Forced landing is a situation where an aircraft unavoidably needs to land, usually regardless of terrain. For instance, a plane lost all engines and had to execute a forced landing on the highway. Sometimes forced landings are not related to emergency.For example, pilots performed a forced landing following a military interception. Precautionary landing is a situation where further flight is possible but inadvisable. It can be connected with technical malfunctions(which don't require Mayday call) or adverse weather. For instance, pilots performed a precaitionary landing due to gradualy developing engine trouble. The difference between PRECAUTIONARY LANDING and FORCED LANDING is that in first case flightcrew has the right to continue the flight whilst in the second case there is no such option.