Narrative:

The aircraft was a winglet equipped B737-300. During the climb to our assigned/planned cruise altitude of FL360; the FMC indicated a max altitude of FL369 and an optimum altitude of approximately FL348. In accordance with the fom; we used the computed altitudes at 1.3G 0.74M (optimum of FL356) for our given gross weight to continue the climb in level change; .74M; smooth air; wings level; zero rudder/wing trim; and climb thrust. At FL355 the pilot flying noted that the aircraft leveled off and then began a very slow climb. At FL357 we noticed the MCP window had a flashing 'a' .74; indicating a minimum speed reversion; as the flight director and aircraft began a slight descent. The pilot flying disengaged the autopilot; and continued the descent as the pilot monitoring immediately coordinated a lower altitude (FL340) and checked the thrust/engines. The pilot flying then selected vertical speed (which indicated -150) and set a -500 descent to ensure control of mach and minimum mach (.691). There was no further abnormal indication/operation. It is our analysis that the flight director followed the (incorrect) data as generated by the FMC.crews should be made aware that the FMC/flight director system does not recognize performance calculator-generated data and will still fly within its own data-derived flight envelope. The company should investigate under what conditions this in-congruence of data/flight envelope may occur and make crews aware of these flight conditions.

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Original NASA ASRS Text

Title: A B737-300 equipped with winglets and a VNAV capable FMC was unable to climb to a flight level which the performance data indicated would be optimum.

Narrative: The aircraft was a winglet equipped B737-300. During the climb to our assigned/planned cruise altitude of FL360; the FMC indicated a max altitude of FL369 and an optimum altitude of approximately FL348. In accordance with the FOM; we used the computed altitudes at 1.3G 0.74M (optimum of FL356) for our given gross weight to continue the climb in Level Change; .74M; smooth air; wings level; zero rudder/wing trim; and climb thrust. At FL355 the Pilot Flying noted that the aircraft leveled off and then began a very slow climb. At FL357 we noticed the MCP window had a flashing 'A' .74; indicating a minimum speed reversion; as the flight director and aircraft began a slight descent. The Pilot Flying disengaged the autopilot; and continued the descent as the Pilot Monitoring immediately coordinated a lower altitude (FL340) and checked the thrust/engines. The Pilot Flying then selected Vertical Speed (which indicated -150) and set a -500 descent to ensure control of Mach and minimum Mach (.691). There was no further abnormal indication/operation. It is our analysis that the flight director followed the (incorrect) data as generated by the FMC.Crews should be made aware that the FMC/flight director system does not recognize performance calculator-generated data and will still fly within its own data-derived flight envelope. The Company should investigate under what conditions this in-congruence of data/flight envelope may occur and make crews aware of these flight conditions.

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.