NEW YORK, United States (AFP) — US regulators said Thursday they continue to work with Boeing to assess an electrical problem that has grounded more than a hundred 737 MAX planes, as airlines expressed hope for a speedy resolution.

The problem, which surfaced during Boeing’s manufacturing process, has so far not significantly affected airlines given the still-lackluster travel volume during COVID-19, but demand is expected to pick up significantly this summer.

The Federal Aviation Administration (FAA) described the problem as “an electrical bonding and grounding issue,” according to an agency notice to civil aviation authorities.

“Boeing notified the FAA on April 7 that it recommended operators temporarily remove these airplanes from service to address a manufacturing issue that could interfere with the operation of a backup power control unit,” an FAA spokeswoman said. “Subsequent analysis and testing showed the issue could involve additional systems.”

Boeing announced publicly on April 9 that it had notified 16 airlines flying its 737 MAX planes of the issue, leading to the immediate grounding of planes.

The problem affects three parts of the plane in models built after Boeing made design changes in early 2019, and caused the grounding of 106 planes worldwide, 71 of which are registered in the United States, the FAA said.

The electrical issue is a new setback after the MAX was finally cleared to return to service in November 2020 after a 20-month grounding following two fatal crashes.

Southwest Airlines Chief Executive Gary Kelly told CNBC there has been no customer impact so far, but that the problem needs to be resolved quickly.

“We need the MAX. It’s just in time because business is starting to pick up,” Kelly said.

The electrical problem is “obviously disappointing,” he said. “It’s fairly minor, fairly straightforward, but we just can’t afford to have any ill consequences.”

American Airlines President Robert Isom said he hoped the FAA would issue an airworthiness directive “in a matter of weeks and not longer.”

The grounding of American’s 18 MAX planes has not affected the company’s schedule, Isom said. 

“I have great confidence that this aircraft is going to be in the skies and the safest and most reliable for years to come,” he said.