Home | Magneto Breakage – Bendix ‘Dual Mag’ on Lycomings

Magneto Breakage – Bendix ‘Dual Mag’ on Lycomings

The FAA has just published SAIB NE-08-26, referring to the Bendix ‘Dual Magneto’ installed on some Lycoming engines. At a glance, it may appear that this SAIB does not apply to any BAC-owner planes. As is often the case, there is wisdom in much of the SAIB text for all of our planes. To my recollection, only our Duchess members have engines that use the D2000 ‘dual mag’. The announcement I received implies that the document only applies to IO360 installations, so our Duchess owners may overlook it (because they have O-360s, not IO-360s). Note that the dual mag refers to the unit containing two magnetos on a single drive; as opposed to the more common two separate magnetos (mounted on two separate drive pads).

It is certainly possible that the problems have first surfaced on IO360 installations, due to the ‘sharper’ vibratory signature. And on the D2000/D3000 magnetos, because of their much heavier weight compared to a standard ‘single’ mag. But the heads-up should put all of us on notice, considering the age of our aircraft.

One of the warnings contained in the SAIB refers to the use of the wrong gaskets under the magneto mounting flange. Some owners and technicians mistakenly use the gaskets with ears on each side. Those gaskets are for use between the magneto impulse coupling adapter and the accessory case. The magneto gasket is supposed to be round, with no ears or tabs on it. This is an age-old problem, with two side effects. The clamp cannot develop full clamping force, as it is being held off of the mounting boss by the ears on the gasket, and the gasket beneath the clamp compresses and erodes. The short-term result is often a mag that won’t hold its timing setting. Someone keeps tightening the clamp (a futile effort), and the result is a damaged clamp and flange. The end result, from vibration wear over time, is that both the clamp and the magneto mounting flange are ruined.

If you are fortunate, this is discovered during a mag inspection. If you aren’t, the mag can break off of the engine, when the clamping flange breaks. Even if no failure occurs before the problem is caught, excessive wear could force you into a new mag housing or mag replacement, at the next mag overhaul. So the sooner this is caught, the less costly it will be. Just replacing the gasket should be quite inexpensive; probably just one or two hours shop rate, to R&R the gaskets and re-time the mags (plus the gasket cost, which is low).

So some cautions for the wise:

– As soon as you can, use a bright light to examine your magneto hold-down clamps. Make sure that there is no sign of gasket material between the base of the clamp and the mounting pad. You only have to check the accessible side. The gaskets have two ears, or no ears at all.

– Each time your cowling is open, see whether you can detect any movement in your mag housing. There should be no movement at all, neither rocking nor rotating movement.

– While the heavier Bendix mags are at greatest risk, it is a good idea to check the mags for security, regardless of model or brand. In theory this gets done at every Annual Inspection; but the owner can also make this check whenever it is convenient. The earlier a loose mag gets caught, the cheaper the fix will be.

[Additional Comment by Mark Gooderum, BAC Technical Director]

There is an improved magneto clamp referenced in this SAIB, “3. Only use the new style magneto clamps, P/N 66M19285.” For what it’s worth the Lycoming part superceded by this new part number is the same part used in our aircraft with the S1200 mags, so the part supercedure applies to our aircraft as well, although they aren’t referenced by the SAID (or the AD out there on dual magneto Mooneys).

I personally suffered slippage (dicussed in a previous thread on this site) that also concurred with a worn flange on the left magneto impulse coupling housing. Whether the wear caused the slippage or was a result of it due to vibration was unclear.

So anyone having their service done I’d highly recommend using the new clamps, they are clearly an improved design over the old ones, two years ago a complete set of four set me back <$100 (they were about $20 each).

Thank you for adding to the resources available for your Fellow BAC Members.