The above code connects to Cosmos DB and retrieves the details for the base airfield that was specified, it then calculates the range of the aircraft in meters by multiplying the endurance (in hours) by the true airspeed in knots (nautical miles per hour) and then multiplying that my 1852 (number of meters in a nautical mile). A Linq query is then run against Cosmos DB using the built-in spatial functions to find airfields within the specified distance. The result is then converted into a JSON array that can be understood by the Google Maps API that is being used on the client side.
The client side uses the Google Maps API to plot the airfields on a map, giving us a view like the one below when given a base airfield of Blackbushe (EGLK), a true airspeed of 100kts and an endurance of 4.5 hours
Click through for .NET code to load and analyze the data.