The open-fan engine has become aviation’s favorite near-future miracle: a sleek, exposed set of blades that promises a step-change in fuel burn, just in time for the mid-2030s clean-sheet airliner cycle. It’s small wonder that industry heavyweights like Airbus and CFM International have been doubling down on the open-fan narrative of dramatic efficiency gains, delivered via a visible symbol of progress, all while offering a tidy answer to ongoing pressure on costs and carbon.

But the industry has been here before. Radical propulsion ideas don’t fail because they’re bad ideas; they come up short because airframes, airports, regulators, and maintenance realities have veto power. And that’s why the most revealing propulsion news this week isn’t another open fan milestone — it’s that CFM has a side-bet, quietly studying a more conventional alternative alongside the concept it has publicly championed. If the open-fan hype is the dream, CFM’s “Plan B” is the fine print.

The Job Posting That Let the Cat Out The Bag

Credit: CFM International

Reuters reported earlier that CFM — the GE Aerospace/Safran joint venture — has been studying a more conventional “advanced ducted” architecture in parallel with its development of open-fan engines as part of its headline-grabbing RISE program. The detail that gives the story its bite: the program surfaced via a job posting, which pointed to the internal existence of a parallel Plan B track, rather than just a casual “what-if” discussion.

CFM’s public centerpiece remains RISE (Revolutionary Innovation for Sustainable Engines), built around an open fan: a very large set of fan blades not enclosed in a traditional nacelle. CFM announced the RISE program in June 2021 as an intended successor of the CFM LEAP turbofan engine, with plans for the engine to enter service in the mid-2030s. It has repeatedly tied the concept to roughly 20% fuel and emissions savings versus today’s best single-aisle engines, and Airbus has lined up an Airbus A380 flying testbed to mature the concept in the second half of this decade.

So if that is Plan A, but we now know there’s a Plan B involving an “advanced ducted” engine, what is that in plain terms? An engine manufacturer insider describes it as the next generation of the enclosed engine architectures we know today:

“Think of it as a next-generation turbofan that still looks like an engine you’d recognize today, a large fan inside a nacelle or a ‘duct’, but it’s being pushed to far more aggressive efficiency levels using many of the same technology building blocks that RISE is currently developing.”

Reuters says this internal track has been referred to as “Advanced Ducted-Large” or ADL, and is viewed as more pragmatic and adaptable than open-fan concepts, even if the ultimate efficiency upside may be less. In other words, CFM is preparing an engine that can fit into a more conventional aircraft design space if the open-fan design/integration/certification puzzle doesn’t come together fast enough for the 2030s.

Why A Plan B Exists At All

Credit: CFM International

The next narrowbody cycle is increasingly framed around the mid-to-late 2030s, with propulsion and wing technology timing being the main gating factor. Airbus has been explicit about targeting the late 2030s for an A320-family successor, and has positioned open-fan engines as a key candidate, while also acknowledging that the next jet must be “evolutionary,” not a moonshot that breaks the business case. Boeing, for its part, has repeatedly signaled that the industry is unlikely to roll out all-new jet designs before the mid-2030s.

The challenge is that a clean-sheet airliner timeline is unforgiving, and engine timelines are even worse. A new engine program doesn’t just require engineering brilliance; it requires manufacturability at scale, supply-chain maturity, durability in grimy real-world cycles, and a certification path that survives the first serious in-service surprises. Recent history is full of engines that looked great on paper but spent years in operational triage once airlines started flying them hard. CFM’s ADL program is therefore a safety net for an industry that is extra-jittery after current engines have created painful shop-visit backlogs and unexpected maintenance cost exposure.

Then there is also the issue of timing. The latest engines on the aircraft we fly today took approximately a decade to advance from their first concepts to entry into service (see below). But keep in mind, each was an evolution of existing engine architectures, whereas open-fan engines are a radically new design. As such, does it seem reasonable that open-fan engines will progress from current first designs and concepts to service-ready engines within nine years?

If Airbus and Boeing plan new aircraft in the mid-2030s, the timing seems uncomfortably tight for open-fan engines. And that is assuming everything progresses at a normal pace. An open-fan architecture that triggers major integration or certification surprises late in the decade could force a manufacturer into an ugly choice: slip the new aircraft timeline, or accept less ambitious propulsion.

It’s not just CFM doing the risk math. Reuters noted that Boeing, along with Pratt & Whitney and Rolls-Royce, has been more skeptical about open-fan engines than Airbus, which has leaned in harder. Competitors have also publicly downplayed the threat and pointed to open-fan-specific complexities, arguing that many of the RISE technology benefits could be achieved without going fully open rotor.

The Open-Fan Challenges That Can’t Be Ignored

Credit: Shutterstock

The appeal of open-fan engines is real: moving a larger mass of air more gently can deliver better propulsive efficiency. But the aircraft has to live with the consequences, and that’s where the hard engineering begins.

Airbus and CFM are trying to answer those questions via flight testing on an A380 testbed by the end of the decade, precisely because modeling can only get you so far when the fan is exposed and the engine-airframe interaction becomes a first-order design driver. There are numerous challenges that keep open-fan engines from being a simple “bolt-on” solution:

• Noise: Exposed blades change how noise propagates and how it can be shielded, yet meeting community and regulatory expectations is non-negotiable.
• Airframe integration: Where the engine sits and how the airflow interacts with the wings and the fuselage drives drag, performance, and structural choices.
• Safety and certification: Traditional turbofans rely on containment assumptions; open architectures will force different design and certification approaches.
• Maintenance and durability: Airlines are already ultra-sensitive to shop-visit surprises; a radical new configuration will be scrutinized through a reliability lens first.
• Program timing risk: If late testing reveals problems, there may not be enough calendar left to redesign without slipping a mid-2030s aircraft launch.

That’s the core message behind CFM having a Plan B with the ADL program: open-fan engines may still win, but the downside of being wrong is enormous. Airbus and Boeing can’t afford to launch a flagship 2030s aircraft around a propulsion bet that might demand major redesign late in the game. So CFM is building a safety net, a more conventional architecture that could deliver meaningful gains with fewer unknowns. Exactly the kind of option manufacturers and airlines quietly crave when the hype has to meet the reality of real-world engineering.