Narrative:

While cruising at fl 410 and just prior to the top of descent the MCP went from VNAV to altitude hold and the IAS/mas window opened and went to 146 knots as commanded by what the FMC was telling it to do. (This was on its own with no pilot input.) the first officer was flying and he immediately caught the anomaly. He clicked the auto throttles off and maintained proper power to maintain proper airspeed. Since we were at top of descent I asked for lower and we started down. The first officer selected level change but the aircraft appeared to not be complying with this command. (The 'B' autopilot was on.) he then went to vertical speed and started our descent with that command. Sometime during the descent; the IAS/mas window showed a flashing 'O' (over speed) at a speed of 270 knots in the window. We needed to get down so he deployed the speed brake to keep the speed below 300 knots. We were nowhere near an over speed as indicated by all three captain's; first officer's and stby mach/airspeed indicators. The flaps were in the proper position of zero; there were no lights on the overhead or fwd panel; and the handle was physically checked (numerous times) to verify it was seated properly. Somewhere around fl 180 the FMC and mach window returned to the normal settings for that phase of flight. On the ground I contacted dispatch and maintenance control and advised them and wrote up the airplane. The maintenance controller was puzzled; as were we; and he informed me that they would send us two mechanics to try and troubleshoot the problem. He suspected it may have been a cold soaked flap sensor.

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Original NASA ASRS Text

Title: A B737-700 may have had a flap position sensor malfunction affecting autoflight IAS/MACH commands from the top of descent down to a lower altitude.

Narrative: While cruising at FL 410 and just prior to the Top of Descent the MCP went from VNAV to ALT HOLD and the IAS/MAS window opened and went to 146 knots as commanded by what the FMC was telling it to do. (This was on its own with no Pilot input.) The First Officer was flying and he immediately caught the anomaly. He clicked the auto throttles off and maintained proper power to maintain proper airspeed. Since we were at Top of Descent I asked for lower and we started down. The First Officer selected Level Change but the aircraft appeared to not be complying with this command. (The 'B' autopilot was on.) He then went to Vertical Speed and started our descent with that command. Sometime during the descent; the IAS/MAS window showed a flashing 'O' (over speed) at a speed of 270 knots in the window. We needed to get down so he deployed the speed brake to keep the speed below 300 knots. We were nowhere near an over speed as indicated by all three Captain's; First Officer's and STBY Mach/Airspeed indicators. The flaps were in the proper position of ZERO; there were no lights on the overhead or FWD panel; and the handle was physically checked (numerous times) to verify it was seated properly. Somewhere around FL 180 the FMC and Mach Window returned to the normal settings for that phase of flight. On the ground I contacted Dispatch and Maintenance Control and advised them and wrote up the airplane. The Maintenance Controller was puzzled; as were we; and he informed me that they would send us two Mechanics to try and troubleshoot the problem. He suspected it may have been a cold soaked flap sensor.

Data retrieved from NASA's ASRS site as of April 2012 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.