Underspeed

An underspeed switch drops the load off if engine speed falls to 380 r.p.m. and an overspeed governor shuts the engine off if its speed rises above 1,700 r.p.m.

These boundaries can vary from prelaunch predictions due to changing atmospheric conditions and variations in SRB and main engine performance. Using this real-time information, a computer and software program called the Abort Region Determinator (ARD) allows MCC to predict the underspeed that will result from a loss of performance (engine failure, throttle lockup, degraded thrust, etc.). This information is then used to predict the actual abort mode boundaries on which the real-time voice calls are made. These boundaries vary from pre-flight predicted boundaries.