There will come a time when a pilot is not able to make it to the planned destination. This can be the result of unpredicted weather conditions, a system malfunction, or poor preflight planning. In any case, the pilot needs to be able to safely and efficiently divert to an alternate destination. Before any cross-country flight, check the charts for airports or suitable landing areas along or near the route of flight. Computing course, time, speed, and distance information in flight requires the same computations used during preflight planning.
PHAK Ch. 16 (34) Flight Diversion
Cessna SOPM 2-72 Diversion
Private Pilot ACS Page 38 Task C. Diversion