Engine Misfire at Altitude.
 

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...

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.

Engine Misfire at Altitude
 

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

Barry
Bingo

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

Are there any other T337 owners that are regularly flying at FL180 or higher? How do your mags do? What did you do to get them to run smoothly?

we do
I make sure the plug gaps are good and the mags are in good shape and timed correctly.
we use to have some issue with occasional misfire but we started using tempest spark plugs and it not much of an issue anymore
as far as pressurized mags I think ram aircraft can fix you up with what you need. I went through the TCM parts manual and ordered all the hard lines and fittings for pressurized mags but never needed to put them on
Kim

A few years ago, I purchased a complete Mag Pressurization kit from Jeff Shapiro at TurboPlus. I believe they still exist in Washington.
If you are experiencing misfiring, might want to check the resistance on your spark plugs. If the resistances are much greater than 1,000 ohms ( >50,000 ) they start misfiring in the outside harness connection at high altitude.
I was experiencing misfiring above 15,000, and spent 4 years and upwards of $3,000 chasing, cleaning, testing and changing pressurization configurations until ( by chance ) we found an article about the problems ( about 6 years ago ) folks were having with AC plugs. We tested all the plugs and most of the resistances were off the scale. We could even see arcing outside the plug.
Sympathetic to my experience, the local rep very kindly provided a new complimentary set of plugs which we installed in the front engine. As an experiment, we switched out the rear plugs for Tempests and then filed for FL220. Both engines purred for about a year, then I started getting "misfires". Found a plug on the FE testing over 1,000,000 ohms ( yes, it was an AC ).
I would be sure that AC has addressed the problem by now, but I would like to see some more hard data......( just the skeptic in me ).....
Or,.... only use the AC plugs in non-pressurized/non-turbocharged aircraft......

This thread does not apply to my naturally aspirated C337G and I do not claim to understand the dynamics that impact mags at high altitude. I can however second the previous post on spark plugs. I finally made the switch 2 months ago from Champion plugs to Tempest plugs and the improvement has been remarkable. For 18 years always purchasing Champion plugs, (for no other reason than habit) I finally made the change. Poplar Grove recently overhauling my front engine, convinced me to do it.

I have always been battling fouled plugs and I have read many articles on the resistance issues in Champion plugs that apparently Tempest plugs do not suffer. After 30 plus hours now of operating with the Tempest plugs, my freshly overhauled front engine, and my 32 year old SFRM and 300+ hour past TBO rear engine have been purring perfectly with not one single fouled plug or plug misfire at anytime. I find the improvement amazing.

Good luck with the resolution to your issue.

Ed

Plugs
 

Ed - thanks for that info on plugs...I have always been a "Champion" guy myself given experience on other aircraft types (and have stayed away from AC plugs). I will give Tempest consideration going forward!
Jeff

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.:D

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! :o

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

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....?

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!:rolleyes:

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

I'm not saying this makes sense - it's a poorly written regulation - but it's clear. The "problem" is the FAR's definition of "pressurized aircraft" - it's any aircraft that can fly over 25,000 ft. So according to 61.31(g) a T337 is a pressurized aircraft, and a P337 is not. There are many recently developed aircraft that you and I would call pressurized that have maximum operating altitudes of 25,000 ft, even though the engine and airframe can go higher. 61.31(g) does not apply to them.

The regulations make much more sense if you replace the word "pressurized" with "high altitude" all through 61.31(g) - but they're written with the word "pressurized."

The FAR is pretty straightforward and clear in the way it defines "pressurized aircraft":

61.31(g)(1): "no person may act as pilot in command of a pressurized aircraft (an aircraft that has a service ceiling or maximum operating altitude, whichever is lower, above 25,000 feet MSL), unless that person has received and logged ground training from an authorized instructor and obtained an endorsement in the person's logbook"

A T337 has a service ceiling over 25,000 ft.
Therefore it is a "pressurized aircraft" according to the FARs. (Hey, I didn't say they made sense.)
Therefore all of 61.31(g) applies.
61.31(g) *never* talks about how high you are actually flying. It only talks about whether or not you're acting PIC.

BTW, the AC doesn't say that you can skip the training if you're operating below 25,000 ft. If the aircraft can go that high, you need the training.

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.

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.

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

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.

Try the fine wire plugs

Dan
67S

I have them. I actually have good performance up to about 19K but I have never taken it up any higher than that, Based on what i have been reading 30K feet without pressurized magnetos seems a stretch, but maybe that's not the case?

The Piper Aerostar’s that I used to fly did fine with the fine wire plugs to 25K. We did eventually put the pressurized option on. Now this was with the 1200 series Magneto’s, which in my opinion are a better mag than the 20 series or the 6300 series Slicks.

Dan

There is no enforcement for UAT above18k
 

In PCA above 18k ft, the world is all 090Mhz. radar, tcas, etc

That said, the heavies up there all have TCAS and can see your basic Mode AC transponder just fine.

As can FAA if within radar coverage as well.

Just remember, your 978 mhz UAT will not see any 1090 transponders directly.

You will onl see 1090 targets when both your UAT and their 1090 are observed by and within range of an ADS B ground station.

if you have dual mode 1090/978 ads B in, you will see any 1090 ads-b out aircraft, 978/1090, but not any basic 1090. mode a/c.

D

That's good info about UAT above 18k, but if you're that high anyway, you'll be IFR and under normal traffic separation rules, so I wouldn't be too concerned about not seeing 1090 equipped traffic. Many of us use a portable 1090/978 in unit for weather and traffic to display on our tablets, so at least there's that. I'm not familiar with "basic 1090" - I guess that would be a 1090 equipped aircraft without ADS-B? I wouldn't imagine there would be many of those around.

Fun fact - Class A airspace used to be called "Positive Control" airspace (IIRC), and it started at 24k, not 18k. So for the longest time you could fly VFR right up to 23.5k.

Basic 1090
 

That’s right

Basic mode a/c a mode A/c transponder without ads b out.

Mode S can be selectively interrogated by radar.
Rather than just interrogating everything in sight.
But even mode S doesn’t have ads b out.

Only mode S extended squitter (ES) replaces w ads vector info.

What I’m hearing is once you get away from major metropolitan airspace,
a lot of smaller aircraft not equipping w ads b

If they don’t go into the airspace that requires it,
they don’t have to get it.

It’s a mixed blessing. Great where everyone has it,
but no guarantee one is seeing everyone else.

My old Bonanza had some sort of a "Transponder sniffer" traffic device that was probably from the 1980's - whenever it sensed a transponder nearby a red light flashed and there was a warning tone. It had HI-LOW sensitivity, you kept it on HI, then if you got an alert switched it to LOW. If you got another alert, you knew the other plane was closing on you. It sure did keep your head on swivel when it alerted - Good times...