The aerospace industry's ambitious efficiency goals clashed with the realities of market economics and propulsion technology on the second day of the Paris air show.
During a morning panel discussion, airframer and supplier executives laid out means by which they hope to reduce carbon emissions and meet a CO2 reduction goal.
That goal, set under the Carbon Offsetting Scheme for International Aviation (CORSIA), calls for the industry to cut emissions to half their 2005 level by 2050.
Executives discussed a host of new technologies.
Dassault Aviation chief technology officer Bruno Stoufflet called hydrogen propulsion systems a potential "low-cost solution" that could propel a "change of paradigm in the design and development of aircraft".
But first, the industry must solve several associated challenges, including those related to distribution, regulation and certification of hydrogen-powered aircraft.
United Technologies is pursing means of replacing more hydraulic and pneumatic systems with electric systems, says chief technology officer Paul Eremenko
The company is also developing a prototype hybrid-electric propulsion system for a De Havilland Canada Dash 8 turboprop.
While "hybridisation can make a meaningful impact", battery limitations make all-electric propulsion for large passenger aircraft a stretch, Eremenko adds.
"Barring a fundamental invention or scientific revolution in energy storage… the all-electric element will be limited to a couple of passengers," he says.
Safran chief technology officer Stephane Cueille agrees. "The key driver is really storage," he says. "It would take something out of the blue… to change the batteries."
Executives discussed other means of improving efficiency, such as use of alternative fuels and improved engine designs. But they were unable to say how they expect to meet the 2050 goal.
The target seems particular challenging in light of airframers' expectation they will deliver thousands more aircraft in the coming decades.
Boeing, for instance, foresees industry demand for some 44,000 new large commercial aircraft through 2038. Most of those aircraft seem likely to be powered by gas turbines.
Engine makers have said they expect that the next-generation of commercial aircraft engines will be turbofans 10-15% more efficient than current-generation engines. The following generation of engines, however, could include other, more efficient designs, such as open-rotor or hybrid-electric propulsion systems, they say.
Asked about the 2050 goal, Boeing chief technology officer Greg Hyslop says: "We don't know how that is going to happen yet. It's a very hard problem."
He adds: "That is why we are all coming together."