Supersonic speeds also threatened inlet stability. The inlet's central shock cone served as a compression surface, diverting air into the annular inlet. As the Lightning accelerated through Mach 1, the shock cone generated an oblique shock positioned forward of the intake lip. Known as a subcritical inlet condition, this was stable, but produced inefficient spillage drag. Around the Design Mach speed, the oblique shock was positioned just forward of the inlet lip and efficiently compressed the air without spillage. When travelling beyond the Design Mach, the oblique shock became supercritical, and supersonic airflow entered the inlet duct, which could only handle subsonic air. In this condition, the engine generated drastically less thrust and may result in surges or compressor stalls, these could cause flameouts or damage.