U.S. AIRCRAFT SAFETY EXPERT JOHN COX:

"This changes a lot because it is now a fleet problem. This is a quality control problem."

ANTHONY BRICKHOUSE, AN AIR SAFETY EXPERT AT EMBRY-RIDDLE AERONAUTICAL UNIVERSITY:

"It was really important to figure out whether it was that specific aircraft from Friday night, whether it was an issue with Alaska Airlines' fleet specifically or whether it's a broader issue that could impact all MAX 9s that have that door. The fact that United has now found some aircraft with loose bolts, that just means that the investigation is going to be expanded.

"It just illustrates how important it was to ground similar MAX 9s and do those inspections. A lot of times in the industry money and profit are placed before safety. And grounding a specific aircraft isn't a light decision. It's a very big decision and it involves money and a lot of cancelled flights. But Alaska Airlines and United Airlines did the right thing in grounding their fleet."

RICHARD ABOULAFIA, MANAGING DIRECTOR OF AERODYNAMIC ADVISORY:

"Yes, it is embarrassing. But it's also there's a certain predictability to it. You have the worst production ramp down ever, and then the fastest production ramp up ever coupled with a very tight labor market.

"There's the bigger cultural issue here Boeing management, needs to take a more hands on approach and get more involved. The FAA, even with their ticketing aircraft, can't check every bolt. In terms of who is responsible, ultimately, it's a Boeing jet, but of course, the work has been done by spirit. You can't really say."

(Reporting By Valerie Insinna and Allison Lampert)