WASHINGTON/CHICAGO - Boeing faces an escalating crisis with its 737 MAX aircraft as both United Airlines and Alaska Airlines report discovering loose parts on multiple grounded MAX 9 planes.

The revelation has raised concerns about the manufacturing process of Boeing's best-selling jet family.

Following the grounding of 171 MAX 9 planes by U.S. regulators after an incident with an Alaska Airlines flight, preliminary checks by both carriers revealed issues. Alaska Airlines found "loose hardware," while United identified bolts needing tightening on several panels.

Boeing is expected to revise inspection guidelines submitted to airlines, pending FAA approval. The company assured it is in close contact with operators to address findings during inspections. This development adds to Boeing's challenges since the 737 MAX family's 20-month grounding in 2019.

Safety expert John Cox highlights the significance, stating, "This is a quality control problem."

The situation may heighten passenger concerns about the aircraft's safety, impacting Boeing further. The latest issue involves a panel, known as a door plug, installed in two stages by supplier Spirit AeroSystems and completed by Boeing. Spirit AeroSystems shares dropped 11% on Monday. The FAA declined to comment on the loose bolt reports.

Flight cancellations are expected as enhanced inspections, involving both left and right cabin door exit plugs, are mandated. Alaska Airlines and United have already grounded their MAX 9 fleets, affecting numerous flights.

Investigators are examining manufacturing and maintenance records to determine the cause of the loose parts. Boeing's shares plummeted by 8% on Monday in response to the unfolding situation.

Amid the uncertainties, the FAA stated that planes will remain grounded until enhanced inspections are completed.

Airlines worldwide, including Turkish Airlines, Copa Airlines, and Aeromexico, have taken precautionary measures. As investigations expand, industry experts emphasize the need for a thorough examination of the entire fleet to address safety concerns.