# Why has structural efficiency (OEW / MTOW) not improved despite increasing use of carbon fiber?

Where can I find information on the weight breakdown of civil aircraft? I am interested primarily in trying to understand why structural efficiency (OEW/MTOW) has not improved despite increasing use of carbon fiber materials.

I'm aware of the following questions:

Unfortunately, none of the answers list any authoritative sources. One links to an older MIT thesis, which includes a fractional weight breakdown of MD-80, DC-10-30, 737-200, 747-100, and A300-B2.

Well, simply consulting Wikipedia you can calculate that an "old" A330 has an empty weight fraction of (in tons) 132/251=0.545. A recent A350 has 142/283=0.50 i.e. some 10% lighter.

As a rule of thumb, a structural piece made of carbon fiber reinforced plastic is some 20% lighter in respect to aluminium. If you have a look at the figure 11 of the paper you quoted, you can see that only half of the empty weight is made up of structure (wing + fuselage + tailplane) the rest being systems, engines and landing gears.

So if you apply the rule of thumb of -20% to the 50% of empty weight composing the structure you end up with a 10% gain which is exactly what we have obtained comparing the previous two ratios for the A330 and the A350.

• I've run into this problem in aircraft design classes - you don't have carbon fiber passengers, fuel, seats, or tires! Commented Apr 12, 2023 at 14:51
• @costrom carbon fiber passengers is a good idea ;O) Commented Apr 12, 2023 at 16:35
• Not entirely true that carbon fibre seats don't exist: compositesworld.com/articles/cfrp-upgrades-economy-seating Commented Apr 13, 2023 at 7:15

To add to sophit's correct answer, calculating structural efficiency as MTOW/OEW is not always informative. It is informative if both aircraft designs aimed to maximize this ratio.

For same weight, MTOW might not increase, if range and payload requirements have been satisfied, and there's no more space or need neither for fuel, nor for cargo.

Extreme examples would be the Cessna 172R with a 70% empty weight fraction, or the Convair B-58 with 31%.

With a better material, the aircraft designer can decrease empty weight, or they can invest the savings into a longer wingspan, better engines (higher bypass ratio for commercial, more power for GA), or something else entirely.