The U.S. aviation regulator will ensure certification of Boeing’s 777X wide-body jet is conducted rigorously, the agency’s head said on Wednesday while reaffirming that its review of the grounded 737 MAX will not be rushed.

Problems with a number of aircraft models have led some airlines to accuse plane and engine makers of over-promising on performance capabilities, while the grounding of Boeing’s 737 MAX after two fatal crashes has increased scrutiny of testing and certification.

The comments from Steve Dickson, administrator of the U.S. Federal Aviation Administration (FAA), come a day after Emirates Boeing’s largest 777X customer demanded that the aircraft be tested for at least 16 months to ensure it is safe to fly and meets performance expectations.

“I did meet with Emirates and we had excellent dialogue,” he said at the Dubai Airshow when asked if the 777X certification would be tougher in light of recent events.

“Perhaps there will be more emphasis on making sure that the systems, as they evolve, are effectively integrated across the entire product and that we are not looking at issues in a fragmented fashion.”

On the continuing review of the 737 MAX, Dickson last week said his team would take “whatever time is needed”, having previously pledged a data-driven methodical analysis, review and validation of the systems and pilot training required to safely return the MAX to commercial service.

According to Reuters, Dickson reiterated on Wednesday that the FAA is not following any timeline for the model’s return to service, adding that time pressure cannot influence the regulatory process.

“We are going to make sure we are very methodical and very diligent. We are not delegating anything,” he told reporters at an airshow briefing.

Boeing has said it expects the FAA to issue an order approving the MAX’s return to service next month, with pilot training requirements for commercial flights to be granted in January.