You want to be able to walk away. If you have two engines you will be able to fly away if you follow the right steps regarding aircraft handling/speed. (But there are different certification requirements for transport category aircraft and light twins). A single engine aircraft is heading down; it is just a matter of where it will land. A high-performance single engine aircraft (military jet trainer) may be able to complete a turnback procedure, but a light single GA aircraft will only have the choice of a small turn (<90*) and gliding to the landing area. In almost every case, if the aircraft was in controlled flight the crash is survivable. Keep cool!