Spirit, which was spun out of Boeing in 2005, is in an increasingly stretched financial position as the U.S. planemaker slows output of its 737 aircraft. 

The planemaker saw revenue fall 8% year-over-year in Q1, in what CEO David Calhoun called a "tough moment" in a letter to employees.

Alaska shares gained 5% in New York after earlier rising as much as 7.1%, the most in more than two years.

One witness said there was a disconnect between Boeing management's words about safety and what workers observe on the factory floor.

Boeing vice president Lisa Fahl says that the number of workers using the company's service to report safety concerns has risen by 500%.

The union message underscores the heightened scrutiny on airline safety following a January incident in which a panel blew off the side of an Alaska Airlines plane mid-flight.

The whistleblower accused the planemaker of subpar safety standards and retaliation for identifying engineering issues.

The bottleneck highlights how Boeing’s manufacturing challenges extend beyond its cash-cow 737 Max jet.

The Boeing 737-800 out of Denver International Airport was supposed to fly to Houston on April 7 before an engine cowling came off and hit the wing flap.

United already had planned several changes, including an additional day of in-person training for pilots starting in May, and a centralized curriculum for new maintenance technicians.