A computer system determines contingency landing sites for an aircraft, piloted or autonomous. The computer system decomposes data from various types of maps into multiple separate risk maps, which can include: one for the risk to people being present on the ground at the landing site; one for the risk of damage to property on the ground at the landing site; one for the risk of damage to the aircraft; and/or one for the risk to people on-board the aircraft. From these maps, the computer system generates a contingency landing site map that the aircraft can use to determine an appropriate landing site in the case of an emergency. The computer system can also perform landing simulations to determine the various risks.