United told Travel + Leisure the airline typically schedules about 200 flights per day on the Max 9, while Alaska's cancellations affect as many as 150 flights per day.

The Federal Aviation Administration (FAA) will keep the beleaguered Boeing 737-9 Max planes grounded until it approves the aircraft manufacturer's inspection and maintenance process.

The FAA said the planes will remain grounded until it approves an "extensive and rigorous inspection and maintenance process." The agency said it currently needs "additional data" from Boeing after reviewing the company's proposed inspection instructions.

"We are working to make sure nothing like this happens again," FAA Administrator Mike Whitaker said in a statement. "Our only concern is the safety of American travelers and the Boeing 737-9 MAX will not return to the skies until we are entirely satisfied it is safe."

The planes were initially grounded after an Alaska Airlines 737-9 MAX aircraft suffered a dramatic mid-air blowout of a plug door panel on a flight from Portland, Oregon, to Ontario, California, on Jan. 5. Prior to the incident, the aircraft had reportedly been restricted from flying to Hawaii after a warning light possibly indicating a pressurization problem had lit up on three different flights.

As part of the initial inspections of their respective 737-9 Max fleets, both Alaska and United Airlines found loose bolts and hardware. Both airlines have canceled hundreds of flights since the incident. United told Travel + Leisure the airline typically schedules about 200 flights per day on the Max 9, while Alaska's cancellations affect as many as 150 flights per day.

In addition, the FAA will increase its oversight of Boeing's production and manufacturing, including auditing the 737-9 Max "production line and its suppliers to evaluate Boeing's compliance with its approved quality procedures." The agency will also increase its monitoring of in-service events related to the aircraft.