The aviation press has all but planted a sloppy wet kiss on emerging full authority digital engine controls-FADECs-but the harsh truth is that this technology is barely beyond the breadboard stage. Well into the 21st century, what still makes airplane engines run is 19th-century magetos and every owner needs two for each engine.
When it comes to engine choices, there are essentially only two and thanks to the FAA certification straightjacket, the choice has already been made for the vast majority of owners: its either Continental or Lycoming. There are only two choices in magnetos, too, either Bendix from Teledyne Continental or Slick from Unison Industries. (Yeah, we know about Eiseman and Case mags, but you wont find them on modern airplanes.)
Fortunately, with only a few exceptions, most engines can use either magneto system but the two arent peas in the same pod in terms of cost or construction; there are differences. In our view, when the decision presents itself, the economics strongly favor Slick mags. But that doesnt mean Bendix is the wrong choice. In this article, well clarify what may appear to be, for us, an unusually waffling conclusion.
Spark is Spark
Well, yes, but some magnetos may make it more energetically than others. When we set out to compare mags, we sought feedback from owners with an open-ended query in our online sister publication, www.avweb.com, and from Aviation Consumer readers. We found owners are divided over which magneto type is better but we found a definite bias in favor of Slicks because theyre less expensive, have fewer ADs and, for mechanics, theyre easier to work on.
Some owners told us that in terms of starting ease, the Slicks were clearly better while others thought Bendix mags were superior. One thing owners and mechanics will agree on, however: when old, tired mags are replaced with new or overhauled units of either type, the engine will run better, lean more smoothly and start easier. Further, dont think it out of the question to either overhaul or replace mags proactively on the way to TBO. Not all engines will need that mid-time infusion of fresh spark but not all will make it without it, either.
Can we put some numbers on the spark energy? Yes, somewhat. One California shop that overhauls mags, Savage Magneto Service in Hayward, California, told us that after overhaul and under test, Bendix mags come in-that is, start making usable spark-at 50 to 70 RPM while Slick models may require 100 RPM or more. Thats a slight difference but perhaps explains why some owners insist that Bendix mags yield quicker starts. In our experience, we recently installed Slick mags and noticed dramatically improved starting and smoother running but, in fairness, the Slicks replaced 1100-hour, 10-year-old Bendix mags. Its an apples and oranges comparison.
As far as performance of these two systems is concerned in cruise flight, our view is that they are about equal if both are in the same state of maintenance. Our survey of owners and mechanics reveals no convincing data that one mag is better than another in igniting the fuel/air mixture.
Cost: Slicks Rule
In a line-by-line comparison of new magneto costs, Slicks are less expensive than Bendix, although they are likely to be more expensive to overhaul than a Bendix. Some two decades ago, Slick magnetos had a reputation for being throw away items and indeed, that was true.
Factory bulletins noted that 4000 and 4100 series Slicks, for instance, were sealed at the factory and contained no parts that could be overhauled or replaced. The only service recommendation was to check the timing every 200 hours and to replace the unit at engine overhaul or based on condition. Were sure some enterprising mechanics and shops cannibalized parts from one to fix another, but the approved procedure was replacement.
Slicks have since suffered a reputation for being unrepairable but this is simply not true. Parts are available for these magnetos and many shops and mechanics routinely fix them. But they dont necessarily overhaul them. This is an distinction with a difference; well get to it in a moment.
But first, consider outright purchase. In this case, Slick magnetos and their accompanying harnesses are cheaper to buy than the equivalent Bendix units, when factory new products are compared. For example, a new Bendix S4LN-21 to fit many Lycoming four-cylinder engines lists for $1849, plus a $150 to $180 core charge. This mag is sold by discount house such as Aircraft Spruce and Specialty for $1313. (Price varies substantially by specific part number, however.)
If Bendix is your wont, the better value option is clearly to buy remanufactured magnetos for less than a third the cost of new. Aircraft Spruce, for instance, sells remanufactured Bendix mags for about $390 and a rebuilt mag in this model directly from TCM sells for $690, plus core.
Remans arent available from Slick so the only valid comparison is new to new or new Slick to the Bendix reman. The equivalent Slick-the 4370 magneto-lists new for $681 and sells discounted for $467.70, a third the cost of a new discounted Bendix and less than a remanufactured Bendix direct from the factory. These price Deltas dont hold across the board with all engine applications-check your own parts numbers-but when comparing new magnetos, Slicks are less expensive.
