2010-10 · NASA ASRS report 913409
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.
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.
In accordance with the FOM; we told ATC we could accept FL360 since we would be flying within the optimum altitude envelope. We did not have any adverse ride reports and the ride had been smooth in the climb. ATC cleared us to climb to FL360.I decided it had to do with the fact that the FMC software is predicated on a -300 with no winglets; which is why we use the performance calculator. Therefore; the FMC thought our airspeed was too slow for the conditions and it commanded minimum speed reversion even though the aircraft was capable of flight at FL360 according to the performance calculator; which recognizes the winglets (the performance calculator output displayed a 'Buffet Low' speed of .691 at FL360.)
The airline has a mixed fleet of this type; some with winglets and some without. The airline's directive is that they should utilize OPC derived performance data for cruise calculations for the winglet equipped aircraft. The reporter stated this event was unique in his experience.
More incidents for this aircraft family
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.
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