Primarily for general aviation discussion, but other aviation topics are also welcome.
Much as it pains me to say, me old mate Northern Trent is right.

Both Bendix (TCM) and Slick (Champion) have Service Bulletins covering inspection and overhaul periods of their magnetos.

Service Bulletins are not legally mandatory. In the days of LAMP (still hanging on in there) one technically assesses continuing airworthiness data to justify not complying with it.

There is also the issue that if it appears in an aircraft manufacturer’s maintenance programme it is mandatory. 500 hour inspections are certainly called up for Cessna 152s for instance but for many years weren’t a requirement on the Piper PA28-161.

Of course, with SDMP/MIP etc coming in, an owner can choose to do what one wants (though not less than MIP).

Slick SB-80C



Slick: 500 hour inspections and overhaul/replace at engine TBO

Bendix: inspection (not overhaul) at 500 hours; overhaul after 4 years in service (or 5 years since manufacture or overhaul or at engine overhaul.

My personal experience is that magnetos don’t last any longer if 500 hour inspections are carried out. That said, it isn’t a bad idea to have a look inside and make sure all is as it should be.

Impulse couplings do wear and if an engine starts to get truculent to start then have the magneto’s impulse coupling(s) looked at.

In a previous life and before the 500 hour inspection was introduced into the PA28-161 maintenance manual, I had a long discussion with our CAA Surveyor as to whether they should be carried out or not. I could provide evidence of Slick mags lasting longer if you left them alone and asked for the CAA’s statistics to prove the safety case to have the inspections done. He quoted a Mooney that crashed on take off in Jersey after the dual mag failed. I pointed out that the aircraft took off with a reportedly rough running engine – the second flight after a 500 hour inspection which may not have been carried out correctly. Had the mag not had the inspection the pilot probably wouldn’t have died – not a great safety case for 500 hour checks.

Some companies like Nicholson-McLaren Aviation carry out 500 hour magneto inspections for a fixed price. Other magneto inspecting companies are available.
Last edited by aerofurb on Wed Feb 15, 2017 10:49 pm, edited 1 time in total.
"Had the mag not had the inspection the pilot probably wouldn’t have died – not a great safety case for 500 hour checks."

True but you can't say "not a great safety case for 500h check". You may say "A rubbish engineer" or "Unprofessional outfit" but you can't blame the call for 500h checks.
Ben wrote:"Had the mag not had the inspection the pilot probably wouldn’t have died – not a great safety case for 500 hour checks."

True but you can't say "not a great safety case for 500h check". You may say "A rubbish engineer" or "Unprofessional outfit" but you can't blame the call for 500h checks.

I was using the 'safety case' comment in conjunction with the CAA Surveyor's 'safety case' rather than a general comment for or against 500 magneto inspections.
dangerous pete wrote:Had our Bendix's done at 500hrs, there was oil starting to penetrate, and they would have failed before long, as it did to a friend of mine, total engine fail at 800ft on takeoff, that was oil ingress. (happy ending though). Every 500h for me.

Wouldn't an occasional in flight mag check detect that sort of problem? Unlikely that both mags would fail simultaneously.
The Slicks were/are known to go downhill fast around the 500h mark to the point that in some cases it is cheaper to buy new.

See earlier post! Lacquer tends to degrade with oil/grease/ damp.........Whilst it really, really gets my goat, when they use their monopoly position to force replacements at extortionate, monopoly prices, it is not unreasonable to mandate replacement of oil-seals at 500 hours. bearings should to UK with a clean/re-lube, but, at a sensible price, possibly cheaper to just replace. I don't know the construction of Aviation HT leads. Carbon (resistance -type) leads deteriorate, as do resistor spark-plugs...copper leads with modern silicone-rubber or plastic insulation, should last indefinitely.

Something often overlooked...strength of Field magnet! A hot environment can weaken them, but it is a quick, easy job for a specialist to re-magnetise them as simple and easy as putting your stuff through a scanner at the airport.

I bow to Aerofurb's expertise in this field but would argue that the internal timing of the mag (the magnetic "break" and the points"break") are far more critical than a worn Impulse-coupling altering the retard by a few degrees....otoh, a broken spring will probably allow the mag to revert to fully -retarded , which a pre-flight drop-test should show instantly.

