<P> The point of no return, sometimes referred to as the PNR, is the point on a flight at which a plane has just enough fuel, plus any mandatory reserve, to return to the airfield from which it departed . Beyond this point that option is closed, and the plane must proceed to some other destination . Alternatively, with respect to a large region without airfields, e.g. an ocean, it can mean the point before which it is closer to turn around and after which it is closer to continue . Similarly, the Equal time point, referred to as the ETP (also Critical point (CP)), is the point in the flight where it would take the same time to continue flying straight, or track back to the departure aerodrome . The ETP is not dependent on fuel, but wind, giving a change in ground speed out from, and back to the departure aerodrome . In Nil wind conditions, the ETP is located halfway between the two aerodromes, but in reality it is shifted depending on the windspeed and direction . </P> <P> The aircraft that is flying across the Ocean for example, would be required to calculate ETPs for one engine inoperative, depressurization, and a normal ETP; all of which could actually be different points along the route . For example, in one engine inoperative and depressurization situations the aircraft would be forced to lower operational altitudes, which would affect its fuel consumption, cruise speed and ground speed . Each situation therefore would have a different ETP . </P> <P> Commercial aircraft are not allowed to operate along a route that is out of range of a suitable place to land if an emergency such as an engine failure occurs . The ETP calculations serve as a planning strategy, so flight crews always have an' out' in an emergency event, allowing a safe diversion to their chosen alternate . </P> <P> The final stage is to note which areas the route will pass through or over, and to make a note of all of the things to be done--which ATC units to contact, the appropriate frequencies, visual reporting points, and so on . It is also important to note which pressure setting regions will be entered, so that the pilot can ask for the QNH (air pressure) of those regions . Finally, the pilot should have in mind some alternative plans in case the route cannot be flown for some reason--unexpected weather conditions being the most common . At times the pilot may be required to file a flight plan for an alternate destination and to carry adequate fuel for this . The more work a pilot can do on the ground prior to departure, the easier it will be in the air . </P>

What type of air navigation uses calculations of speed and time