Boeing is facing new scrutiny over the safety of its passenger jets after a frightening door mishap mid-flight earlier this month.
Boeing's CEO Dave Calhoun is meeting with lawmakers on Capitol Hill Wednesday as two major airlines outline their growing concerns in the wake of the Alaska Airlines mid-air blowout on a 737 Max 9.
Alaska Airlines CEO Ben Minicucci is also speaking out, saying he's "frustrated" and "angry," adding that inspectors have now found loose bolts on "many" of the Max 9 jets grounded since the incident.
Minicucci is sending his own inspectors to audit Boeing's quality-control systems and acknowledged that his airline was lucky no one was seated next to that door.
"There were only seven open seats and we had a guardian angel, honestly, on that airplane," he said.
Now, there are more problems for Boeing.
The US Federal Aviation Administration urged airlines on Sunday to inspect so-called door plugs on an earlier version of Boeing 737 airplanes. FAA opened a formal investigation into Boeing's quality control around two weeks ago. The agency said it continued to review data collected from inspections of 40 sample aircraft as it considered how to determine if the planes were safe to fly again.
United Airlines, which also flies the Max 9, says it's also found loose bolts and -- due to quality concerns - is now re-thinking its commitment to Boeing for future orders.
The CEO of United Airlines, one of the biggest buyers of Boeing jets, also expressed frustration with the company.
"I'm disappointed that... this keeps happening at Boeing. This isn't new," said Scott Kirby, CEO of United, in an interview Tuesday. "We need Boeing to succeed. But they've been having these consistent manufacturing challenges. They need to take action here."
United has 79 of the Max 9s, more than any other airline, and had originally scheduled nearly 8,000 flights with the plane for this month before the incident, according to Cirium, an aviation analytics firm. Boeing's future is uncertain regarding orders for the 737 Max 10, a newer, larger version and more expensive version of the 737 Max that has yet to be certified by the FAA.
"I think the Max 9 grounding is probably the straw that broke the camel's back for us," said Kirby. "We're going to build a plan that doesn't have the Max 10 in it."
Boeing says workers at its 737 factory will stop work Thursday at its Renton, Washington facility to hold workshops on quality control.
"During the session, production, delivery and support teams will pause for a day so employees can take part in working sessions focused on quality. This is part of the immediate quality actions recently shared by Boeing Commercial Airplanes CEO Stan Deal," the company said in a statement.
The internal notice Boeing sent to its employees indicates that the 737 stand down is the first of several that it will hold at its facilities.
Meanwhile in Atlanta, a tire fell off a Delta Boeing jet before takeoff. Experts say it was likely a maintenance issue.
Passengers were forced to board a replacement plane.
In a statement, Stan Deal, the CEO of Boeing Commercial Airplanes, said: "We have let down our airline customers and are deeply sorry for the significant disruption to them, their employees and their passengers. We are taking action on a comprehensive plan to bring these airplanes safely back to service and to improve our quality and delivery performance. We will follow the lead of the FAA and support our customers every step of the way."
Travel website Kayak has now added a feature that allows users to filter out the 737 Max 9 when searching for flights. But, for now, all Max 9s remain grounded.
CNN contributed to this post.