Preflight Inspections - look harder!

[Guest Maj Millard]

That's correct Biggles, a wider than normal gap can cause the plug to miss and ultimately would cause it not to fire at all. On the subject of uc bolts...they are highly stressed bolts in their normal use, and many aircraft over the years have suffered regular failures requiring periodic bolt replacement as part of normal servicing. The Partenavias were particularly subsceptical to bolt failures and maybe some Tecnams also.

 

Undercarriage bolts lead a rough life and for sure need to be part of every per-flight and occasionally a more through check over or replacement..................Maj..

 

 

[Jabiru7252]

A wide gap will place more load on the electrical system i.e a greater electrical potential so it would be possible in theory to have a rough running engine while under load.. As electricity will take the path of least resistance and may very well try to track across a bit of carbon around the plug. Thats the way I understand it anyway.

Well I would dearly love somebody (who has the knowledge) to explain how a wide gap can load the electrical system. I must be losing it in my old age....

 

 

[frank marriott]

Ross

 

Are you sending from your new iPad or still on your home computer

 

 

[Jabiru7252]

RossAre you sending from your new iPad or still on your home computer

Home computer. Why, what nefarious activities have you planned? I don't have a new iPad, perhaps you have me confused with somebody else?

 

 

[frank marriott]

Home computer. Why, what nefarious activities have you planned? I don't have a new iPad, perhaps you have me confused with somebody else?

Sorry , the problem with user names, I had a conversation with Ross (maj Millard) this morning and it was a toung in cheek comment to him. I should have used his 'site' name to avoid confusion.

 

 

[Guest Maj Millard]

Yes Frank ...breaking in the new mini IPad....or is it a case of it breaking me in ....????????..................Maj...

 

 

[Guest Maj Millard]

Jab7252, Any ignition system is designed to operate at a certain amperage range, it requires more amperage to 'push' a spark across a larger plug gap than the design requires. This can make the ignition system work harder and create more heat than it can handle eventually damaging a component. This is one reason an engine manufacturer specifies a certain plug gap. To match the designed output capability of the ignition system without overloading or over heating it....................Maj.....

 

 

[motzartmerv]

Thanx maj. Yea I get how plug gaping works, but I struggle to believe it could suddenly cause a partial engine failure. A drop in rpm yea, even slightly rough running. But an efato? Hhmmmmm

 

 

[Guest Maj Millard]

Well to really understand that Motz you would have to get inside the computerised sealed ignition boxes themselves, and frankly that's a bit outside of my area of expertise!.......I do know however from personally testing this on a spark plug testing machine, that if you keep opening up a plugs' gap, It will eventually stop firing.....Maj...

 

 

[motzartmerv]

Yea of course. I presume eventually the gap would be too great for the arc to leap across, similar to an air break insulator. But I'm thinkin that's why we have 2 plugs per pot. Is there any reason a plug gap would suddenly spontaneously open up to a gap that prevented it firing?

 

 

[Hongie]

As the plug goes through numerous hot/cold (or maybe less hot) cycles every second, its not inconceivable that the gap could open up or close up due to impurities in the metallurgy, and from high voltage jumping across the gap, erosion will occur.

 

 

[turboplanner]

Yea of course. I presume eventually the gap would be too great for the arc to leap across, similar to an air break insulator. But I'm thinkin that's why we have 2 plugs per pot. Is there any reason a plug gap would suddenly spontaneously open up to a gap that prevented it firing?

As well as having to be two plugs at once.

 

 

[Guest Maj Millard]

Well the plugs should be changed well before it occurres through normal wear. One example of where miss firing may eventually occur was where recently I found a set of installed plugs gapped at .032 instead of the recommended .024-5 thou. They were gapped and installed new at the larger gap. So with say a hundred hours of use they may well get to the point where they could misfire especially at the higher rpms..........Maj....

 

 

[Guest Maj Millard]

As well as having to be two plugs at once.

Even with only one plug of the two firing you would still get a drop in rpms and performance. Just as

you get a drop in rpms when doing a mag check...the combustion burn efficiency is different with one instead of two firing. The combustion is more complete when two plugs are firing vs only one............M aj......

 

 

[Jabiru7252]

Jab7252, Any ignition system is designed to operate at a certain amperage range, it requires more amperage to 'push' a spark across a larger plug gap than the design requires. This can make the ignition system work harder and create more heat than it can handle eventually damaging a component. This is one reason an engine manufacturer specifies a certain plug gap. To match the designed output capability of the ignition system without overloading or over heating it....................Maj.....

Where did you get that information from? All those lessons I did on circuit analysis/AC theory and resonant circuits and I never heard that. If you want to 'push' a spark across a wider gap you need a higher voltage, but making a gap wider is not going to make the voltage or current go up, it simply means the sparks won't fly. The ONLY possible explanation is some fancy computerised system that can somehow detect the plug is not firing properly and ups the ergs accordingly, but I doubt that very much on our relatively simple recreational aircraft.

 

 

[alf jessup]

Andy

 

Yea of course. I presume eventually the gap would be too great for the arc to leap across, similar to an air break insulator. But I'm thinkin that's why we have 2 plugs per pot. Is there any reason a plug gap would suddenly spontaneously open up to a gap that prevented it firing?

Andy,

 

You must been the unluckiest bloke flying when you had your Jabs, wrong plug gap, now that seems like a furphy from the factory, it wouldn't be the plugs were a higher quality built than the donk itself that caused the failure???

 

Alf

 

 

[Thirsty]

And if it only had 40 something hours on it that means the gaps were wrong from the factory presumably?