The ignition characteristics of a plasma jet, a commercial jet and a spark gap igniter have been studied in a combustion bomb at sub-atmospheric pressures. It was found that the plasma jet igniter promoted increased flame growth, particularly for stoichiometric mixtures and extended the flammability limits to both richer and leaner mixture strengths. The rapid formation of ignition kernels large enough to be self-sustaining at low pressures may be of importance in the restarting of jet engines at high altitudes.