Who flies when it gets dangerous ?
- the highest-ranking pilot present in the cockpit given his experience
- the SIC (second in command), for the reasons quoted below
- whoever was PF when the incident began continues flying
- or no specific rules, the pilots must make a joint decision
Just beware that either pilot may be at the controls during a particular incident. In fact, while protocols are different [from] carrier to carrier, it's not unusual during emergencies or other abnormal situations, for the captain to delegate hands-on flying duties to the copilot, so that the captain can concentrate on communications, troubleshooting, coordinating the checklists, etc. (source)
Case in point:
In the AF447 crash, the captain was not in the cockpit when the incident began. When returning to the cockpit, he did not take over control but sat down behind the 2 First Officers.
Please mention in your answer
- which person (if any) is designated by the regulations, airline rules and best practices
- for what reason
- and whether these rules/recommendations have often been not followed in crashes