Narrative:

In the climb approaching FL350; I noticed a very slight vibration. I also noticed that the airspeed was increasing above the programmed .74 mach. I called the airspeed to my first officer's attention and he verbally responded. During that time I saw us approaching FL354 and also noticed the vibration turning into a rumble. I looked down and the airspeed was now around .77 mach and a buffet alert appeared on the FMS. I called for an immediate airspeed reduction and my first officer with his hand on the throttles and yoke again verbally responded giving me the impression he was taking corrective action. The aircraft started pitching over to level flight but I saw or noticed no airspeed reduction as the alfa symbol started flashing in the airspeed window. With the autopilot still engaged; I was ready to take control when I commanded my first officer to turn off the autopilot; reduce thrust; and fly the aircraft. He did; but as he reduced thrust; he started a descent. Again; and ready to take control; I commanded to level off. My first officer arrested the descent around FL350; after losing about 400 ft. During this event; center called us and requested a verification of our altitude. I told the controller that we may have experienced an automation problem triggering the loss of 400 ft; but since have turned off the automation and are able to reestablish a climb to FL360. I instructed my first officer to manually initiate a climb back up to FL360; but it appeared that my first officer was struggling with getting a manual climb established. I was starting to question if I may have misinterpreted the performance computer; FMC; and/or the aircraft's optimum climb and cruise performance; and therefore to increase our margin of safety; I requested a decent to FL340 rather than a climb to FL360. My first officer started a descent and leveled off manually at FL340. All automation was reengaged and the performance data was reviewed and it all was correct. Since everything checked; we continued to our destination and the remaining flight went uneventful. After the event; my first officer stated that he was trying to correct the airspeed deviation with automation and did not understand why the flight director would not command a higher pitch up to increase the rate of climb to maintain the programmed .74 mach. He also stated that the autopilot pitched the aircraft over. I would like to sum the event up by saying maybe I should have taken control of the aircraft. It is easy to criticize and second guess when you're sitting next to someone or observing; but the fact is that things happened very rapidly and this event did border line my comfort level. My question to my first officer was why he hesitated so long to reduce thrust and disengage the autopilot when the automation was obviously not working for him. I wanted to give my first officer the chance to properly correct the situation; but was at all times ready to take control if he would not have turned off the autopilot and reduced trust when I commanded him to do so.

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Original NASA ASRS Text

Title: A B737-300 crew reported that having calculated an optimum cruise altitude of FL363; the aircraft would not climb above FL355 and with the 'A' alfa alert displayed speed increased to MACH .77 with a BUFFETT ALERT. The flight descended to FL340 where flight directors cycled OFF/ON which reset the system to normal operation.

Narrative: In the climb approaching FL350; I noticed a very slight vibration. I also noticed that the airspeed was increasing above the programmed .74 Mach. I called the airspeed to my First Officer's attention and he verbally responded. During that time I saw us approaching FL354 and also noticed the vibration turning into a rumble. I looked down and the airspeed was now around .77 Mach and a buffet alert appeared on the FMS. I called for an immediate airspeed reduction and my First Officer with his hand on the throttles and yoke again verbally responded giving me the impression he was taking corrective action. The aircraft started pitching over to level flight but I saw or noticed no airspeed reduction as the Alfa symbol started flashing in the airspeed window. With the autopilot still engaged; I was ready to take control when I commanded my First Officer to turn off the autopilot; reduce thrust; and fly the aircraft. He did; but as he reduced thrust; he started a descent. Again; and ready to take control; I commanded to level off. My First Officer arrested the descent around FL350; after losing about 400 FT. During this event; Center called us and requested a verification of our altitude. I told the Controller that we may have experienced an automation problem triggering the loss of 400 FT; but since have turned off the automation and are able to reestablish a climb to FL360. I instructed my First Officer to manually initiate a climb back up to FL360; but it appeared that my First Officer was struggling with getting a manual climb established. I was starting to question if I may have misinterpreted the performance computer; FMC; and/or the aircraft's optimum climb and cruise performance; and therefore to increase our margin of safety; I requested a decent to FL340 rather than a climb to FL360. My First Officer started a descent and leveled off manually at FL340. All automation was reengaged and the performance data was reviewed and it all was correct. Since everything checked; we continued to our destination and the remaining flight went uneventful. After the event; my First Officer stated that he was trying to correct the airspeed deviation with automation and did not understand why the flight director would not command a higher pitch up to increase the rate of climb to maintain the programmed .74 Mach. He also stated that the autopilot pitched the aircraft over. I would like to sum the event up by saying maybe I should have taken control of the aircraft. It is easy to criticize and second guess when you're sitting next to someone or observing; but the fact is that things happened very rapidly and this event did border line my comfort level. My question to my First Officer was why he hesitated so long to reduce thrust and disengage the autopilot when the automation was obviously not working for him. I wanted to give my First Officer the chance to properly correct the situation; but was at all times ready to take control if he would not have turned off the autopilot and reduced trust when I commanded him to do so.

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.