B757-200
Flight Crew Total 18800
Confusion
Aircraft Equipment Problem Less Severe; Deviation - Altitude Crossing Restriction Not Met; Deviation - Altitude Overshoot; Deviation - Speed All Types; Deviation - Track / Heading All Types; Inflight Event / Encounter Weather / Turbulence
Air Traffic Control Issued New Clearance; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Returned To Clearance; Flight Crew Overcame Equipment Problem; General Maintenance Action
Procedure; Aircraft
I was the pilot flying as we were descending on the RNAV Arrival; approaching ZZZZZ Intersection which has a 210 KT at 9000' restriction. We were in night IMC conditions in moderate turbulence and rain. We had been cleared to delete all speed restrictions and maintain 250 knots. The left autopilot was engaged in LNAV and VNAV with speed intervene 250 knots selected. As we came within a few miles of ZZZZZ Intersection our speed started decelerating and I asked the first officer to ask Approach Control if he still wanted us to maintain 250 knots. As he was asking; the aircraft started a rapid pitch down going through 9100'. I disconnected the autopilot and found the aircraft was trimmed significantly nose down as I tried to level off at 9000' nearing ZZZZZ Intersection. At the same time we received a TCAS traffic advisory and I glanced down at the TCAS display on the center console but found that the turbulence rendered the display unreadable. Approach Control said he still wanted 250 knots; and still needed 9000' at ZZZZZ Intersection. I was unable to reverse the descent until between 8700' and 8600'. The first officer told the controller that we had an issue with the autopilot; and we were immediately cleared to 7000' as we were climbing back to 9000' over ZZZZZ Intersection. After I manually stabilized on course to ZZZZZ1 Intersection descending to 7000'; I re-engaged the left autopilot in LNAV and Flight Level Change. Shortly thereafter the aircraft started turning right and diverging from track. The flight director was commanding a turn left back towards the course while the aircraft was increasing its bank angle to the right. I selected the HDG Select roll mode and selected a heading to turn left to re-intercept the course. The aircraft continued its right turn without starting a roll reversal to the left; so I disconnected the autopilot again. We then were issued a speed reduction to 170 knots; descent to 3000'; and a 90 degree left turn vector for a base leg. After stabilizing the aircraft again manually; I engaged the right autopilot which worked normally for the remainder of the approach including a coupled ILS in VMC down to 200'AGL.I called the TRACON supervisor on the phone and asked if he was familiar with my flight number and the problem over ZZZZZ Intersection earlier. He indicated that he was; and that they had 'no issues.'Maintenance technicians told me that they did not find any stored faults in the avionics; but that the aircraft had a history of left autopilot write-ups with no faults found. I had written up a similar incident a few weeks earlier with the same roll mode faults; but no vertical mode issues. I believe; but could not confirm that it was the same aircraft. Technicians determined that the left flight control computer should be replaced. I was informed that this aircraft had a history of left autopilot write-ups where trouble-shooting found no faults and the aircraft was returned to service. A high percentage of my write-ups are repeats of earlier discrepancies noted by other crews or myself. From my perspective; this issue is becoming more common as these aircraft age. I suggest that Maintenance management initiate a tracking methodology that identifies repeat write-ups and requires more extensive troubleshooting. For critical systems such as autopilot malfunctions; the corrective action; 'Checked per [SOP] Okay for continued service' should not be allowed.
[Report narrative contained no additional information.]
B757 flight crew reported an autopilot malfunction which resulted in a missed crossing restriction and a track deviation. Reengaging the left autopilot caused more problems; but the right autopilot functioned normally.