Engine Trouble

[Nev]

We attended a Bearhawk fly-away last week, and while returning home over water between the islands experienced a loss of engine power. We made a precautionary landing and discovered 3 cylinder studs had sheared off above the #4 cylinder. Several of the remaining ones were loose.

My reason for mentioning it here is that with the benefit of hindsight there were some early warning signs that I had missed, and that knowledge might benefit others. It's not a Bearhawk specific issue, but rather an engine issue. Mine is an IO540 that was rebuilt specifically for my Bearhawk.

It now appears that the sequence of events was that the studs probably sheared some time earlier, resulting in a small oil leak. We subsequently also found the left exhaust cracked through in several places.

The final straw was actually the spark plugs failing on the affected cylinder with accompanying rough running, power loss, CHT & EGT indications decreased abruptly as the precursor to the precautionary landing.

Several studs were also found to be loose on other cylinders, so we will be checking all of them and torquing to Lycoming specs.

Next time I'll be paying more attention to any oil leaks no matter how small, and also casting an eye over those cylinder nuts which are visible.

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90 minutes earlier in the flight, unaware of the issue.

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Flight track from Gisborne across Hawkes Bay and Cook Strait. We landed at Kaikoura on the east coast of the South Island.

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[Battson]

Sorry to hear about this Nev. Glad everyone is OK.

Certainly, it would appear that under-torqued nuts on those studs was the problem. It explains everything you've seen, neatly.

[rkennell]

Glad you had a safe landing Nev and everyone is safe. I appreciate you sharing the pictures and your experience. Thanks for being a vigilant safety ambassador. Hope the repairs go well and you are back in the air soon.

[Richard E]

Nev. A failing engine at 820ft over Cook Strait is not a happy scenario. Pleased to hear that all ended well.

Just off to the hangar to check my studs and exhaust.

[whee]

Glad you’re safe Nev!

Is that some sort of black sealant on the cylinder base? Can’t see it clearly but it looks like remnants of black rtv type sealant on the edge of the base. Also looks like it’s squishing out of the neighboring cylinder. Hope I’m wrong, that’s a serious no no and would necessitate removal of all cylinders to clean that stuff off and install the cylinders properly.

edit for clarification: If that is some sort of sealant on the cylinder bases then you know the cause of your loose and broken cylinder studs. There should be nothing between the cylinder base flange and the case. Sealant or even paint will eventually cause the studs to loosen and fail.

[Utah-Jay]

Wow! Firstly glad you’re safe!

Thanks for the heads up on something else to check

[Bissetg]

The seriousness of this can not be overstated, the good outcome was a function of Lady Luck smiling at the right time and some good decision making.

A real heads up for the rest of us to pay attention to any oil leaks and preflight check those studs that can be seen.

I think Whee raises a really good point too.

Looking forward to seeing you back in the air Nev.

[svyolo]

Glad you and your family are safe. And thanks for sharing.

[AKKen07]

Dang Amigo, nice airmanship in a stressful moment.

[Mark Dickens]

Ho many hours does this engine have on it? As I recall, this is a recently overhauled "Bob" engine. You should not be having these issues at this point, I'd think. Have you talked to Bob about this?

[Redneckmech]

Glad to hear everyone is safe! Thank you for sharing your experience as it is a good reminder for all of us to look beyond the small things we may find during pre-flight and ensure that there is not a bigger issue.

[bkyser]

Were any of the nuts on the through-bolts loose? If so, that can lead to bigger issues in the bottom end (case fretting, spun main bearing(s), etc).

[Nev]

Thanks for your kind words everyone.

Whee - the engineers noticed a contaminant on the cylinder flanges too. The #4 & #5 cylinders have been taken to South Air in Dunedin for inspection and crack testing. They will decide if they can go back on the engine.

Mark - the engine has 361 Tachometer and 458 Hobbs. I spoke to Bob yesterday and he was very helpful. Bob said they use a torque of 60 ftlbs on the large studs and 30 ftlbs on the smaller ones. He felt the Lycoming specc'd torque of 50/25 ftlbs was not quite enough.

bkyser - some of the remaining nuts were loose, but the studs/ through bolts were all tight. We had to remove the #5 cylinder to extract one of the through bolts. We found it tight and secure, but it requires replacement due to the #4 (left side) end being sheared.

The oil filter is being removed and checked today, and the remaining cylinders are being Boroscoped.

[Battson]

A clarification here. Studs and through-bolts are different things and are installed in different parts of the engine. I think we're at risk of confusing the two, for some readers' benefit. If I understand the above posts, the concern is case fretting / bearing issues, which comes from through-bolts which hold the case halves together.

[bkyser]

Battson. Yes, some of the cylinder hold down nuts are on studs and some of them are on through bolts. The through bolts provide the required clamping force for the main bearings. If an engine is operated without the proper torque on one or more through bolts, bearings can shift (spin) and the cases halves may fret.

[Nev]

One of the thru bolts was sheared off, on the side that it is threaded into the block. So we had to remove the opposite cylinder to extract that through bolt. They've opened the oil filter and fortunately found it clean.

[Zzz]

Bob said they use a torque of 60 ftlbs on the large studs and 30 ftlbs on the smaller ones. He felt the Lycoming specc'd torque of 50/25 ftlbs was not quite enough.

This seems like something you shouldn't second-guess engineers on. Torquing applies a calculated "stretch" or elastic deformation to a fastener, and this "stretch" has a profile associated with it that I can only think is to maximize retention. I know Bob is a mechanical engineer and I'd like to hear his justification of that change.

Maybe time for some torque seal?

[Bissetg]

I think this highlights a point discussed on another thread.

An experimental engine is exactly that, it’s not a Lycoming that is overhauled by an uncertified organisation, although that is the misunderstanding that a lot of folks, me included, have had.

When I had my experimental engine issues my mindset was wrong. I expected an overhauled Lycoming engine that included Lycoming recommended practices with respect to RTV use, Torque values, parts compatibility etc. It was an eye opener to me that that wasn’t the case.

I’m not saying that it’s wrong to do those things on an experimental engine, it’s just wrong to think that an experimental engine will or must be assembled to a certified standard.

My understanding is (and I’m not suggesting that this is the case here) but if for example RTV is used where it’s not usually recommended, an increased Torque on the bolts might be seen as an appropriate action without full exploration of the unintended consequences. Whereas in certified world that would have to be a rigorously tested procedure that was shown to work consistently before being adopted.

Nev’s experience should be a wake up call for all of us.

I think we all need to be vigilant and monitor our engines for weeps, leaks or changes.