The Point of No Return (PNR) is a crucial concept in aviation. It refers to the moment during a flight when an aircraft can no longer safely return to the airfield from which it departed due to fuel limitations. Once the aircraft crosses this point, it must continue to an alternate destination. Several factors influence the determination of the PNR. These include the aircraft’s configuration, wind conditions, and its altitude.
Calculating the PNR is often necessary for various reasons. For instance, if the cargo or passengers are sensitive, it may be diplomatically important to land only at the intended destination or the departure point. This could apply to politically sensitive passengers or cargo that requires special handling. Additionally, if a technical issue arises, it may be best to resolve it at the home base. The PNR calculation also becomes critical when the destination is remote and lacks suitable diversion options.
When calculating the PNR, pilots must consider several factors. They might add extra fuel to the minimum required for overhead to allow for a diversion back to the departure airfield if needed. Assumptions may also be made about potential issues, such as one engine becoming inoperative or losing cabin pressure after reaching the PNR.
To perform a basic calculation of PNR, use the following variables: E represents safe endurance in hours, calculated by taking the fuel available at take-off, subtracting the necessary minimum overhead fuel, and dividing by the average fuel burn per hour. H denotes the groundspeed when returning to the departure airfield, while O indicates the groundspeed when heading to the destination airfield.
For scenarios involving an inoperative engine, the distance to PNR can be calculated using the formula: Distance to PNR = Fuel available / (Specific Fuel Flow Out + Specific Fuel Flow Back). Here, Specific Fuel Flow is determined by the average fuel flow divided by ground speed, and Fuel Available is the fuel at take-off minus the required minimum overhead fuel.
