Continental Engine AD - Big Troubles?
 

Hey again all, just been reading about Cirrus grounding their entire fleet until further notice due to an upcoming Continental AD. The part that got me was:

'Continental Aerospace Technologies is working with the FAA on possibly developing an Airworthiness Directive (AD) on aircraft powered by late models of several of its engine models. These include 360-, 470-, 520- and 550-series engines. Continental is urging owners of planes with any of these engines with a manufacturing date between June 1, 2021, and February 7, 2023, to voluntarily ground their planes “until further information is available.'

This is an enormous call, particularly as it will ground most aircraft with an engine less than 200 hours old. Keen to see what you all think.

Cheers, Mach1

Not the entire fleet. As you quoted, just aircraft with engines manufactured between June 1, 2021 and February 7, 2023 and it is currently a "voluntary" grounding.

Currently dates between June 1, 2021 to February 7, 2023 and with less than 200hrs on the engine. Also applies to OH engines with a new Crankshaft between said dates. The issue is centered around Crankshaft counterweight pins and the circlips that hold them.

looks like a two day mx input to pull a few cylinders and inspect.

Brought on by three in service engine failures (not Cirrus airframes). Cirrus were the first manufacturer to break cover and start to ground (or pause, as they called it) their own fleet that fell between the initial dates. The AD will probably give five hours grace to ferry with “essential crew” crew only to a mx shop for checks and rectification.

probably those caught up in this already know everything mentioned above. Took a while to tease the actual issue out of Continental.

You might want to read this AD.

https://www.federalregister.gov/docu...with-a-certain

https://www.regulations.gov/docket/F...-0027/document

I noticed that, but it refers to a much shorter period of manufacture - only valves produced for a few months of 2022, so it's unlikely (though not impossible) that those could be flying on aircraft built in 2021.

FYI: this is not the AD in question for the OP issue. There is no AD currently in the works but an SB in the works which will contain S/Ns. TCM came out with a notice yesterday. Word is supposedly the retaining rings/circlips for the crankshaft dampener bushings/pins were orientated opposite of the installation requirements. If you want to see the area in question look in the old SB 99-3 which addressed a different issue. Given they have it narrowed down to less than 200 hrs TIS engines they probably have a good idea of what is the cause. Regardless, my guess there more to this than a reversed circlip as most have no specific orientation requirements.

The circlip seems to have a "sharp" and a "non-sharp" edge, where the sharp edge needs to be positioned "outwards", according SB 99-3. Mount it the other way around, and it'll probably get some slack, in the long run loosening up the balance weight mounting, in the end, destroying the engine, once one of the balance weights departs its mounting.

So, it could be, one specific mechanic did install the circlips consequently the wrong way around, and it seems to be known from experience, that the engine destroys itself within 200 TIS, when the circlip(s) is/are mounted wrongly.

Retrieve the engine serials this mechanic worked on, follow the delivery trail, and it is known which engines need to be opened up and potentially will need a crankshaft rebuild/revision. A complication can be, that crankshafts might be prebuilt/assembled, leaving the challenge, which crankshaft ended up in which engine. An issue is to reliable reach all aircraft owners/users with affected engines.

It does and is a result the stamping manufacture process. Common AN/NAS steel washers have the same feature. Supposedly the weight pin retainer plate is stamped also. One theory is the sharp side of the clip and the sharp side of the retainer plate may cause interference and prevent complete seating of the clip. However the clip ear dimension check should have caught that, if it was checked.
The groove dimension is spec to the clip thickness. Either way there should be no play unless not seated. Plus theres a note that the retainer plate should not have any play at the pin. Perhaps when the SB is released it will provide more on the reason.

While they might possibly exist, I've never encountered a circlip that came with instructions regarding which way round it should face.

Presumably if they do, they would have to feature some unambiguous marking on one face, or have some other asymmetry so that the correct orientation could be guaranteed.

Sounds a tad unlikely to me.

The circlip won't have those instructions as they are considered a standard part. Its the aircraft maintenance instructions that will dictate that orientation as it does for this weight pin installation as shown below. Most clip installs are not direction specific in my experience. Even standard washers may have an install requirement by the OEM whether the smooth or sharp side faces one specific direction. There's also no mark on the clip as one can tell by feel or sight which side has a sharp edge or round edge quite easily.

https://cimg7.ibsrv.net/gimg/pprune....3af90dcae5.jpg

Is it just me or does that diagram make it look like the rounded side is outboard?

Looks like, though, I think, that "rounded thing" visible is part of the balance weight "behind" the plain of the diagram. To give it a kind of 3D appearance.

We thought the same. But if that was the case then it should have been black below the "diagonal" line. However, it could have been mistaken by the person installing the clips?? The larger picture associated with this close up doesn't show that line. The full diagram is in the SB 99-3 link above.

Yes, maybe, this is a special spec circlip. I could even imagine, the "rounded" side is extra rounded to match (or to avoid damaging) a rounded edge on the inner-inside of the groove. A rounded inner-side edge to avoid a sharp edge and a potential fatigue starting point.

The important aspect in this is, the manufacturer documentation explicitly mentions the way the circlip needs to be mounted.

I think, the clip ear dimension check/spec is there to have an extra check, whether the circlip is mounted correctly. Though, a precise mechanic might have used "force" to enforce the proper clear ear size.
When you have mating mechanical parts like the groove/circlip, these either have some play, or are tightly connected. And, when there is (a little, microns) play and a lot of force, eventually, the play will get bigger.

The no-play remark for the retainer pin suggests, the mounting of the circlip creates maybe a somewhat spring-loaded construction, to have the whole in the normal situation "without play". Another argument, to have the sharp edge of the circlip on the outside and being forced somewhat into the groove, together with the clip ear spec to check on this. Temperature effects to be seen, though.

The SB/AD would be an interesting one.

These clips are standard and cost about $0.70 USD each. Nothing special about them. The ring grooves and retainer plates are checked as part of overhaul with the bushings changed as well. Usually one buys a ring kit for OH and gets new circlips for this application.

I’ve been rebuilding engines (mainly air cooled motorcycle types) since the early 1970s and have always been aware that the side of a circlip with the sharp edge should be the thrust bearing side. They don’t come with instructions! A good example is those holding gudgeon (wrist) pins in pistons. The last thing you need is a loose clip and the end of the pin scraping the bore. The rounded edge of the circlip can ride up the (square edged) groove and pop out the same way it was incorrectly put in. Wire circlips are different in that they have rounded grooves.

Probably, though the mechanic might have applied "general" knowledge (nothing special) and now, we do have panic, including ac/incidents, around these engines, focusing on the circlips.
For this engine, it got explicitly specified. Probably not to nag the mechanics, but because of genuine reasons, which could be the reasons you specify or maybe additional ones. I've seen so many detailed instructions, thinking "why the hell do like that", though sometimes finding out later, there were genuine reasons to do so. So, for me, manufacturer instructions are the leading instructions.

For decades mechanics knew how to install circlips the right way. And a FAA approved SB drawing from 1999 shows it in the wrong direction.

Just a pilot, fix a lot of things around the house. Having a clip that fits both ways, no obvious markings or instructions what the correct way is, and the engine blows up within 200 hours if you do it wrong sounds like an unacceptable risk.

Maybe it’s just that some less experienced engineers/mechanics these days (possibly a very small number) don’t have certain basic “nouse” about what’s right and what’s not.