The Boeing boss was in D.C. Tuesday -- where he was appearing before U.S. senators for a hearing meant to address all the issues at Boeing lately -- and he was asked straight-up about whistleblowers speaking truth to power ... acknowledging for a fact retaliation happens.

Pressed if Boeing fired any staffers who were responsible for the retaliation, Calhoun was unable to give any specific numbers ... but he said retaliation is a thing at Boeing and people have been fired as a result. He promised Senator Blumenthal to get him specifics.

It's pretty significant testimony, given what's happened to some whistleblowers in recent months.

As we reported ... Boeing whistleblower John Barnett died back in March from an apparent suicide, smack in the middle of depositions for his lawsuit against the company -- something his attorney was highly suspicious of when it happened.

A few weeks after John's death, a second whistleblower died from an infection despite reportedly showing no symptoms in the weeks prior ... and again, it raised alarms.

Calhoun's not saying the deaths in these instances are related or the result of retaliation, but it's interesting nonetheless. Bigger picture -- he's fessing up to the fact that Boeing has had a history of trying to silence folks in their own ranks for pointing out issues with the planes.

The Boeing chief also apologized to people in the room who'd been affected by the company's safety issues -- including family members who'd perished in crashes.

Quite an eventful day ... and it sounds like Boeing is under pressure to make changes.