A Cozy Mk IV light aircraft crashed after 3D-printed part was weakened by heat

Material choice and properties

• Thread focuses on the claimed CF‑ABS material vs. what testing showed: glass transition temperature (Tg) 53–54°C, which commenters say is typical of PLA/PLA‑CF, not ABS‑CF (100°C).
• Several point out the owner misunderstood Tg: comparing thermoplastic Tg to thermoset epoxy Tg is invalid. Thermoplastics soften and creep under load; thermoset composites stay largely dimensionally stable below Tg.
• Others note datasheets and marketing for filaments often exaggerate Tg/HDT, and that HDT under load is more relevant than bare Tg.

Part design and failure mechanism

• The failed component was an intake air induction elbow, under continuous suction and located in a hot engine bay.
• Original plans specified fiberglass/epoxy plus a short aluminum tube at the inlet to provide temperature‑insensitive structural support. The 3D‑printed part omitted the aluminum tube.
• Commenters suggest progressive softening and creep at temperature, increased restriction → more suction → sudden collapse.

Regulation, disclosure, and responsibility

• Aircraft is a homebuilt, experimental‑class Cozy Mk IV. In that category, wide latitude is allowed; owners effectively sign off their own airworthiness.
• The modification was classified “minor” by the LAA based on an incomplete description; the 3D‑printed elbow was not disclosed, which commenters see as a “trust‑don’t‑verify” failure.
• Debate over blame: installer/owner for poor judgment and nondisclosure; vendor for misrepresenting or mishandling material; LAA for superficial approval.

3D printing vs. engineering rigor

• Strong pushback on blaming 3D printing itself: the real failure was material selection, lack of testing, and absence of proper engineering analysis. An injection‑molded thermoplastic of the same polymer would likely have failed similarly.
• Others argue that cheap FDM lowers the barrier for unqualified people to make serious parts, analogous to “vibe coding” in software: outputs that look professional without underlying validation.
• Multiple comments note that aerospace and automotive already use additive manufacturing (including metals and high‑temp polymers like PEEK/Ultem), but only with stringent qualification and traceability.

Experimental aviation culture & LAA reaction

• Several emphasize that experimental/homebuilt aviation tolerates high tinkering and risk; Cozy builders are likened to highly hands‑on “hacker” communities.
• Some fear the LAA’s planned “3D‑printed parts” alert may overgeneralize, penalizing properly engineered high‑temperature printed parts rather than focusing on qualification and testing.