Emirates President Tim Clark said he may not reconsider Airbus SE’s largest A350 model until the end of the decade because he wants to see evidence that the engine that powers the variant meet the airline’s specifications.

Speaking at an event in London, Clark said he’d need to be convinced that the Trent XWB-97 engine on the A350-1000 has “the durability we’d need,” ideally by studying its performance in a rival’s fleet. If satisfied, Emirates would consider the plane “at the back end of the decade,” he said.

Clark openly criticized engine maker Rolls-Royce Holdings Plc last week at the Dubai Airshow, calling the powerplant on the largest A350 variant “defective” because the maintenance cycles were too short. Emirates ended up ordering just 15 of the smaller A350-900 variant at the event, far below the as many as 50 A350-1000s Clark had said he’d want.

Rolls-Royce is the sole engine manufacturer on the A350. Despite Clark’s criticism, the -1000 variant has been especially popular with airlines this year, who have bought it in record numbers to benefit from low fuel consumption and its ability to fly long distances. Airbus and Rolls-Royce have defended the model, saying it works just fine, and that reliability will improve over time.

To be sure, Clark has changed his mind on aircraft before, openly criticizing a model’s performance before turning around and buying the plane in large numbers.

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