Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 145; Flight Crew Type 12000
Human-Machine Interface; Situational Awareness
Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude; Deviation - Speed All Types; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Loss Of Aircraft Control
Air Traffic Control Issued New Clearance; Flight Crew Became Reoriented; Flight Crew Regained Aircraft Control; Flight Crew Returned To Clearance
Aircraft
With the FMC Optimum Altitude indicating FL358; we elected to climb from FL340 to FL360 for a smoother ride. We used Level Change with .74 in the MCP to accomplish the climb. The aircraft leveled at FL355 and began to accelerate yet didn't climb as our airspeed touched the high speed buffet margin as indicated in the HUD. I elected to utilize the Vertical Speed mode of the MCP to trade the airspeed for altitude and reach FL360. The MCP immediately reverted to Level Change with .72 flashing in the window. I tried again but with the same result. With airspeed continuing to increase; I began to wonder if we were in a 'Mach Tuck' situation. I reduced thrust as the airspeed exceeded .76 and the aircraft began to descend. Even as early as the beginning of the event; I considered disengaging the autopilot and climbing; but was concerned that I would be overriding a built-in safety feature of the aircraft; potentially risking an upset condition. We coordinated our un-commanded descent to FL340 and began to investigate the reason for the event. We consulted the charts and compared our N1 and fuel flow indications. Chart N1's were 80.5 percent; yet ours were between 84 percent and 85 percent. Chart fuel flow expectations were around 2;100 PPH; yet the aircraft was indicating between 2;400 and 2;500 PPH. We then deduced that the likely cause was due to a combination of being heavier than we thought and light turbulence. Later we discovered that the event was called an 'Alpha Floor Excursion' and that it is covered in Distance Learning with the recommendation of disengaging the autopilot and continuing to climb. 1. An informal poll of fellow Pilots shows that few are aware of this; as most have yet to complete this training. Most of those who do know about it have had it happen to them! Apparently it was explained in an early (year) edition of (Company Publication); but that hardly seems like an effective way to communicate such an important issue. Publish a (Company Bulletin) explaining the anomaly.2. Providing a solution that requires disengaging the autopilot in RVSM airspace doesn't seem prudent. Recommendation: Fix the software. Until then; limit the older Fleet to FL350.3. Unexpected anomalies at high altitude present a certain hazard. Risk management would dictate that Company policies; procedures and equipment provide features intended to enhance safety. I recommend that items contrary to this theme be clearly and quickly communicated to the Pilots.
B737-300 Captain reports attempting to climb to FL360 with the FMC indicating the optimum altitude is FL358. For unknown reasons the aircraft will not climb above FL355 with the autopilot engaged and begins accelerating. The crew requests FL340 and is cleared to descend. Post flight investigation indicates that an alpha floor event was triggered.