Engine Misfire at Altitude.

[basloane]

We have a 1974 T337G. In late 2011 we installed a Factory Remanufactured engine in the front position, and reinstalled the ( incumbent ) magneto pressurization kit.

First flight up to FL210, we experienced misfire "kicks" above about 17,000 feet.

We checked out both pressurization systems, confirmed that they allow air to flow and that they will hold pressure. On the next flight, we experienced the same issue above 16,000 feet.

Then,... one day I noticed that the pressurization systems were drawing air from a fitting located on the venturi throat of the cabin pressurization educator. We tested it with a pressure gauge and ( sure enough ) it would draw a vacuum when we ground ran the engine.

We relocated the pressure tap to an area where we were sure that we had high pressure air, and believed we had solved the problem. On the next flight however while climbing up to FL210, same problem while climbing through 16,000 feet.

We then attempted to identify a specific magneto. On the front engine, we experienced very rough operation at altitude on the right magneto. Other magnetos seemed fine. We dismantled the right magneto, found a bit of oil and pitting on the rotor electrodes but otherwise nothing significant. Cleaned it up and put it back together. On the next flight, while climbing through the 15,000 to 16,000 range, same problem ( kicks ) on the same ( front-right ) magneto.

At the annual inspections, all plugs are cleaned, re-gapped, tested and re-installed.

Same problem continues starting at 16,000 feet.

Has anybody experienced this problem? This one is driving us nuts...

[hharney]

From my IA:

I’d send the suspect magneto to Wayland Bruce at Magneto ER (easier said than done on the front engine of a T337).

A few years ago I had a similar issue with the rear mags on a P337G and Magneto ER was able to pinpoint the problem and fix it quickly and inexpensively. The mags have been running for a few hundred hours with no more issues.

If the magneto itself checks good then the pressurized air line is the next suspect. Is that magneto getting hit with oil residue from the pressurized air? Make sure there’s a good filter installed in-line (RAM Aircraft sells them).

A lot of shops have a high-tension ignition lead tester. I’d doubt that the ignition harness is a cause, but if a tester is available, it’s a very quick and easy test to rule out the possibility.

[basloane]

Thanks Herb.

Is the organization ( and contact information ) you are referring to?

Magneto ER Inc. 2505 South Main St. Searcy, AR 72143 Phone: 501-268-9950

Regards,...... Barry

[hharney]

Barry
Bingo

[JimC]

Is it possible to get pressurized magnetos on a non-pressurized turbo Skymaster? I took my T337E up to FL180 for the first time yesterday and had bad misfiring, so I went back to 16,000 where it ran well. I'd like to be able to use all the FLs I can, especially eastbound.

[hharney]

Originally Posted by JimC

Is it possible to get pressurized magnetos on a non-pressurized turbo Skymaster? I took my T337E up to FL180 for the first time yesterday and had bad misfiring, so I went back to 16,000 where it ran well. I'd like to be able to use all the FLs I can, especially eastbound.

Sure, it wouldn’t be too difficult. You’d have to tap into the upper deck pressure line…the pressurized Skymasters have a nipple welded to the steel pressure line that runs between cylinders 1 & 2, then a rubber hose comes from that nipple to a tee fitting, then rubber hoses going to the barb fitting on each mag. You’d also want to add an inline filter (1396-2 from RAM Aircraft) just before the tee fitting to make sure you’re not pumping crud into the mags.
You’d have to remove one plastic cap on each mag and add a barb fitting. Also add a gasket between the ignition harness & the mag. Whelen Bruce at Magneto ER is very familiar with this stuff and could hook someone up with the necessary parts if he was given the mag model #’s.
I’m not sure how to do this legally, but I don’t think the actual work would be that much.

Craig Griswold, Director of Maintenance
Griswold Aviation llc
1243 S Kalamazoo Ave Ste C
Marshall, MI 49068
(269) 213-6746 office
(269) 330-2908 cell

[B2C2]

I have a 1969 T337D, non pressurized and no pressurized mags. I have tempest fine wire plugs and GAMI injectors. I have also replaced all the ignition wiring and rebuilt the magnetos. The airplane flys nicely LOP with this setup. I occasionally have experienced bumps when operating at altitude above about 17K LOP. The highest I have flown it is 21K and it was the same experience, occasional ignition bumps, but not rough operation. I have found going to ROP stops this. You don't mention if you are running LOP or ROP but you might want to see how this effects the performance.

[JimC]

I'm still ROP. I haven't upgraded *anything* yet - I don't have an engine monitor, and I have the plugs, wires & mags that came with it when I bought it 4 months ago. It does run well LOP at 13K or so.

