shaft via the mag cap and this carbon brush. A strangely complicated way of doing things, but typically British. In servicing the point gaps I’d either introduced a speck of dirt or moved the carbon brush enough that it didn’t properly disable one of the Mags.
Now before removing the plug points cap, I remove all the plugs and plug leads before touching anything on the Mags. You need to know an individual aircraft’s systems and design properly to know its failure modes.
And as far as the Auster carburettor goes, while it has a mixture control it doesn’t have an idle cut-off - Much like the Stromberg carburettors that are typically on Continental A-65’s and C-85’s etc. So my standard shutdown here is to idle, (500-600RPM) the engine for 30-60 seconds, which helps cool down the cylinder heads, turn both mags off and as the prop slows below an idle open the throttle wide to help create a lean condition where I don’t get dieseling.
Over the years of flying together Dad & I had plenty of learning experiences that involved failed mags, intermittent failures and broken P-leads etc. Any time I move a prop I expect the engine to fire. I never move the prop ‘just because’. I never lean on props and don’t allow anyone near me to do so either. If I have to move the prop for servicing etc. all plug leads are removed/disconnected and most times both set’s of plugs are out too unless the other plug has to be in the cylinder for differential compression tests.
And to keep this vaguely RV related, on our RV-6, I check each mag at 1000RPM by turning the [key]switch to R then L, confirm I get a RPM drop on each then to completely off and back on before pulling the mixture to idle cut-off.
I will be using the mag earth screw and an Adel cushion clamp to support the P-lead as DanH showed above. Such a brilliantly simple idea. Thank you DanH
Don H
1946 AusterJ4
1998 RV-6
There are a number of posts in this thread that triggered thoughts in me.
First - Idle cut-off. Yes we typically shutdown by shutting off the fuel supply in the carburettor. I’ve always understood this is done for two reasons;
1) Safety - Remove the fuel from the intake so there is nothing to burn. More on this in a sec
2) Create a lean condition where any possible hot-spots in a cylinder can’t ignite the remaining air/fuel after shutdown leading to the engine running on even when the magnetos/ignition are off (dieseling).
Now as for no.1 above. I grew up around vintage aircraft. When I was a kid I was thoroughly educated by my father about the danger of props. It was probably about the time that golfer walked into a prop, when I remember Dad demonstrating how you could never trust the engine not to start. He shutdown the Franklin engine on his Stinson with the idle cut-off. He then explained what that meant and why we did it. He then left the mixture at idle-cutoff, turned the Mags on and cranked the engine. It fired up and ran a few blades. - Certainly enough to drive home the point he was making.
Secondly; Another one of teachable moments came not long after I got my first aeroplane flying, A British built 1946 Auster J4 with an inverted inline Cirrus Minor, with no electric starter, and British BTH Magnetos. I’d checked, gapped the points and replaced the points cap and it’s P-lead. When I started the engine next, by hand-propping, while ‘sucking in’, (priming the cylinders after flooding the intake manifold for a cold start), The engine ‘popped’ or fired as it went through compression and the impulse let go. It didn’t run just popped enough to really get my attention. I checked the mag toggles were off, checked the wiring and earthing all were good and not faulty. I then went on to start the engine normally with the impulse Mag on (the right Mag on this aeroplane). I then double checked each mag shutdown the engine. No problems were obvious.
A little while later it dawned on me. The British BTH SG-4 mags earth the points through the mag cap with a carbon brush connected to the P-lead that rubs on the centre shaft of the points cam. The body of the points are earthed and the centre shaft is insulated and you earth the centre shaft via the mag cap and this carbon brush. A strangely complicated way of doing things, but typically British. In servicing the point gaps I’d either introduced a speck of dirt or moved the carbon brush enough that it didn’t properly disable one of the Mags.
Now before removing the plug points cap, I remove all the plugs and plug leads before touching anything on the Mags. You need to know an individual aircraft’s systems and design properly to know its failure modes.
And as far as the Auster carburettor goes, while it has a mixture control it doesn’t have an idle cut-off - Much like the Stromberg carburettors that are typically on Continental A-65’s and C-85’s etc. So my standard shutdown here is to idle, (500-600RPM) the engine for 30-60 seconds, which helps cool down the cylinder heads, turn both mags off and as the prop slows below an idle open the throttle wide to help create a lean condition where I don’t get dieseling.
Over the years of flying together Dad & I had plenty of learning experiences that involved failed mags, intermittent failures and broken P-leads etc. Any time I move a prop I expect the engine to fire. I never move the prop ‘just because’. I never lean on props and don’t allow anyone near me to do so either. If I have to move the prop for servicing etc. all plug leads are removed/disconnected and most times both set’s of plugs are out too unless the other plug has to be in the cylinder for differential compression tests.
And to keep this vaguely RV related, on our RV-6, I check each mag at 1000RPM by turning the [key]switch to R then L, confirm I get a RPM drop on each then to completely off and back on before pulling the mixture to idle cut-off.
I will be using the mag earth screw and an Adel cushion clamp to support the P-lead as DanH showed above. Such a brilliantly simple idea. Thank you DanH
Don H
1946 AusterJ4
1998 RV-6
Norman CYYJ
Well Known Member
Mags and guns are the same. Both are always loaded.
Switches, mags & props
Respect those propellers and their operational arcs at all times.
A pilot brought the 260 horsepower Lycoming back from the run up area for take off. He said it wasn't running right. Troubleshooting narrowed it down to Ignition and half the spark plugs were out and the Eiseman lead checker was attached to the suspect lead and button pressed and the engine fired.
The tester current went through the magneto through another lead and connected sparkplug. Fortunetly 3 people were out of the propeller arc.
Very somber day for us all.
Respect those propellers and their operational arcs at all times.
A pilot brought the 260 horsepower Lycoming back from the run up area for take off. He said it wasn't running right. Troubleshooting narrowed it down to Ignition and half the spark plugs were out and the Eiseman lead checker was attached to the suspect lead and button pressed and the engine fired.
The tester current went through the magneto through another lead and connected sparkplug. Fortunetly 3 people were out of the propeller arc.
Very somber day for us all.
Dr_Dave_MT
Well Known Member
Ha, that sounds like my old college roomate.
I would add, "all dogs bite" to the list as well...
