We have seen Airbus and Boeing going all the way for composites on their A350 and 787, but when it comes to smaller planes like the Embraer E-jets or the Bombardier C-series, they choose to use aluminium, I'd like to know what are the reasons for this to happen, why is more cost-effective?
There are some all-composite business jets like the newest bombardier Learjet. The design choice is about trade-offs in manufacturing(supply chain, number to be built, available tools and workers, legal certification[old methods are considered proven so require far less testing data], existing techniques, cost/percentage of build flaws.) market price of the end product, needed performance and lifespan. T
hen there is a question of available qualified and certified after market mechanics for repair inspection and maintenance. Downtime to repair minor damage like bird strikes and examining internal damage from loose carts or cargo that is a visible dent on metal but only looks like a scuff on composite but could be internal de-lamination. (Boeing had to consider and develop solutions for all of these external demands. Some solutions only made sense for large carriers with major maintenance divisions.)
For example if a significant mistake is made on a lamination layer, the whole part is pretty much landfill garbage, while an aluminum panel or rib can be recycled and replaced as fairly low cost and effort. On the other side; business jets like Lear, generally have far different flight profiles over their life compared to commercial jets, operate at different cabin pressure are more concerned with altitude and speed, may consider small airports a constraint, pennies per seat mile is a much lower consideration. Cost of downtime for repair or inspection is a much different priority as well.
Then there are the aspects of fire, repainting(paint stripping spec.), effect of fuel any that may leak into the structure, and of course weather certifications for lightening and icing conditions. For de-icing and anti-icing aluminium can be easily warmed from the inside while plastic is a poor conductor and may need another method. For lightening protection the 787 has a metal cloth for the outer layer and electrical bonding between all the parts, this adds weight and thickness; as a percentage a widebody jet is less effected by the extra material than a narrow body(the circumference doubles but the cross section area is quadrupled) Not all private aircraft are required to have full lightening protection if they will not be flying near such conditions, maybe just the fuel tanks will have static and lightening protection. Also useful payload is far less concern for business jets than for regional carriers who often sell extra baggage capacity to freight or parcel carriers like UPS/FedEx.
The C-Series has a carbon wing box. The fuselage is aluminum but is a newer fairly exotic aluminum lithium alloy that is a bit lighter than 2024. The decision to go with aluminum for the fuselage was mostly development and manufacturing cost (a fuselage requires a massive autoclave and it's development is fairly high risk the first time around) and ease of repair (the fuselage takes the brunt of vehicle damage on the ramp, especially on smaller aircraft) compared to carbon.
Cost is probably the number one driver. They stuck with things like bleed air services, and wisely decided to avoid lithium ion batteries (Boeing had to use L-ion with the level of electric services). The C-Series is overall a more conservative design than the 787, and it still almost broke the company, which was forced to give the airplane to Airbus for free or go under.
The E-jet is more or less the old 190 with a complete systems state of the art revamp (and I believe a new airfoil) that make it more or less a smaller C-series system wise without the risks of composites development. An even cheaper choice, and a wise one on Embraer's part.