Flight Crew Last 90 Days 217
Other / Unknown
Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude
Air Traffic Control Issued Advisory / Alert; Flight Crew Overcame Equipment Problem; Flight Crew Returned To Clearance; General Maintenance Action
Aircraft; Human Factors
At cruise flight; the Captain and I experienced an uncommanded descent and an uncommande altitude deviation of 600 FT from FL320 to FL314.  This altitude loss of 600 FT occurred (by my estimation) in less than 10 seconds. This event occurred so quickly that the Pilot Flying (Captain) had his hands on the controls; and he was attempting to correct the loss well before the 600 FT deviation took place by disengaging the autopilot to arrest the descent.  I attempted to assist the Captain between 400 FT and 500 FT of altitude deviation and I noted a rather strong nose down force present by how heavy the control wheel felt.  It appeared as though the autopilot had trimmed the aircraft nose low during cruise and then simply quit working.  It was also evidenced during the altitude recovery accomplished by the Captain that he had to input a fair amount of nose-up trim.Initially; I had assumed that the autopilot had entered into the Control Wheel Steering mode.  However; 32;000 FT was visible in the Altitude window of the MCP; and no changes or inputs had been made by either the PF or me (the PNF/PM). Shortly after this event took place; Center queried us regarding the altitude deviation.  In the heat of the moment I responded to the Center Controller that; 'We've just experienced an UNSCHEDULED descent'.  What I meant to say was; 'we've just experienced an UNCOMMANDED descent'. The Center Controller working our flight was helpful and asked us if we were in trouble and in need of further attention.  I explained that we had recovered the lost altitude by returning to FL320. An aircraft logbook entry was made regarding the nature and circumstance of this event; and the aircraft was met by Maintenance.  Absolute vigilance. Even during a low workload environment such as cruise flight.  This event has been somewhat unsettling to me.  In several thousand hours of flight time; I have NEVER experienced an uncommanded flight deviation of this nature outside of windshear.
B737-300 First Officer experiences a sudden pitch down at FL320 with the Captain flying. When the autopilot disengages great effort is required to arrest the descent at FL314 along with substantial nose up trim.