Originally Posted by
flyboyike
I'm curious what US carrier requires "letters from Captains you've flown with" to qualify for upgrade. I'm not aware of any, at least not to become a line Captain. Now, if you're talking about becoming an instructor, that's another matter.
Some airlines ask the line captains if an FO is ready for upgrade, not major airlines, but smaller companies who upgrade on merit rather than seniority. (I worked for at least 4 of those)