Dave Calhoun, the CEO of Boeing, has recognized the company's mistakes as more than 170 planes remained grounded after one of their flights lost a door in mid-air.

He warned employees that an accident like the mid-air Alaska Airlines panel burst "can never happen again."

PHOTO | COURTESY Boeing (BA.N) CEO Dave Calhoun

Stan Deal, the company's top planemaker, also told a somber town hall meeting at its Renton, Washington 737 production that Boeing recognizes "the real seriousness of the accident" as it conducts investigations into its quality controls and systems.

His comments marked Boeing's first public admission of error since a "door plug" snapped off the fuselage of a largely full 737 MAX 9 on Friday, leaving a gaping hole next to a strangely vacant seat.

Calhoun said he was "shaken to the bone" by the disaster, which reignited concerns about Boeing's troublesome small plane family over five years after a full-fledged MAX safety crisis prompted by catastrophic crashes in Indonesia and Ethiopia.

PHOTO | COURTESY Boeing (BA.N) CEO Dave Calhoun

Alaska Airlines (ALK.N) and United Airlines (UAL.O), the two US carriers that employ the temporarily grounded jets, have discovered loose parts on identical planes, increasing concerns that a similar occurrence could occur again.

According to people familiar with the situation, Boeing told employees in a meeting on Tuesday that the results were being considered a "quality control issue" and that tests were being conducted at Boeing and fuselage supplier Spirit AeroSystems (SPR.N).

On Tuesday, Boeing shares dipped 1.4% as United canceled 225 daily flights, or 8% of its total, and Alaska Airlines canceled 109 flights, or 18%. On Wednesday, similar cancellations were expected.