ATC Flight Plan — Page 242, Lesson 253

Alright, let's start a new topic. I want to walk you through the Point of Equal Time, or PET. The Point of Equal Time, which you might also hear called the Critical Point (CP) or the Equal Time Point (ETP), is a specific position along your planned track. It's defined in relation to two suitable airfields. From that exact point, the time it would take for the aircraft to fly to either of those two airfields is exactly the same. Those two airfields could be your departure and destination airfields, or they could be any two airfields that are suitably positioned in relation to your track. Now, why is this important? The PET is a decision-making tool. It allows the pilot to quickly determine which of the two diversion airfields is closer in terms of time, not distance. This becomes critical if you have a failure of an engine or a major system, or another event like a serious illness on board. The fuel you load for the flight—that's your trip fuel, contingency allowance, holding fuel, alternate fuel, and so on—must always be sufficient for the aircraft to fly from the PET to either of those nominated airfields. It's crucial to understand that the PET is a time problem. To make the time HOME from the PET equal to the time ON from the PET, the two distances will generally be different. The only exception is when there is zero wind; in that case, the distances are equal. You'll most often need to calculate a PET for routes over oceans or remote parts of the world. In those areas, there is a scarcity of suitable en-route diversion airfields within a reasonable flying time from any point on your proposed track. So, you calculate a PET between your departure and destination airfields, and also between those and any adequate airfields along the way. For example, there is a limit set on how far a twin-engine aircraft can be from an adequate airfield. That distance is equal to one hour's flight time, in still air, at the normal one-engine-inoperative cruise speed. Any operation planned beyond that distance from an adequate aerodrome is considered to be Extended Range Operations, or EROPS, which is also known as Extended Twin Operations, or ETOPS. Approved ETOPS operations specifically require the calculation of PETs between adequate airfields. Let's look at the diagram here. This figure illustrates the all-engine Point of Equal Time, or critical point. You can see the track between two airfields, and the PET marked at the position where the time to either airfield is equal.