Boeing Co. cautioned that the aerospace industry could suffer from supply disruptions for more than half a decade, delaying deliveries to airlines and hampering the rebound from the COVID-19 pandemic.

“I can see supply constraints for a very long time,” chief executive officer Dave Calhoun, appearing on a panel alongside Qatar Airways CEO Akbar Al Baker, said at the Qatar Economic Forum, which is underwritten by the state of Qatar and sponsored by Bloomberg, in Doha on May 23. “We have backlogs that go out five to six years, so if the backlogs would suggest supply constraints that far, that means it’s even further.”

Read more: Airbus, Safran Warn Supply Chain Snarls Will Drag Into 2024

Aircraft manufacturers have struggled to increase production at a time when airlines are clamoring for new jets to meet the surge in travel demand. Component shortages have restricted output as Boeing and arch-rival Airbus SE seek to scale up production. Calhoun said only after the industry has regained what he called stability — a process that will take about a year and a half — can it really ramp up output rates.

Al Baker said that along with delays of new jet deliveries, supply constraints are a hurdle for existing fleets, forcing airlines to ground some planes that need spare parts for everything from engines to avionics.

“This is all the consequence of the pandemic,” Al Baker said. 

Read more: Boeing Wins $40bn Order from Ryanair for 737 Max 10 Jets

Calhoun said he doesn’t expect either Boeing or Airbus to build a brand-new aircraft model until 2035 because required propulsion systems won’t be available sooner to justify the huge outlays needed to improve fuel efficiency by 20% to 30%.

The two executives dismissed the notion that hydrogen-powered aircraft could soon be available to fulfill the industry’s net zero carbon ambitions, predicting instead that the technology will only be mature in the second half of this century. Until then, the industry will need to rely on so-called sustainable aviation fuel, they said, which is chemically similar to normal jet fuel but doesn’t require extraction of more petroleum.