underspeed

From under- +‎ speed.

underspeed (uncountable)

1. An excessively-low speed; a speed lower than the minimum operating speed.
   • 1961 August, “The English Electric Type 5 3,300 h.p. "Deltic" diesels for British Railways”, in Trains Illustrated, page 469:

     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.

   • 2006 October 10, NASA, “2.2.1 Voice Calls”, in Intact Ascent Aborts Workbook 21002‎, archived from the original on 25 June 2022, page 2-3:

     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.

• overspeed