B737 Undifferentiated or Other Model
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 150
Confusion
Aircraft Equipment Problem Less Severe; Deviation - Altitude Undershoot
Air Traffic Control Issued New Clearance; Flight Crew FLC complied w / Automation / Advisory; General Maintenance Action
Aircraft
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.
Filed for FL370. Upon climbout reaching FL366; the airspeed window opened and began flashing 'A.' The airplane leveled off and began a gradual descent. I was the PF. The PM immediately informed Center we needed to descend to FL350. The airplane was still in a descent and by the time we actually received clearance back down to FL350; we were nearly there anyway. Center asked if everything was ok and we indicated that we appeared to be operating normally; but couldn't achieve the planned altitude; and that we were troubleshooting and would get back to him.We double checked everything. We communicated with Dispatch and asked if freight had been loaded without informing the crew. They checked and said no. We checked everything we could think that would have this effect on the airplane. We checked the performance computer and it indicated FL370 was optimal. The weights were entered right; the atmospheric conditions were normal; nothing was adhering to the airplane; every switch was in the normal position.We came to the conclusion that we were either had additional weight on us and didn't know about it; or there was a flaw in the automation or sensing system that needed calibration; because every bit of information we had said we should have been able to achieve FL370 normally. I felt my Captain and I worked very well as a team and used the available resources to troubleshoot as best we could. The remainder of the flight was normal.
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.