B737 flight crew reported being unable to achieve FL370 at a weight of 110.5 and with the FMC indica...

Attributes
ACN	1291626
Time
Date	201508
Local Time Of Day	0601-1200
Place
Locale Reference	IAH.Airport
State Reference	TX
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	B737 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	Climb
Flight Plan	IFR
Component
Aircraft Component	Angle of Attack Vane
Person 1
Function	Captain Pilot Not Flying
Qualification	Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 150
Person 2
Function	Pilot Flying First Officer
Qualification	Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 197
Events
Anomaly	Aircraft Equipment Problem Less Severe Deviation - Altitude Undershoot

Narrative:

We were cleared to climb to FL370. I checked the CDU and saw that the opt and maximum altitudes were 370. I checked the performance computer and the optimum altitude was also 370. We weighed 110.5 and were climbing through 36;500 ft MSL. We reached 36;600' and the IAS window on the MCP opened and displayed an 'a.' the autopilot then commanded a descent as it thought that we were in an under speed condition. We asked for and received a lower altitude and leveled at FL350. I cross checked the loadsheet with the inputed numbers and they all matched. I contacted dispatch via ACARS and informed him of the situation. He contacted ops and asked if there was freight or any other load that was not input into the loading schedule. Dispatch then informed us that there was no extra freight or bags and this was also confirmed upon arrival at the gate. Maintenance was called and I explained the situation to them and the outbound captain. Maintenance had no idea as to what to check and the captain accepted the aircraft with no further discussion.

Original NASA ASRS Text

Title: B737 flight crew reported being unable to achieve FL370 at a weight of 110.5 and with the FMC indicating that FL370 as optimum. At FL366 the airspeed window on the MCP opened and displayed an 'A' and the autopilot begin a gradual descent.

Narrative: We were cleared to climb to FL370. I checked the CDU and saw that the OPT and MAX altitudes were 370. I checked the performance computer and the optimum altitude was also 370. We weighed 110.5 and were climbing through 36;500 ft MSL. We reached 36;600' and the IAS window on the MCP opened and displayed an 'A.' The autopilot then commanded a descent as it thought that we were in an under speed condition. We asked for and received a lower altitude and leveled at FL350. I cross checked the loadsheet with the inputed numbers and they all matched. I contacted Dispatch via ACARS and informed him of the situation. He contacted Ops and asked if there was freight or any other load that was not input into the loading schedule. Dispatch then informed us that there was no extra freight or bags and this was also confirmed upon arrival at the gate. Maintenance was called and I explained the situation to them and the outbound Captain. Maintenance had no idea as to what to check and the Captain accepted the aircraft with no further discussion.

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 flight crew reported being unable to achieve FL370 at a weight of 110.5 and with the FMC indicating that FL370 as optimum. At FL366 the airspeed window on the MCP opened and displayed an 'a' and the autopilot begin a gradual descent.