Turbine engine ignition systems usually operate for a brief period because the combustor only needs an initial spark. Once ignited, the ignition system is switched off and the fuel remains burning. However, there is potential for the engine to flame out— it'll stop burning. Because of this possibility, continuous ignition is used during critical periods, such as during takeoff, landing, and in emergency situations. This is different from a reciprocating engine, in which the sparks are timed to ignite during various times in the operating cycles.