Dr. Bill Heybrook:
The Bendix mag (magneto) impulse coupling AD is coming due. It was replaced 490 hours ago, and it’s a 500-hour AD. Anyone know of a replacement mag (or part) that will terminate this recurring AD? Are the Slick mags any better (or worse) than the Bendix mags?
Technical Editor:
I had the same question some time ago, but decided that living with the Bendix mag AD is the best way to go. There is an excellent article in the August 2003 Light Plane Maintenance (check your library?) that
addresses this very question. What it boils down to is that the Bendix mags on this era of IO360’s/O360’s are some of the best mags ever made. In my 1977 Sierra, the left mag is an S4LN-1227 (impulse), and the right mag is an S4LN-1209; I suspect that yours are the same ones. They provide very good performance compared to most other models and series of magnetos, including starting and at altitude. They are very repairable by a decent field shop with the right tooling, as opposed to requiring frequent major overhauls or outright replacement from larger magneto shops. For example,
Northeast Aircraft Maintenance at Craig Field here in Jacksonville, FL does a good job of 500-hour servicing on these mags.
You can pay quite a bit of money for a Slick conversion, but then you’ll be faced with the likelihood of reduced spark performance and more frequent repair and replacement. The later Slick mags are far
better than the earlier units, but are still reported as having more wrong with them when opened up at 500 hours. Here is a direct quote from the LPM article: “Support shops responsible for general aviation
aircraft in several different countries have developed maintenance programs that reveal and confirm that when Slick magnetos accumulate 500 hours, they are found to develop a variety of failure points that
need immediate maintenance or they will face catastrophic failure”. There is also the option of the Unison (Slick) LASAR conversion, which is even much more expensive than just the Slick magneto
conversion. However, the LASAR is still based on the newer Slick mags, which will still require similar servicing intervals. I haven’t yet seen much in the way of performance and reliability feedback for the LASAR system; relatively few seem to have been sold.
To be fair, the newer Slicks are better, but a good friend with them on a Mooney still has a lot of trouble with his. My Bendix mags have never let me down, with only minor maintenance (points, condenser, and maybe a small bearing or seal) at the 500 hour intervals. LPM’s key point is that at the 500 hour point the magnetos should go though a thorough bench inspection and minor repair, regardless of whether they are subject to the impulse coupling AD or not (and regardless of brand). I had been looking forward to the smaller physical size of the Slicks, as things are so tight behind the engine, but I decided that the reliability and repairability of the 1200 series Bendix units made them “keepers”.