There are several calculations that are made based on en route scenarios. The PNR, or Point of No Return, can be established by three different choices - returning back to the departure, to the nearest ETP airport, or to a separate airport. The next two calculations, one engine inoperative and depressurization, are both utilized when there is a performance issue with the aircraft. In those cases, the aircraft typically drops to a lower altitude in an effort to conserve fuel, oxygen, or time. In the final medical scenario, the aircraft needs to land as quickly as possible but is not limited in any way.
Extended-range Twin-engine Operations Performance Standards (ETOPS) utilize a time period - typically 90, 120, or 180 minutes - to ensure that an aircraft is within a suitable distance to a diversion airport. The certification level depends on aircraft capabilities with just one performing engine.
ETPs and ETOPS are most frequently included in flight plans over the Atlantic and Pacific Oceans where there are fewer diversion options and inclement weather is common. However, some operators elect to include the calculations on transcontinental flights that are over sparse regions. They are generally used by scheduled Charter (Part 135) and Commercial (Part 125) operators as a requirement, however, many General Aviation (Part 91) operators elect to use them as part of their own internal procedures.
Selecting ETP and ETOPS Airports