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How do piston engines fail

Another great article from Mike Busch

He makes some points which are perhaps less than well known.

One is that crankshafts last a huge amount of time or hours – 14k hours or 50 years is quoted. I know people will point out that both Lyco and Conti have had QA issues on cranks, but if a crank makes 200hrs it is probably OK. And indeed I have never heard of any evidence that Lyco’s 12 year crank life limit was based on any engineering data; it seems to have been a purely legal liability limiting maneuver.

Another is that the 500hr inspection or overhaul on mags does reduce the failure rate, and mags do fail quite often. I overhaul mine at 500hrs – I have a spare (single shaft dual mag) on the shelf, at a cost of $2500, so one can be away overhauled (I would not use any company in Europe) while I fly on the other. So people making use of the 500hr inspection being theoretically optional (on US reg) are not doing the right thing.

Other things do fail but less often and when they do it is rarely a total loss of power.

A lot of issues are caused by bad workmanship e.g. under- or over-tightening of bolts.

Shoreham EGKA, United Kingdom

“The top-end components—pistons, cylinders, valves, etc.—are considerably less robust”

But that’s not such a bad thing, and shouldn’t be improved, because “it works”.

A good article indeed.

Does any significant number of the N-reg. operators really totally skip 500h magneto inspections? I doubt it.

Mainz (EDFZ), Germany

Probably as many as skip ADs – loads and loads and loads, N-reg, G-reg, D-reg, you name it. It depends on the diligence of those involved. I get peripherally involved in TB pre-buys (usually I refer people to an A&P/IA/EASA66 guy I work with on mine) and don’t recall a single one that was 100% right.

Shoreham EGKA, United Kingdom

Does any significant number of the N-reg. operators really totally skip 500h magneto inspections?

About 50% would be my estimate. Mags are possibly the least reliable part of the engine because of marginal lubrication, but there are two of them and I think owners rely too much on that.

Some owners prefer to run Slicks to 700 hrs or some other number and then toss ’em (for replacement by new mags). Bendix mags make more sense to overhaul indefinitely.

Re ADs, my experience is that the ones most often found in non-compliance are those where complexity and multiple revisions make it hard to figure out what is required here and now,

Last Edited by Silvaire at 25 Apr 15:10

Not everybody has two entirely separate mags

I am sure you are right about AD compliance – because one needs to subscribe to various databases to find out about these, and these cost $$$.

The FAA has/had a mailing service whereby they send ADs to owners but, from vague memory, something changed on that a few years ago and anyway most owners are not involved in maintenance so they will not know what to do with these emails (assuming their spam filters let them through). AFAIK most engineers subscribe to AD update services, but if you work on multiple types you may not want to pay for it. Most freelance engineers I know get their AD feed from the company where they work during the day

In theory anybody doing maintenance should have the current data but in practice this is widely disregarded (the ATP subscription is $1000/year per aircraft type) and provided the chap is familiar with the type, etc, this is not a safety issue. Also I think a subscription to Commercial Data Service X cannot be legally enforced, so long as you demonstrate that the data you have has not been superseded.

ADs and mandatory-lifed parts noncompliance are a legal issue but for as long as the same person is doing the work, nobody is going to check

Last Edited by Peter at 25 Apr 15:13
Shoreham EGKA, United Kingdom

I’d be careful with that mag

When I bought both my planes, I physically inspected all component serial numbers, then the IA and I searched the FAA database and made a AD compliance spreadsheet to be clipped into the logbooks. That makes the AD check paperwork task relatively easy each year because mainly we’re just looking for new ones.


Last Edited by Silvaire at 25 Apr 15:24

Some owners prefer to run Slicks to 700 hrs or some other number and then toss ’em (for replacement by new mags). Bendix mags make more sense to overhaul indefinitely.

FWIW the advice of Mike Busch on this is that mags should NOT automatically be overhauled, but Inspected and Repaired as Necessary (IRAN).

Here is a link to his webinar:

Last Edited by Jonzarno at 25 Apr 15:47

I have asked people in the business about that.

An inspection is cheaper than an overhaul, but with an overhaul you get NDT of the casing which is probably important because cracks in the casing, near the point where the mag is secured to the engine, are not unheard of.

If such a crack propagates far enough, on a single drive dual mag, the engine will stop.

With single mags, he is probably right on the tradeoff between infant mortality of newly fitted parts (in the overhaul) and getting the NDT done.

Shoreham EGKA, United Kingdom

I still find it remarkable that dual mags were able to meet certification requirements. It is a perversion of redundancy. I have one, too and I very much dislike it. The statistics don’t show increased in fight engine shutdowns for these aircraft though.

My aircraft has a dual mag because a drive pad was required for the turbocharger oil suction pump.

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