February 2005
Thirty-two years ago my start in the engine business was as chief pilot test flying, delivering engines, airplanes and customers. Since that was in the sun-drenched wonderland of eastern Long Island at Mattituck, N.Y., I needed a rainy/snowy day job in the shop where I could fill my time between flights and storms. Assignment to the accessory shop was the answer and magnetos became my game.
In the thirty-some months’ tenure in that seat I believe I overhauled or repaired well in excess of 1,000 magnetos. Thankfully that predated much dealing with the dreaded “Siamese” or dual magneto, so most were Bendix S20 and S200 series as well as quite a few of the old 600 series Slick and a smattering of antique stuff like Eiseman, Case, Bendix SF and a couple of sets of S-600 low tension units.
Back in those days—with the exception of replacing the then “throw away” Slicks with new—the bulk of the magnetos were overhauled. Parts were relatively inexpensive, so it was economically practical.
The horrific parts price increases for all mag parts and the evolution of the newer series of Slick magnetos has changed all that. Today in the engine overhaul business, with the exception of dual magnetos, specific customer requests for Bendix and a few odd installations where Bendix are required, it is standard fare to replace both Bendix and runout Slicks with new Slick magnetos.
As often as not, the old Bendix 20 and 200 series mags are beyond economic repair and it is not practical to overhaul a current production Slick mag in complete compliance with the overhaul manual.
My view is that if I can provide customers with a brand new, relatively trouble-free product with few to no ongoing airworthiness directives, it’s a better deal than overhauling the old Bendix.
Maintenance
It does appear that mags are going without proper maintenance in the field. To their credit we see a lot of Slicks that appear to have run to TBO without ever being taken apart. Good as the mags are, there are maintenance procedures called out by the manufacturer at 100 and 600 hours. We don’t really pay much attention to condition, as they are headed for the crusher.
Bendix mags usually go the same way but we find that those sent for overhaul often don’t make the cut. This also goes for Bendix mags that are not timed out but are on engines which are in the shop for things like prop strike inspections.
Wear and Tear
I have seen an impulse cam that came off a mag on a repair job engine. The shiny area adjacent to the round rivets on either side indicates severe wear of the rivet or the impulse pawl itself. This the subject of an old AD (78-09-07), and I’d say that this coupling and the one on the other magneto would not have passed the required inspection for many hundreds and perhaps a thousand hours. Another magneto with flat sides on it was sentenced to the scrap heap by AD 73-07-04. That’s 31 years ago! Both mags on this engine were essentially junk.
Failure of the impulse coupling can lead to catastrophic failure of the engine if the steel parts fall into the wrong place when it self-destructs. You may feel the redundancy of two mags negates the necessity of serious maintenance but one mag failing can be pretty dicey at an inopportune time.
Ignition Harness
The ignition harness, while not as critical as the magnetos, should be the subject of a reasonable level of inspection and replacement when it starts to look like the one pictured. The same goes for spark plugs. Plugs should be properly cleaned, gapped and rotated at reasonable intervals.
In my opinion nothing makes a conforming ignition system (that is, one with nothing else wrong with it) run better than clean properly gapped (or new) spark plugs, a good ignition harness and properly maintained and timed magnetos. You may have two, but when one quits—assuming similar maintenance or lack of the other one—do you want to trust it to get you there?
A mechanical “shower of sparks”
As an aside to mags, let me mention what I believe to be a product that has not received the praise it is due. That is the Slick Start. The theoretical best magneto system for starting is the “shower of sparks” that is an outgrowth of the battery ignition systems of the 1930s that came into common use in the late 1950s or early 1960s.
For an engine to start without kicking back it is necessary to retard the timing delaying the spark until the piston is almost at the top of its travel. The tractor technology method of doing this is using an impulse coupling. It works however it provides one little spark per plug on each compression stroke.
The shower of sparks system uses a mechanical vibrator operated through a separate set of point to provide a “hot” shower of many sparks at the appropriate time. It is a good system until the vibrator goes south. Then even God can’t start the engine, particularly if it is fuel injected and hot.
The Slick start combines the impulse with a much more powerful shower of sparks generated electronically rather than mechanically. If it fails (which I have never heard of), the mag simply continues with the old mechanical impulse allowing the engine to start. I installed them along with Slick mags on my Twin Comanche over a thousand hours ago and have yet to miss a start.
That reminds me, it’s about time to do the 500-hour inspection on the mags again.
Charles Melot is an A&P who has been involved in the engine business since 1972. He personally overhauled about 900 magnetos from 1972 to 1974. Melot is president and founder of Zephyr Aircraft Engines, and is a 10,000-hour pilot with commercial, multi-engine, and seaplane ratings.
