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Pressurised magnetos - when required?

The distribution of fuel through the air is not uniform. A simple model says that to avoid misfire, the spark has to be big enough to reach an area of slightly rich mixture, which is probably further from the spark plug with leaner average mixture. The spark might exist regardless of cylinder pressure, but with weak mags it’s just not big enough to do the job.

The same model can explain why the development of lean misfire looks like a cliff relative to air to fuel ratio, while rich misfire looks like a gradual slope.

Last Edited by Silvaire at 18 Mar 15:30

The reason why LOP is supposedly a possible extra stress on magnetos is that the flashover voltage of a spark gap depends on the gas pressure.

The type of gas also affects it greatly, which is made use of in high voltage switchgear, but in an engine you get to choose that

However I don’t understand why the fuel % affects it because the mixture is around 14:1 air:fuel so going from no fuel at all to fully rich should change the gas pressure at the spark gap by only 7%. It would to be an extremely marginal system for leaning the fuel to expose the problem.

Administrator
Shoreham EGKA, United Kingdom

So, yes probably possible to fly high altitude with no misfires as stated by Peter_G, but not sure if that’s LoP or not.

I have always flown LoP and not had problems.
I do, though, have GAMIjectors: I don’t know whether because these create a very exact and equal mixture in each cylinder whether this helps.

Rochester, UK, United Kingdom

Long time since I’ve taken my plane (TR182) up to nosebleed altitudes, but never seen a problem due to its unpressurised mags. I took it to Colorado several years ago at FL200 without problems, and have often taken it up to the low teens.

LFMD, France

Sorry for pitching in late to the discussion. But adding a bit more nuanced picture here to the questions and other comments made. Having flown a TA IV for more than 10 years, I can attest that the problems with high alt (let’s say from 10/13 k feet) misfires started only when we started to lean for LoP operation. We installed pressurized mags, and the problem went away immediately. No prob to fly LoP at high altitude in the FLs after that.

So, yes probably possible to fly high altitude with no misfires as stated by Peter_G, but not sure if that’s LoP or not.
We did not have any particular issues with the pressurized mag set up. There was IIRC however an airfilter that needed change every 100 or 200h, but the additional cost of that in the maintenance was really ‘nothing’.

Hope this helps adding colour to the topic.

LSGL (currently) KMMU ESMS ESSB

Peter wrote:

Are they pressurised?

Yes, they are. Part.-No. 6361

Berlin, Germany

Are they pressurised?

Administrator
Shoreham EGKA, United Kingdom

My Cessna T182T ( service Ceiling 20000 ft) has Slick Magnetos. They must be checked every five hundred hous. I`ve never had any problems with Slick Magnetos.

Berlin, Germany

Our Mooney has pressurized mags. The TSIO360MB had it and it was replace by a TSIO360SB which has it as well. We have the mags IRANed every 500 hours, that’s all. No issue in eight years so far.

Our new mags were affected by an SB and we had to send them to a repair station last year after only 70 hours in service. I pulled and reinstalled them under the supervision of my A&P and it was quite an effort. I wouldn’t want to do it every 100 hours.

EDIT: I have to add that the mags are really hard to reach with our installation. Quite a few parts have to be removed to reach them. With other airframes and engines it’s much easier.

Last Edited by terbang at 01 Mar 09:32
EDFM (Mannheim), Germany

Presumably the mags used on an engine are those which the engine was originally certified with.

The Q is why.

It is suggested in that article that small mags are more prone to flashover (well, obviously) and thus the whole business of mags is really close to the margin, at whatever max altitude the engine is intended to be operated at.

I can’t believe pressurised mags need an inspection every 100hrs. That is really onerous and means every 2nd service is a big expensive one.

Which GA planes have pressurised mags?

Administrator
Shoreham EGKA, United Kingdom
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