B737-700 captain experienced an over speed during descent in VNAV with the speed programed to .78/28...

Attributes
ACN	1322986
Time
Date	201601
Local Time Of Day	1201-1800
Place
Locale Reference	ZMA.ARTCC
State Reference	FL
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	B737-700
Operating Under FAR Part	Part 121
Flight Phase	Descent
Flight Plan	IFR
Component
Aircraft Component	Autoflight System
Person 1
Function	Captain Pilot Not Flying
Qualification	Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 129
Events
Anomaly	Aircraft Equipment Problem Less Severe Deviation - Speed All Types

Narrative:

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.

Original NASA ASRS Text

Title: 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.

Narrative: 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.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.

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.