Equivalent Time Point (ETP)
The Equivalent Time Point (ETP), also known as the Critical Point (CP), is the point at which the aircraft's position is equidistant (the same flying time) from two suitable en-route Alternates (accounting for wind and temperature conditions). ETPs created for legs/segments during the flight are useful for managing worst case scenarios.
The formula for calculating ground ETP distance (DETP) and ETP time is given above. It uses the Ground Speeds of the Return-to Alternate (GSr) and Continue-to Alternate (GSc) and the Trip/Leg distance (Dtrip).
Point of No Return (PNR)
The Point of No Return (PNR) is the point on the flight path at which the aircraft becomes unable to return to the departure aerodrome due to fuel constraints. Beyond this point the aircraft must proceed to another other destination. Calculation of the PNR is mandatory for flights overwater or those without Alternates on route.