What is an emergency?
It is what the pilot determines. It is what the pilot declares. It is what the pilot decides that the state of the aircraft is.
There are many factors and different circumstances that determine whether an urgency or an emergency exists or not.
An engine failure with a four-engine aircraft is very different from an engine failure of a single-engine aircraft.
Therefore in general, it is the pilot in command who decides, what he thinks that the situation is for his aircraft in his situation: A minor failure, an urgency or an emergency.
Two states of emergency
a) Distress:
A condition of being threatened by serious and/or imminent danger and of requiring immediate assistance.
Examples : ditching, crash landing imminent, total engine failure...
b) Urgency:
A condition concerning the safety of an aircraft or some persons on board but which does not required imediate assistance.
Examples : lost, fuel shortage, partial engine failure, navigation system failure...
