By Bjorn Fehrm
August 28, 2026, ©. Leeham News: Last week we looked at thermoset prepreg tape layup with different machine types, what tape types they use, and how the molds could be simple flat plates or complex rotational symmetric mandrels with cutouts for stringers, collapsible after the autoclave cure so that the mandrel could be extracted from the composite barrel (Figure 1).
Figure 1. The tape laydown system for the 787 nose section at Spirit AeroSystems, now Boeing. Source: Boeing.
The problem with the thermoset prepreg and autoclave process is that it is slow and expensive. Airliner OEMs have been looking for a faster, less costly process since the development of the Boeing 787 and Airbus A350.
An optimized composite structure
We have covered prepreg tape-laid composite production over the last few Corners. It’s the most widely used method for producing large, aeronautical-quality parts for our airliners. The Boeing 787, 777, and military aircraft use it, as do the Airbus A350, A340, A330, and even the A300 vertical tail; this method is also used for the A400M wingbox production.
While the method produces a tightly controlled, very high-quality carbon fiber composite, it’s slow and costly. For the full cost of the airframe structures produced with this method, including all lightning protection and electrical return-path arrangements, the cost is considerably higher than if the same structure was made in aluminum.
For a wing, there is a tangible mass benefit when the wingbox uses carbon composites for the spars and wing covers with its stringers. For a fuselage, experience shows the mass gains are marginal.
The reason is that the wing’s parts that are replaced with composite parts are large, relatively few, and highly loaded in tension and compression. The advantages of composite construction there deliver tangible benefits in design freedom, mass savings, and reduced maintenance inspections.
For the fuselage, the advantage is less clear. A lot of the fuselage shell structure is not dimensioned by tension or compression forces but by toughness requirements. The thin skins required by stress calculations can’t handle the bird strike, hail, or ramp-rash requirements for the surfaces.
The brittle carbon composite needs to be thicker than envisaged, and the mass advantage of using carbon composites disappears. That’s why Airbus decided to build the A350 cockpit section in aluminum (Figure 2).
Figure 2. The A350 cockpit section is aluminum instead of composite. Source: Airbus.
The lower corrosion inspection requirement is the remaining advantage of a composite fuselage, albeit at a high production cost.
The case for Dry Fiber Resin Infusion
The autoclave combination of high pressure and temperature squeezes trapped air out of an AFP- or ATL-laid-down prepreg tape. However, we have seen that an autoclave cure requires long cycle times: the nitrogen atmosphere pressure must be gradually raised, held at around seven bars for hours, then cycled down before the molds and composites can be extracted and new parts loaded.
For the A350 wingbox cover with stringers, we have 13 + 8 = 21 hours, excluding the handling time to get molds and parts in and out. There has been a desire to replace the autoclave with an oven cure where parts can move in and out hot, i.e., the oven has a constant temperature of 180°C with doors that open and close while molds and layups transfer, Figure 3.
The problem is that the Out-of-Autoclave (OOA) cure does not work well with prepreg tape laydown. The one-bar vacuum hold-down pressure on a bagged part leaves too much trapped air, with inclusions exceeding the target of below 1%.
Figure 3. An oven for curing composites with a roll-in, roll-out port. Source: Infrared Heating Technologies.
It’s why OOA processes use dry fiber infusion processes. The dry fiber infusion stage starts by pulling a vacuum on bagged, dry, porous carbon fiber layup. Because no sticky resin blocks the air pathways, virtually 100% of the air is evacuated from the dry layup before introducing any liquid resin. The liquid resin then flows in to fill the gaps, resulting in very low void content (
With dry fiber infusion, we now have a pre-stage before curing that fits an OOA curing process. The challenge in dry fiber infusion and OOA curing has been producing a dry-fiber layup that meets the final part form, especially when the part is more than a flat laminate.
Many methods have been developed for dry fiber infusion. The development and testing of these methods have intensified over the last years as Airbus and Boeing have sought OOA methods for next-generation aircraft.
The manual fabric layup for flat parts we described in Part 13 is the straightforward method. But if you want a more complex 3D form, how can the dry fiber attain that shape and stay intact when a vacuum bag presses at minus one bar and a resin flow front passes through the layup?
We will look at several different such methods in the next Corner.
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