I ran a different continental TSIO up to 25K smoothly with pressurized mags, so I know that the pressurized mags make a big difference.

PS - How do you have a 1969 T337G? The model number & year don't match.

[B2C2]

Oops, sorry its a T337D. edited my below post to reflect this

[B2C2]

I was thinking about this and was wondering how Cessna gave a service ceiling of 30K feet for the T337D. With non pressurized magnetos this seems unattainable. Did they offer these as an option on the plane when they sold it? otherwise it seems like there is no way to achieve that altitude in operation.

[mshac]

My new-to-me '78 P337H has pressurized mags (PMs) that aren't original. The turbo outlet "plenum" was tapped and a hose was connected from there to the PMs, with an in-line filter. I found no mention of PMs in the log books (its possible I missed it). That said, I flew it home at 17.5K and the engines were smooth as glass.

Anyone wanting some photos of the system, shoot me a PM.

My understanding is the T337 had PMs, but the P337 did not, for whatever reason. T337 is certified to something like 30K, whereas P337 is limited to 20K. Did not having PMs cause the ceiling to be 10,000 lower for P-series?

If I flew my P337 above FL20, even a little, would the FAA come crash my door down? I understand the cabin altitude restrictions.

Right now I have a Uavionix ADS-B tail beacon that has 978 UAT, meaning a hard altitude limit of 18k. Thought about selling it and upgrading the GTX330 to GTX330ES, which would give me 1090ES ADS-B with no altitude limitations. Not sure its worth it, even at low/no cost, just to gain 2k in possible altitude. And its not really even 2k more, because you won't be flying westerly headings at FL20 very often, so FL19 eastbound is likely the highest you'll go. Hope that made sense!

If I knew I could go to say, FL22-24 occasionally, it would be far more tempting to upgrade the ADS-B out.

[basloane]

I would not be too concerned with the FAA swat team crashing your door down for flying above FL200 ( unless they are dressed in bunny rabbit suits ).
I have known of P337s that have flown as high as FL240 for a short periods, and I have done a couple of long flights at FL210 and FL220.
I have been told that the skymaster is probably good for about FL260, but have not tried it. I am guessing that performance begins to drop off significantly at those levels.
The biggest risk is the cabin altitude... When I was a 30-40s something person in reasonable physical condition, non smoker with no health issues I could regularly fly my non-pressurized T-Arrow daytime at 15,000 with no ill effects. When I reached my early 50s, I tried a flight at 15,000 in a Bonanza and ended up with a pounding headache.
In any case, best to have Oxygen standing by......
As for ADS-B, we installed a GTX345. For most flights, 978 UAT is probably sufficient, but one day you are going to want to fly over weather at FL200......
Does anybody know what happens with 978 UAT above FL180....?

[mshac]

Well I found out I do have an STC for the pressurized mags - ACD Corp STC SE4303NM 4/25/1988 if anyone is interested.

Thanks basloane for your info on flying above FL200. Seems logical that with pressurized mags, the P337 should be good for FL240 at least. I would carry a pony bottle with me in case of cabin pressure loss. The original owner pulled the factory chemical emergency O2 system. I wouldn't mind having it back in service if they still sell the 02 charges.

Pressurized A/C with a ceiling above FL250 require the pilot to have a high altitude endorsement, I wonder if that played into Cessna's thinking.

If I can get up and play in the low 20's, I may have to upgrade my ADS-B.

And that begs the question - What happens when you fly a 978 UAT aircraft above FL180??? Do your wings immediately turn into fireballs? Do controllers screens start flashing red with your tail number??? Curious minds want to know!

[JimC]

Originally Posted by mshac

Pressurized A/C with a ceiling above FL250 require the pilot to have a high altitude endorsement, I wonder if that played into Cessna's thinking.

ALL aircraft with a ceiling above FL250 require the endorsement. The FARs define a "pressurized aircraft" as an aircraft with a ceiling higher than 25,000 ft. That means all T337s, period.

I'm still getting the misfiring...haven't fixed it yet. Kim, do you still have all that pressurize mag hardware?

[mshac]

Originally Posted by JimC

ALL aircraft with a ceiling above FL250 require the endorsement. The FARs define a "pressurized aircraft" as an aircraft with a ceiling higher than 25,000 ft. That means all T337s, period.

I'm still getting the misfiring...haven't fixed it yet. Kim, do you still have all that pressurize mag hardware?

Nope, that's wrong. From the FAA's own Advisory Circular: "As required by 14 CFR § 61.31, pilots who fly at altitudes at or above FL250 in a pressurized aircraft must receive training in the critical factors relating to safe flight operations under those circumstances."

Link: https://www.faa.gov/documentLibrary/...AC_61-107B.pdf