Why do composite materials not decrease the OEW

Question

Boeing states in an article, that the 787 Dreamliner saves around 20% of weight in a comparison to an aluminium based aircraft. Boeing 787 Dreamliner

When comparing the 787 Dreamliner to other "recently" introduced aircraft such as the A330-900 it does not seem that any weight has been saved. The A330-900 is an aluminium aircraft built on the A330-300.

Aircraft	OEW	MTOW	Passenger Exit Limit	OEW/MTOW	OEW/Exit Limit
B787-10	135t	254t	440	0.531	0.3068
A330-900	135t	251t	460	0.537	0.2935

When comparing the OEW to the MTOW it seems, that both values are pretty similar. Given the same OEW, the A330-900 can carry 20 passengers more.

What could be the reason, why the A330-900 has a same structural efficiency as the 787 Dreamliner, despite not using any composites? Doesn't the 787 Dreamliner should have decreased the OEW by 20% hence having a much better ratio of OEW/Exit Limit?

Very much related: What is the average weight difference between composite and metal airliners?. It probably answers your question. — DeltaLima, Jun 07 '23 at 10:46
If anybody wants to flag or vote to close as a duplicate, that would be good. I won't because that would mod-hammer it. — DeltaLima, Jun 07 '23 at 10:56
Does this answer your question? What is the average weight difference between composite and metal airliners? — 300D7309EF17, Jun 07 '23 at 11:03
This does not quite answer my questions, as it indicates, that weight savings from composites were used to build larger wings. Comparing the B787 and the A330-900 this is actually the opposite. The A330-900 has a 4m larger wingspan, using aluminium and still having a better structural efficiency. — rohrerph, Jun 07 '23 at 11:22
Boeing's relationship with truth isn't not akin to an open marriage. — Therac, Jun 07 '23 at 17:27
Does this answer your question? Why has structural efficiency (OEW / MTOW) not improved despite increasing use of carbon fiber? — user33651, Jun 08 '23 at 06:54

Therac · Answer 1 · 2023-06-07T17:58:08.053

Composite fuselage weight savings aren't as good as a straight beam-to-beam comparison indicates. There are design constraints other than structural load, which limit what can be achieved.

Meanwhile, Airbus wasn't just doing nothing, small improvements do get implemented even on existing models of aircraft.

The 787-10 is a slightly larger aircraft than the A330-900. It's got fewer exits, but through an extra 9" in width and a longer cabin, packs more seats in practice.

It's hard to compare apples to apples, because airlines don't operate two competing types at once. That said, the 787-10 tends to pack up to 310-344 seats in long haul, while the A330-900 typically does 290-350 (various airlines). The 787-10 also carries up to 40 LD3 containers vs 33 on the A330-900.

So the few percent of improvement from composites is used to get space for more cargo and pax, at the expense of somewhat less range compared to the A330.

Another factor is that the knowledge of the theoretical structural savings tends to cause weight creep on other areas, as an almost subconscious relaxing of weight control. Sort of a moral hazard effect combined with "death by a thousand cuts" as tiny weight increments in a thousand different places add up. — John K, Jun 07 '23 at 17:56

Why do composite materials not decrease the OEW

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