B737-700
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 129
Confusion
Aircraft Equipment Problem Less Severe; Deviation - Speed All Types
Flight Crew Took Evasive Action
Aircraft
Descending from FL300; we were in VNAV with a programmed speed of Mach .780/284 knots. I was running the landing numbers and we were in a normal descent rate. In the scope of 10-15 seconds; the plane went from .780 through .82 and right to the clacker. There was no wind shift; nor was engine de-ice used. The First Officer disconnected the autopilot and (too) abruptly pulled back on the speedbrake. It was as if the plane thought it was higher than it actually was; and sped up to compensate; which threw us right under the bus. This isn't the first time I've seen this occur; but this was the most abrupt. The rest of the flight was uneventful. Either lower the cost index or adjust the software to handle this reoccurring problem.
B737-700 Captain experienced an over speed during descent in VNAV with the speed programed to .78/284 for no apparent reason. Speed increased from .78 to .82 Mach; without turning on the engine anti ice or a wind shift.