Preventative 500-hourly "overhauls" as above, should dee a mag make as the SB's seem to hint, Engine manufacturer's TBO. there are plenty of marine and stationary engines that do thousands of hours on magneto ignition, with scant attention. (Forget ancient British motorbikes, if the oil didn't kill the mag, the vibration did! :twisted: )
My impulse coupling comment was not intended to be taken as the most important reason for carrying out mag inspections - it was merely an observation.

The 500 hour inspections of Bendix and Slick mags are inspections - they are not overhauls.
I bought an aircraft that was advertised as having had the mags recently overhauled.

My sharp-eyed inspector spotted that the Bakelite covers were broken - they'd been overtightened. The aircraft started well on the first occasion of the day, but would not start thereafter so we pulled the magnetos.

The first thing the reputable overhauler did was to put them on a test rack. After 30 minutes, one stopped working and after about an hour the other one packed it in as well and refused to work again until it had cooled down. Change of coils, capacitors. Some mags can have the magnets re-strengthened but mine didn't need it. Lubrication was applied. Bearings were renewed. After that they worked indefinitely.

It wasn't a small sum of money, but I think it was very well spent. And thanks again to my careful inspector who found a few things that might have spoiled my day!
Ben wrote:True but you can't say "not a great safety case for 500h check". You may say "A rubbish engineer" or "Unprofessional outfit" but you can't blame the call for 500h checks.

I can't remember where I heard the story now, I think it was at one of the safety days or something. There was something amiss about American aircraft in WWII which operated in the 8th Airforce. Someone noticed that many failures seemed to happen after maintenance. They plotted it all out and it made sense. The maintenance intervals were increased greatly, and the failures decreased proportionately.

If anyone knows the full story, please jump in.
ZLHGLP 's post, re-overhaul (and subsequent posts) really call into question the whole system of regulation and oversight.
I'm afraid I would have kicked-up a real stink at the "overhaulers"....if, indeed, that had been their remit.

the very fact the covers were overtightened to the point of breakage, points to a lack of mechanical empathy, if not a distinct lack of skill and aptitude. AIUI, the qualified person who signs off the work is ultimately responsible. Shoddy, shameful and a potential killer.
A friend used to "sub-contract" overhaul/rebuild starters and alternators. His work was superior to the "qualified" sources and far,far cheaper (he had a fully-equipped Auto-electrician business)
To my knowledge, no come-backs in 15 years. The man who put his name to the work, knew it was his own neck on the line and was happy to do -so.

A cautionary tale.....the big Mercedes V-8 had a very long timing-chain . customer neglected replacement, tensioner reached stop, chain flapped around on the "loose" run - (think push-bike, top run of chain =tight bottom-run slack) - chewed the teeth off the first cam-sprocket and eventually jumped, wrecking some, the cam sprocket was accessible, the crankshaft sprocket replace it would triple the job-cost. As he was selling the car, customer elected to leave the worn crank-sprocket in place

Some months later, the irate new owner phoned , threatening to sue for motorway tow-off, wrecked pistons and valves....blah blah...... I explained that i'd done a cheap job FOR THE PREVIOUS OWNER,and had no contract with him, but was happy to discuss what had been done ,with his repairer.

Moral...make sure the vendor will give a personal guarantee to any claimed repairs or overhauls, otherwise, as in ZLHGLP's case, you could get a big hole in your pocket with no redress.
Thank you, my learned Southern friend :thumleft:

We all know the solution for Permit aeroplanes.....

P-Mags !

Dual electronic ignition, third blade starts hot or cold, mapping available, robust auto based electronic ignition backed up by internal alternator to satisfy regs. No inspections, 2000 hour life.

Brad is close to certifying the 6 cylinder version.
cockney steve liked this
The seller won't tell me who did the original overhaul, which I suspect means it was him: I grant you I should have asked for more details prior to buying. Anyway, he's now running a children's charity so can't be all bad (TFIC).

I was impressed by the gentleman who repaired the mags for me: clearly knew what he was doing and had a nice little business serving the classic car/tractor/aircraft industry.
cockney steve liked this