Emirates ruled out an immediate deal to buy Airbus A350-1000 jets on Tuesday, blaming a dispute with engine maker Rolls-Royce over the durability of its engines and leaving the European planemaker without a major showcase order at the Dubai Airshow.

Speaking a day after placing a $50 billion order for Boeing 777X jets, the president of the Dubai airline, Tim Clark, said the A350-1000’s engine would offer only a quarter of the time between maintenance visits compared to Emirates’ needs.

He told reporters Emirates would be prepared to order between 35 and 50 of the jets if Rolls-Royce improved both the durability and maintenance costs.

Rolls-Royce said: “Emirates is a valued customer and we look forward to supporting their future fleet growth plans.”

While airplane orders dominate headlines at air shows, airlines typically negotiate to buy the airplane and long-term repair services from the engine suppliers separately, meaning big-ticket plane announcements can hinge on unseen engine talks.

Airbus Chief Commercial Officer Christian Scherer defended Rolls-Royce’s Trent XWB-97 engine.

“It’s a perfectly fine engine operated by many customers around the world. Tim does not operate it,” he said while announcing an order from Egyptair for 10 smaller A350-900s.

Emirates has ordered 50 of the A350-900 version, which uses a different Rolls variant, but has yet to take deliveries.

Reuters reported on Monday that an order from Emirates for the largest version of the A350 appeared to be on hold over terms of engine guarantees with Rolls-Royce.

It is the latest example of disagreements in aerospace over the performance designed to save fuel at the expense of ever-hotter internal temperatures that require more maintenance.

Engines face particular challenges in hot and sandy or dusty conditions like the Middle East and India.

Emirates’s order for 90 more GE-powered Boeing 777X jets dominated the opening of the week-long Dubai Airshow on Monday.

With plans for an Emirates A350 order off the table for now, Airbus also saw a second major order from Turkish Airlines (THY) (THYAO.IS) slip off the show’s agenda, industry sources said.

Airbus said on Monday it had reached agreement “in principle” on a significant THY order.

But it added the deal needed to be ratified in the coming days, in what sources saw as a sign it would need Turkish government approval and was unlikely to appear at the show.