"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.
Polymaker's ABS is dubious too because it is blended with PETG. They are coming out with a Pro version that has a higher Tg and requires way higher chamber temps to print properly.
> 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
What a misunderstanding -- glass transition temperature means different things for thermoplastics (i.e. anything that comes out of an FDM printer like the CF-ABS in question) and for thermosetting resins like epoxy that actually undergo molecular cross-linking during the curing phase. Thermoplastics will get soft and can deform without limit, while thermosets get rubbery but still more or less hold their formed shape.
I think an extended quote shows that this was a really bad call:
“ 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.“
> The epoxy resin specified for the laminate has a glass transition temperature of 84°C
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?
There are some incredibly low Tg epoxies out there, such as West Systems 105 where "TG onset" is 54°C and the heat deflection temperature is even lower.
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 (carbon fibre – acrylonitrile butadiene styrene) filament material,
with a glass transition temperature3
of 105°C.
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.
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.
It was an intake manifold, so it's continuously under suction. At the temperatures in an engine bay the plastic probably gradually creeped to a point where the restriction increased the suction and suddenly it collapses completely.
I wonder who installed it. Was the pilot home 3D printing mods for their plane? And is that even allowed? Super concerning if there was a company behind the installation.
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.
Nearly anything is allowed for experimental amateur-built aircraft like the one in this incident. Unapproved modifications to certified aircraft are forbidden in most parts of the world.
The LAA classified the proposed modification pf the overall fuel system as minor based on the owner’s description of it, and approved it on that basis. But the owner lied and did not disclose the fact that the induction elbow was modified at all or that it was 3D printed. The report does not discuss any required inspection by a person authorized by the LAA prior to returning the aircraft to service. So it seems like the LAA modification process is a trust-don’t-verify policy.
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.