This, of course, makes them a favorite of overhaul shops. These days, field shops work on tighter margins than ever and the lower price of Slick magnetos helps them remain competitive. Charlie Melot of Zephyr Engines tells us that new Bendix mags on an overhaul are a non-starter unless the customer will bear the cost premium. Its either overhauled Bendix units or new Slicks, both of which cost the same to the owner at overhaul.
If Bendix mags cost more than twice as much as Slick mags, are they twice as good? Our view is no, they arent, but not everyone we contacted agrees with that assessment. On the other hand, were hard pressed to detect any consensus strongly in favor of Bendix magnetos, from a value or serviceability perspective. They do, nonetheless, have their champions.
Savage Magnetos Al Marcucci views Bendix mags as the hands down winner, this from the perspective of a shop that deals with these gadgets every day. He believes the Bendix mag is fundamentally more robust, produces stronger spark-at least when new-and is imminently more overhaulable. Another accessory shop we checked with agrees with that view, as well it should since it overhauls only Bendix magnetos.
Slick magnetos are not economically overhaulable, the shop owner told us. By the time you pay for all of the parts, youre better off buying new mags. Unison obviously prices new magnetos and parts to encourage owners to go with new rather than repaired or overhauled units. Although Slick mags are fixable, an overhaul requires replacement of practically every part in the magneto, except the case halves and the rotating magnet. The parts arent cheap and are bundled in a way that tilts toward replacement of the entire component. For instance, you can buy a spring for a Bendix mag impulse coupler but you have to replace the entire coupler in a Slick.
In speaking to accessory overhaul shops, owners and mechanics, we found an interesting dichotomy between overhaul and repair. Accessory shops often dont overhaul Slicks for the above stated reasons, they thus lean toward Bendix mags as being the better product thats also easier to work on.
But mechanics who pop open magnetos to do inspections and repairs short of major overhauls seem to prefer the Slicks as being easier to access and being of a more modern design.
For instance, Bendix magnetos have old-style bicycle-type bearings that require manual lubrication while Slicks have sealed bearings that are easier to handle and install. Further, the rotor shaft in Bendix mags requires shimming to adjust end play-a procedure straight out a 1930s generator shop-while the Slicks use a simple spring washer.
Continuing ADs are a bugaboo of Bendix magnetos, which Charlie Melot jokingly refers to as the AD of the month. Bendix mags have a range of ADs that require inspection of various components ranging from impulse couplings to coils. The requirement for inspection at 500 hours is by factory service directive, not AD. Mechanics hate dealing with the Bendix AD chase, as much for the paperwork as anything else.
Bendix has used the same part number so the AD research provides a long list of things to research and provide an historical list to, says A&P/IA Ron Rich. As initial research, it becomes long and laborious, going through the log books that have not had good entries. It helps when the mag has been overhauled by a reputable shop that provides good records when the component is returned.
As routine maintenance goes, its a good idea to open up the mags and have a look at them every 500 hours, given that the curtain comes down if they dont perform as expected. A little routine maintenance at this juncture can save grief and cost later on.
Were wary of claims that Bendix mags are a slam dunk over Slicks or that Slicks are throwaway junk. Shops tell us that Slick magnetos can make engine TBO, if properly maintained, even if some dont.
Given the price disparity between the two types, we think the decision to go with one over the other calls for some careful analysis based on your situation. For an engine about to undergo overhaul, new Bendix mags generally arent an economic choice; remanufactured units are a better value. Thats why many field shops offer new Slick mags instead. Theyre simply more cost effective.
If theyre properly maintained, the Slicks should deliver reliable, long-term service. Even if they require mid-time repair or replacement, new Slick mags have a lower life cycle cost than do new Bendix mags. (Dont forget the AD compliance hassle with Bendix mags.)
Properly maintained for a Slick mag, according to one accessory shop we spoke to, may mean inspection, cleaning and adjustment at more frequent intervals than 500 hours. But if this preventive maintenance extends the life of the mag, the money is well spent.
If, as we recently did, you face replacement or overhaul of Bendix mags at mid-time, the economics may be less clear cut. Once again, you can buy new Slicks for the cost of the overhauled Bendix units, making a compelling argument for the Slicks and, probably, less downtime. In this case, overhauling the Bendix mags might not be the wrong choice but its hardly a compelling good deal, in our view.
Either way, we think the best advice we can give is to pay the money to have the mags inspected several times during a TBO run and fix what needs fixing. The mags will work better and are more likely to make TBO without mid-time replacement, especially the Slicks.
Also With This Article
-Danny Schultz assisted with the preparation of this article.