CF-ABS (or so claimed)
[1] https://www.gov.uk/aaib-reports/aaib-investigation-to-cozy-m...
Material was CF-ABS
The whole point of 3D printing is that the material is moldable when hot but rigid when it cools. And people really should be aware that engines get hot.
Like gunshows, it’s a magnet for bad ideas.
Also it's insane that they used a bolted joint with plastics on a critical place, the plastic will creep under the clamp load and will lose clamp force.
Well, no, it's in the UK. It also has a gross weight of around 2000lbs, so it's probably not subject to any of the relaxed regulations anywhere, although I don't know how the UK homebuilt rules work these days.
Just because a part has the shape of an engineered part does not make it compatible, strong, safe, and fit for purpose. This part could have likely been fine if it used a different material such as Ultem.
The original part was fiberglass/epoxy with the epoxy having a Tg of 84C.
HDT does, kind of, but that’s already covered by the load being defined for the various conditions. HDT is always defined at a specific load so it also does not change with load (since load is fixed).
https://duckduckgo.com/?q=plastic+air+induction+elbow&ia=ima...
so, if you were thinking "who would use a 3D-printed part", remember that it may otherwise also have been made with some liquid material, but using a mold, and perhaps two parts using a mold that are joined with re-heating etc. - and now it no longer sounds so outlandish.
I did a quick search and found that many plastics are governed by ISO 11357 test standard [1]. Some of the plastics I have worked with used this standard.
A spec sheet for that material is here [2].
[1]: https://www.iso.org/standard/83904.html
[2]: https://um-support-files.ultimaker.com/materials/1.75mm/tds/...
Really it seems like a problem of not understanding the environment, and testing (with margins) your replacement in it... 3D printing seems nearly entirely unrelated apart from enabling people to make parts.
An injection molded part, for a close more traditional analogue, would presumably have failed the same way here.
Also the glass transition temperature reported in the report is suspiciously low for ABS and the only source on the material is the owner saying the person they bought it from said... I wonder if it was just outright made out of the wrong material by accident.
Edit:
edit: nvm, i found my answer in the actual report.
> The aircraft owner who installed the modified fuel system stated that the 3D-printed induction elbow was purchased in the USA at an airshow, and he understood from the vendor that it was printed from CF-ABS
CarVac•31m ago
edit: It was ABS-CF, which shouldn't be used under stress long-term in higher temperatures than maybe 65-70°C, or lower depending on the blend.
gostsamo•29m ago
the_mitsuhiko•28m ago
> An alternative construction method for the air induction elbow, shown in the Cozy Mk IV plans, is a lamination of four layers of bi-directional glassfibre cloth with epoxy resin. The epoxy resin specified for the laminate has a glass transition temperature of 84°C, after the finished part has been post-cured. The aircraft owner stated that as the glass transition temperature listed for the CF-ABS material was higher than the epoxy resin, he was satisfied the component was fit for use in this application when it was installed
https://assets.publishing.service.gov.uk/media/69297a4e345e3...
ohazi•24m ago
buildbot•24m ago
“ The aircraft owner stated that as the glass transition temperature listed for the CF-ABS material was higher than the epoxy resin, he was satisfied the component was fit for use in this application when it was installed. A review of the design of the laminated induction elbow in the Cozy Mk IV plans showed that it featured a section of thin-walled aluminium tube at the inlet end of the elbow, where the air filter is attached. The aluminium tube provides a degree of temperature-insensitive structural support for the inlet end of the elbow. The 3D-printed induction elbow on G-BYLZ did not include a similar section of aluminium tube at the inlet end. Tests and research Two samples from the air induction elbow were subjected to testing, using a heat-flux differential scanning calorimeter, to determine their glass transition temperature. The measured glass transition temperature for the first sample was 52.8°C, and 54.0°C for the second sample.“
jandrese•16m ago
This seems very low for the kinds of epoxy I've used. I wonder if the manufacturer specs are highly conservative? Or maybe the material has a shortened lifespan with even moderate temperatures?
I was thinking about the ABS in the article and wondering if I would have made the same mistake. Close to every car manufactured today has plastic intakes, usually bolted right on top of the engine. The incoming air should help keep it cool, especially on aircraft. Maybe it was the radiant heat from a nearby cylinder that melted it?
CarVac•12m ago
bluerooibos•28m ago
I'd think any semi competent engineer would know better.
Edit: from the report - "A modification application was made to the LAA in 2019, by the aircraft owner2 , to replace the engine’s throttle body fuel injector with a mechanical fuel injection system. This system consisted of a fuel controller, high-pressure engine-driven fuel pump, electric auxiliary fuel pump, fuel flow transducer and associated fuel hoses, filters and fittings. Following flight testing, the modified fuel system was approved by the LAA in 2022. The modified fuel injection system had accumulated 37 hours in service when the accident occurred."
So the pilot himself and the LAA were incompetent. LAA is an association for amateur pilots though so I'm not sure what level of rigour they "approve" things with.
Zak•25m ago
proee•24m ago
duskwuff•18m ago
MBCook•27m ago
Certainly seems questionable to use any 3-D printed plastic material for exhaust. That’s absolutely going to be too hot.
CarVac•24m ago
brovonov•23m ago
"Two samples from the air induction elbow were subjected to testing, using a heat-flux differential scanning calorimeter, to determine their glass transition temperature. The measured glass transition temperature for the first sample was 52.8°C, and 54.0°C for the second sample"
Yeah, they might have used ABS-CF filament, but unless they got it from a good brand that uses good resin and proper printing parameters, the actual Tg will be lower, plus the stress from the vibration/load could have made the part fail if it was not for the heat later in flight.
CarVac•20m ago
Polymaker Polylite ABS has a claimed Tg of 101°C but the HDT curve clearly shows it starting to lose strength at 50°C, for example.
brovonov•16m ago