Air India said it’s demanding compensation from Boeing for delivering 787 Dreamliner aircraft that don’t meet promised fuel-efficiency targets because the planes are heavier than planned.

Design changes made after targets were set meant some 787s are heavier than required, Air India Chairman Rohit Nandan said in an interview in Doha, Qatar. The national carrier and the plane maker are negotiating a formula for compensation, which may be finalized after the 18th Dreamliner is delivered to the Mumbai-based carrier, he said.

In February, Boeing said Air India was not happy with the reliability of the aircraft and the plane maker was upgrading software and changing components on some planes owned by the flag carrier whenever they could be taken out of service. The model has experienced a series of malfunctions since commercial debut 3 1/2 years late in 2011, including a three-month grounding of the global fleet last year after battery meltdowns.

“The worst part is over,” Nandan said. “The initial ones did have some difficulty of engineering, in terms of service, but I think they have all been upgraded now.”

The 787-9 variant of the Dreamliner can seat as many as 280 passengers and the twin-aisle plane has a range of 15,372 kilometers, according to Boeing’s website. It has a maximum takeoff weight of 250,836 kilograms.

Unprofitable Air India ordered 27 Dreamliners on the promise of a more fuel-efficient operation. The Indian carrier has sparred with Boeing over compensation for tardy deliveries.

Besides fuel efficiency, Air India had to face other issues with the 787s. The company had experienced 136 snags with the plane as of November last year, India’s aviation ministry said previously. One of its planes was diverted to Kuala Lumpur in February as a precaution because of a software fault. In other incidents, some windshields cracked, the Times of India reported last month.

“For the last few months, we have no problems with them,” Nandan said, referring to the 787. “An occasional problem can happen to any aircraft. There’s nothing specific, there’s no pattern as well.”