Dual mag?

[jasong]

Hi all,

I am considering moving up to a J after my F was totalled in a windstorm last year.  I've often seen references to the "dual mag" of the J model here on the forums but I've never been able to find out exactly what that means or what the practical implications are.

Can someone break it down for me?

Jason

[Rustler]

Jason--

The "single mag" engines actually have two magnetos, but they are driven by a single, common shaft. Therefore, if that shaft breaks, you effectively have no magnetos. On the "J" models, the engine nomenclature is IO360A3B6D, where the "D" indicates the "single" mag. The Cessna Cardinal, among others, used a similar setup. There were problems with that mag configuration when it was introduced, but many miles have been flown safely since they ironed out the difficulties.

At some point, Mooney decided to use the IO360A3B6 engine (without the "D"), which has two discreet mags.

Any "J" model can be upgraded at overhaul to the dual mag engine if desired. I think Jimmy Garrison at All American indicates it adds about $4000 to the value of the aircraft to do the upgrade.

[KSMooniac]

I'll add that TCM has dropped support of the "D" magneto, so the future is uncertain at this point in terms of parts availability.  There are a couple of aftermarket replacements in work, but not ready for prime time yet.  Hopefully those will get to market before the spare parts are completely gone.  IMO, it would be a good idea to look for a plane without the dual mag, but I imagine the vast majority of J's are still flying with one.  The fleet is big enough that I bet we won't find ourselves grounded due to spares, but who knows for sure...

The conventional wisdom that I've gathered over my 4.5 years as an owner indicate that the -A3B6D engine (and mag) are very good, but the 500 hr maintenance intervals should be strictly adhered to with the mag.  I bought my plane with less than 10 hours on an overhauled mag, and just did a 500 hr service last summer and haven't had a lick of trouble with mine.  <knocking on wood>

[Piloto]

Dual mags have a very good safety record. Never heard of a broken shaft. You can look at this from another view point. You only have one oil pump drive shaft and only one cam shaft. Anything double has twice the probability of failure than that of single. A six cylinder engine has a 50% higher probability of a cylinder failure than that of a 4 cylinder engine. A twin engine plane has twice the probability of engine trouble than that of a single engine. This is why the Cessna Caravan twin Soloy conversion was not popular. It impaired the high dispatchability rate the plane had already with a single engine.

José

    

[jetdriven]

Thats a good explanation, Jose.  The dual mag has a couple things that make it less reliable. First is there is a lot of moving parts inside the dual mag, including one set of breaker points that if retightened, come loose and cause engine failure.  The other, is the hold down nut isnt saftied, which can allow the mag to loosen and fall out.

[Jeev]

Rustler - Did you need an approval from your local FSDO for the change?  I am coming up on overhaul and was rold by the engine shop it would not be more expensive to swtich.

[Parker_Woodruff]

Quote: Jeev

Rustler - Did you need an approval from your local FSDO for the change?  I am coming up on overhaul and was rold by the engine shop it would not be more expensive to swtich.

[Jeev]

Quote: Parker_Woodruff

Hey Jeev,

I think the switch is done at the factory, and was told it costs about $3000 more.  Did you decide to keep your M20J or go for a turbocharged Mooney?

[Parker_Woodruff]

Quote: Jeev

Hey Parker - Im going to keep her .  I just put 23 hrs on it last week flying all over the western US from coastal areas, flat hot deserts and high DA Mt areas and have come to the conclusion that the J meets my needs very well.  I just wanted something new and flashy...   I am not going to step up until I can go substantialy higher such as a TKS Ovation or even a Piper Meridian or Mirage.  I will be doing the MAG upgrade when it comes time and will prob put an Aspen and paint in it during the next two years.

[fantom]

Quote: Parker_Woodruff

[Piloto]

Quote: jetdriven

Thats a good explanation, Jose.  The dual mag has a couple things that make it less reliable. First is there is a lot of moving parts inside the dual mag, including one set of breaker points that if retightened, come loose and cause engine failure.  The other, is the hold down nut isnt saftied, which can allow the mag to loosen and fall out.

[Jeev]

Quote: Piloto

José

 

[Jeev]

Quote: Parker_Woodruff

http://www.controller.com/listingsdetail/aircraft-for-sale/MOONEY-M20K-252TSE/1981-MOONEY-M20K-252TSE/1208717.htm?

 

 

[Rustler]

Jeev--

No FSDO approval required. We installed the engine, which is approved for the M20J, and made the appropriate logbook entry.

The cost delta was zero. My overhaul was done by Western Skyways, and they happened to want an IO360A3B6D for an upcoming overhaul, so it was an even swap. I'm glad I did it and would do it again. Installing it was a non event, except for some swearing at the prop governor until we got the correct one.

Please let me emphasize that, from what I can determine, the "dual mag" system is proven and has thousands of trouble-free hours in the logbooks of multiple aircraft. I simply am more comfortable with actual redundancy, which I do not consider extant with the "dual mag." Piloto's statistical inference is correct, by the way, as far as it goes.

[Jeev]

Quote: Rustler

Jeev--

No FSDO approval required. We installed the engine, which is approved for the M20J, and made the appropriate logbook entry.

