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Mag Failure

Old 27th Jul 2012, 06:38
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Mag Failure

I had my magnetos overhauled by a reputable organisation near MK approximately 2 years and 200 hours ago. I installed a new ignition harness at the same time. They are timed to fire at 20degs BTDC. Because I time them myself and check them at annual, they are within a hairs breadth and I normally see a 40rpm drop on both. I have an electronic tacho and rarely do a dead-cut as the tacho will tell me if either mag does not ground out during mag checks at the hold.

Yesterday I flew 5 eventless sorties and had three more planned. I arrived at the hold and went through my pre-takeoff checks. 1700 rpm Left mag - a standard 40 drop and commensurate engine note change. Right mag - engine cut.

Silly senile old Stik, I thought - could not have had the mags on BOTH initially on the key type starter. So turned the key one click further counterclockwise suspecting it would not go but it did and visually it showed mags as OFF, throttle back to idle and three clicks clockwise back to BOTH and she was running like a sewing machine.

I powered up to 1200 and repeated my earlier check - LEFT (the key grounds out the right mag and forces the engine to run on the left mag) a slight drop and minor engine note change. RIGHT (the key grounds out the left mag and forces the engine to run on the right mag) = total dead cut. I taxied back to dispersal and shut down.

I was slightly confused as in 30 years I have never had this! I've had faulty plugs, faulty leads, decaying condensers, etc.

My initial thoughts were, perhaps the P lead to the right mag has abraded and is permanently grounding out. After uncowling the engine it became clear what had occured. The Bendix mags are effectively three parts, the ignition harness screws into the casing. The casing is two parts, held together with 4 machine screws and these are sealed at overhaul.

The machine screws had vibrated loose and the casing had split by about 3/8". I can only assume that with the engine started (on the left impulse couple mag) the spark could bridge the gap and fire at the plugs, thus causing the audible/visible mag drop when I ground out the right mag but when I ground out the left mag, it had insufficient spark to keep it running.

Anyway the moral of the story is that perhaps had I given the top of the mag casing a shake when replenishing oil earlier in the week, I may have detected it as being loose and could have prevented yesterday's inconvenience. From now on I'll be giving them a shake - hopefully for the next 30 years!

I am heading back to the airfield where I left the aeroplane this evening with a spare zero-timed magneto, a tool box and my trusty 1942War Department mag timer in it's wooden case.


Stik
stiknruda is offline  
Old 27th Jul 2012, 10:56
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Yup,

I've had this a few times with various aircraft I've been sent to ferry back. It's amazing that for all the requirements for self locking fasteners around a plane, these screws are not locked at all in most cases. The odd time you'll see "witness paint" on them, though that fills in the slot, and makes them hard to remove for maintenance next time.

On one occasion, I had no choice but to tie together the mag case with plastic tiewraps. Not approved, but it worked fine.

Yes, preflighting the security of the mags, in the whole sense, is a good idea!
Pilot DAR is offline  
Old 27th Jul 2012, 11:08
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There have been mag failures where the Bendix mag came loose because the bolt holding its retaining bracket came loose.

As Pilot-DAR says, it isn't wirelocked, which is stupid, especially if it is one of these which, if it works loose, stops the engine totally.

I always check the retaining bracket, on a preflight. Fortunately it is visible via the prop TKS refill cover.

There are also some muppets working in the maintenance business... I overhaul my dual mag every 500hrs, by this firm. I have a spare one so just swap them over at the Annual.
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Old 27th Jul 2012, 11:13
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Is this not a problem of Bendix?
If I am correct, certain parts that must be replaced at overhaul are either not available or are difficult to obtain. It makes me wonder
I think a change to slick which can be purchased new would be better, if possible for your engine.
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Old 27th Jul 2012, 14:08
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There was a temporary issue with the Series 3000 single shaft dual mags (mainly) whereby TCM stopped making the mags and while most parts were available under PMA, not all of them were, which resulted in a temporary parts shortage for overhauls.

Under FAA Part 91, there is normally a 500hr inspection, not an overhaul, and that dealt with it in the meantime.

When I heard of the impending demise of the mag parts, I quickly bought a whole factory overhauled mag ($2.5k) so I have a spare on the shelf, which is damn useful.

Recently, during the parts shortage, the mag came up for an overhaul, and the key part (an impulse coupling spring) was listed at a certain well known UK EASA145 firm near Oxford. So I sent them the mag for an overhaul. Afterwards they refused to let me have a copy of the work pack... I complained to the CAA who were totally (suprisingly actively, I thought) disinterested. Obviously I wasn't going to fly with this suspect mag, so despite having just paid 800 for the OH I sent it to the USA to be re-done (by then the parts were available). It's back on the shelf now

I prefer to overhaul my mag at 500hrs because the casing is then NDTd, which is important because a cracked housing near the mounting bracket can cause it to come off the engine...
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Old 27th Jul 2012, 19:04
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There have been mag failures where the Bendix mag came loose because the bolt holding its retaining bracket came loose.
As Pilot-DAR says, it isn't wirelocked, which is stupid, especially if it is
one of these which, if it works loose, stops the engine totally.

They're not the same thing - you can't lock a screw if it's sitting in a recess or has a countersunk head. The magneto-to-accessory case nuts have locking washers to secure them; wirelocking is completely impracticable in most cases because of access. I actually installed a magneto today on an IO 540 and you can barely see the inner clamp nut, never mind wirelock it.

I'm puzzled as to why you weren't happy with the Form 1 that the overhaul facility supplied with your magneto, Peter337 (and I know which company it is; we use them all the time and they're excellent in all respects...) - the paperwork certifies that the overhaul/repair has been carried out iaw procedures and all SBs/ADs have been complied with.

Apropos of wirelocking, it's not always necessary on aircraft: look at your cylinder base nuts. Many hydraulic, fuel, oil and pneumatic hose couplings aren't locked either.
stevef is offline  
Old 27th Jul 2012, 19:17
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Ever heard of Loctite,or bathroom sealant...?
sycamore is offline  
Old 27th Jul 2012, 20:51
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the paperwork certifies that the overhaul/repair has been carried out iaw procedures and all SBs/ADs have been complied with.


(sorry couldn't resist, but will spare you my usual comments on certification regimes, starting with ISO9000)
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Old 29th Jul 2012, 07:30
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stik,

A very interesting thread about a problem I had never heard of. I shall pay much more attention to the mags during pre-flight now.

Could you post a picture, pointing out the screws in question?
India Four Two is offline  
Old 29th Jul 2012, 07:50
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I24

I'm really good at posting pics or vids of my formation team as the images are already on the 'net!

The bendix mag has 4 screws that hold the ignition harness into the casing. There are then 5 screws that hold both casing halves together (I didn't check but they appear to be about 1.5" 6-32UNC). It was these that all worked themselves loose. The right mag on the Pitts S2A is a bastard to see as it's obscured by the oil/water seperator, the VACpad oil adapter and the engine mount.

I normally bolt the bare engine onto the basket. Then install and time the mags before coupling up all of the controls, fuel, oil lines and electrics. Removing and replacing mags onto an engine that is all "hooked up" incites my Tourretes Syndrome as it is uber-fiddly!

Hope this helps.

Stik
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