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Pressurised magnetos - when required?

The TB20 and the TB21 use the same D3000 magneto which is unpressurised, despite the 25000ft ceiling of the TB21.

Yet other turbo pistons, which all have a ~25000ft ceiling, use pressurised mags.

Is it that the D3000 has better insulation than the single mags (from both Bendix or Slick) or is there some other factor?

Pressurisation is AFAIK done only to make the air inside withstand a higher electric stress before it flashes over.

Administrator
Shoreham EGKA, United Kingdom

Piper Turbo Arrow with the stock 6-cyl Conti TSIO-360 as well as early Mooney 231 with virtually the same engine often have ignition spark arcing problems at altitude. The 231 got pressurized mags as one of the pre-252 refinements to overcome the problem, and I read that some Turbo Arrow owners have done the same to their engines.
(The Mooney 252, called “the fixed 231” by some, is certified a 20K type like the 231, but with numerous modifications mostly to the engine.)
For the Turbo Arrow, I believe arcing problems are reported from about 13.000 ft and up, without mag pressurization.

Last Edited by huv at 27 Feb 21:39
huv
EKRK, Denmark

huv wrote:

Piper Turbo Arrow with the stock 6-cyl Conti TSIO-360 . . . often have ignition spark arcing problems at altitude.

Although I have known pilots who regularly fly over The Rockies fitting pressurized mags, or a larger magneto, it is not all gain as this article in Avweb explains. The regular servicing of pressurized magnetos (every 100 hours), and/or the contamination that can occur because of ‘pressure’ make some choose a larger magneto, or not change at all.
My usual cruising level in Europe, in my Turbo Arrow IV, is between 10,000’ – 14,000’ and I have never had any problems.
Last year, I flew for 5 hours at 17,000’ over The Alps without a single misfire on ‘ordinary’ magnetos..
I would have thought that for most flying in Europe (What our American friends would call ‘lowland flying’) pressurized mags are not necessary and, as mentioned above, could cause more problems than they cure.
I would, though, appreciate, other’s comments.

Rochester, UK, United Kingdom

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

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

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

Are they pressurised?

Administrator
Shoreham EGKA, United Kingdom

Peter wrote:

Are they pressurised?

Yes, they are. Part.-No. 6361

Berlin, Germany

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

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.

KPAO, United States
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