The cost delta was zero. My overhaul was done by Western Skyways, and they happened to want an IO360A3B6D for an upcoming overhaul, so it was an even swap. I'm glad I did it and would do it again. Installing it was a non event, except for some swearing at the prop governor until we got the correct one.

Please let me emphasize that, from what I can determine, the "dual mag" system is proven and has thousands of trouble-free hours in the logbooks of multiple aircraft. I simply am more comfortable with actual redundancy, which I do not consider extant with the "dual mag." Piloto's statistical inference is correct, by the way, as far as it goes.

[mooniac58]

I am installing a Lycoming factory reman in my M20J model next month.  I decided to go for the two single mags to replace my A3B6D engine with the single mag housing.  The "dual" mag label is confusing though and some people think this means it has two separate mags & housings and others think you mean the all-in-one housing with a single shaft.

I had to pay a $3800 fee for a "dissimilar core" from Lycoming - they would not budge on this at all - and we fought them forever. The really unnerving thing is the "D" core is considered more valuable (new engine price is more)...so there is no logic in this other than Lycoming sees an opportunity to gauge you and takes it.

I really want the extra redundancy of a second mag with all of its own parts.  I agree that in some ways this increases the odds of failure (like more cylinders or more engines) - however what happens when you have the failure is very different.  With the "D" engine your engine is going to quit - with the separate mags you will be running with 50% of your spark plugs out.  For this same reason I would like a second alternator - especially with an all-glass no-vacuum system ;-)

[Cruiser]

I looked at this option when I replaced my engine three years ago. Lycoming had the same charge then.

I decided to stay with the "D" version a single dual mag. The only way it can fail both mags is the common shaft (or if it is not mounted properly and comes loose from the engine). Check the NTSB database for the number of accidents related to the single dual mag ............ Nope, I couldn't find them either.

In the end I rationalized that the reason for spending the extra money was not valid and decided to spend it elsewhere. NOW, it seems these are no longer being produced and parts could become a problem..........

Doesn't your Aspen system have an external 30 minute battery backup in case the ships power fails?

 

[smccray]

When my engine was replaced in 2003 two owners ago, the dual mag was replaced with two separate mags.  The story I was told by the prior owner is that the owner responsible for the overhaul had an accident traced back to the dual mag.  I don't have any additional detail supporting the story.

[jetdriven]

There have been several accidents related to the single mag.  Most common is the mounting nut getting loose and the mag falling out of the accessory case.  But also there can be crossfiring inside the mag causing both sides to fail.  Impulse couple springs that break causing both sides of the mag to retard timing and lose power.  Cam retaining screw being retightened without the proper new nut installed causing complete mag failure within a few minutes.  Capacitor failure. The "single point failure" of this mag is not limited to the drive shaft, in fact the distributor gears are plastic.

here is some more info.

Statistically speaking I would guess it has a small deviation from the mean GA fleet accident rate as a whole but I can't find any accident reports due to a failure of a single mag in a conventional setup.

Anyways, the D4LN-2021 mag what we have and for 4K it costs to convert it, it's staying there. But this is the one thing on our airplane I don't completely trust and I hope the last shop that did the 500-hour was a good one.

If I could preflight only one thing, it would be the mag hold down nut.

[mooniac58]

Quote: Cruiser

.....

Doesn't your Aspen system have an external 30 minute battery backup in case the ships power fails?

 

 There is actually 2 hours (conservatively) of battery backup (required by the FAA to be my backups).  The PFD has a 30 minute battery built in (as all Aspen tubes do) and the MFD has a 90 minute battery blind mounted in the panel.  I actually had a regulator failure just after takeoff (cough, cough) VFR on a 1.5 hour flight and went to backup batteries - I got 55 minutes from the PFD and after 45 minutes my MFD showed 88% capacity on the large battery.  Talking to Peter Lyons the FAA requires them to test the batteries in the most extreme of temperatures which is why the estimates are so low.

However, despite all this a second alternator would still be added peace of mind in a 100% electrical ship.  I don't think I can do it anyways - but it is surely one of the appealing things in a 252.

 

[jetdriven]

Overall Craig, from seeing your 201 at Oshkosh you have a really, really nice machine.  Worse problems to have I suppose.

[John Pleisse]

Quote: jetdriven

There have been several accidents related to the single mag.  Most common is the mounting nut getting loose and the mag falling out of the accessory case.  But also there can be crossfiring inside the mag causing both sides to fail.  Impulse couple springs that break causing both sides of the mag to retard timing and lose power.  Cam retaining screw being retightened without the proper new nut installed causing complete mag failure within a few minutes.  Capacitor failure. The "single point failure" of this mag is not limited to the drive shaft, in fact the distributor gears are plastic.

[Cruiser]

Quote: mooniac58

 

[mooniac58]

Haha, no I think I will refrain from that upgrade.  However, my next upgrades will be KSN770 and Aspen Connected Panel (when all of these products are ready that is!) - and a Turbo Normalizer at some point!

[fantom]

Quote: mooniac58

I am installing a Lycoming factory reman in my M20J model next month.