While
Pilot DAR hasn't said exactly what this is about I've assumed a light a/c that doesn't transmit extended information?
Have a look at this thread, and the dots will join up:
Cessna C208B down near Seattle, 4 deceased
From the ADS-B information presented in the link, either the Caravan was flying one knot faster than its 175 KIAS Vmo, at a hell of a rate down, or, if the descent rate is
not factored into the ADS-B "groundspeed", it was flying at an IAS significantly faster than Vmo. In either case, the plane could not stand what happened to it.