B737-300
Flight Crew Last 90 Days 281; Flight Crew Type 8550
Workload; Human-Machine Interface; Communication Breakdown; Confusion; Distraction
Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude; Deviation - Speed All Types; Deviation / Discrepancy - Procedural Clearance
Aircraft Automation Overrode Flight Crew; Flight Crew Overcame Equipment Problem; Flight Crew Took Evasive Action
Aircraft; Procedure
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.
We were initially filed for FL340. During the climb; we determined that the optimum altitude was FL363; so we requested FL360 with ATC. Passing FL350; I noticed that the aircraft was climbing very slowly because it was accelerating back to the Mach .74 that I had selected in VNAV (LNAV and VNAV were engaged with autopilot on). Around FL355 I noticed that the airspeed had increased to over Mach .77 but the aircraft was still climbing slowly. I monitored the aircraft; but did not take any action. I expected the autopilot to pitch up to reduce the speed and climb the extra 500 FT to cruise altitude. It did not and we received a buffet alert; and shortly thereafter an 'A' appeared in the airspeed window. I pulled the power back a bit at this point but was hesitant to disconnect the autopilot and climb manually. At this point the autopilot started a rapid descent. I disconnected it and smoothly leveled the aircraft around 35;000 FT. ATC called and asked if we were climbing. We let them know we were having autopilot problems and were going to try to continue the climb. I manually flew the aircraft back up toward FL360 at Mach .73-.74 and asked the Captain to set the flight director/autopilot back up so I could engage it. The autopilot would not re-engage and the 'A' was still flashing in the airspeed window. I asked the Captain to get clearance back to FL340. In the descent; we were able to re-engage the autopilot and flew the remainder of the trip uneventfully. ATC gave us a number to call when we landed and we did so. It seemed like they were more concerned with the autopilot failure then an altitude deviation. The autopilot seemed to be working; but slow to correct for large airspeed variations. There may have been a change in the winds around that altitude also.If I'd pulled the power back earlier; it may have prevented the buffet mode and the autopilot descent. That just seemed counterintuitive when I still wanted to climb 500 FT. I could also have disconnect the autopilot earlier and hand flown it to altitude. I just hadn't seen the autopilot that unresponsive before. Autopilot went into buffet mode and would not climb up to altitude.
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.