BRUSSELS (Reuters) - The top procurement official at one of Europe's largest airlines urged GE Aerospace and other engine makers on Wednesday not to overlook durability when it comes to designing the next generation of fuel-saving engines.

Problems with durability - or the gap between scheduled maintenance visits to overcome wear and tear - have resulted in long waiting times at engine repair shops in recent years, leaving hundreds of airplanes grounded and pushing up fares.

"Give me unlimited time on wing and I buy now," Lufthansa Chief Technology Officer Grazia Vittadini said at a Brussels aviation policy forum hosted by GE Aerospace.

The world's largest aerospace group is developing a technology demonstrator called RISE aimed at delivering 20% savings in fuel and emissions for the next generation of aircraft due late next decade.

Vittadini's comments reflect a tug of war between efficiency - or fuel savings - and durability, or the time between repairs.

To achieve the fuel savings promised to airlines in the most recent generation, experts say many engines have run hotter and push new materials to the limit. But doing so imposes extra wear and tear and cuts the time between maintenance visits.

The engine industry has been criticised for not planning for this by investing in adequate maintenance capacity, notably at GE competitor Pratt & Whitney which supplies some of the aircraft facing disruption at Lufthansa Group.

Pratt & Whitney has reported progress on the issue.

For the next round of engines, GE said it was making progress in testing and supercomputer research on RISE.

GE executive Roman Seele said CFM, co-owned by GE and France's Safran, had carried out more than 250 tests on the demonstrator, which aims to prepare the ground for an engine to be offered for the next Airbus and Boeing ( BA ) medium-haul jets.

Vittadini also highlighted delays in development and delivery of larger jets, saying Lufthansa expected to become the first operator of the GE-powered Boeing 777X next year.

That compares with an original plan to take the first delivery in 2020 and have all its long-haul 777X airplanes flying by some time in 2024, she added.

The world's largest twin-engined jet has been repeatedly pushed back due in part to certification delays. Boeing ( BA ) said last year it was delaying the first delivery to 2026 from 2025.