Look at a few facts -
A F/O word going against a captain's "superior" experience, seniority and status...?
Generally does not work too well. Some captains can get away with murder.
Chief pilot might have a few beers with that captain at the pub once a week.
Wish you had a flight engineer. In the old days, F/Os and F/Es could report facts together.
So, that F/O should not try to go at it "alone"...
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Get name/phone number of all F/Os who flew with him recently.
Call them and discuss the matter.
Are other F/Os saying same facts about that captain...?
Are they willing to make an individual report to the chief pilot...?
A dozen of written reports from various F/Os would help.
Reports should include name of cabin staff on these flights.
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The F/Os should refuse to fly with him, and mention that fact in the written report.
The only decent "weapon" F/Os have. "I request not to fly with Captain X..."
Start report with the famous sentence "In the interest of flight safety..."
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Happy contrails