The aircraft was cruising at flight level (FL) 350 with the autopilot engaged and 35,000 ft pre-selected in the altitude display on the autopilot mode control panel (MCP). The crew observed the aircraft make an uncommanded climb from FL350 to FL351, where it remained for approximately 20 seconds until the autopilot returned the aircraft to FL350. The crew then noticed that the `Mach Trim Fail' warning light had illuminated. The `Mach Trim Fail' light extinguished for a short time, re-illuminated, then extinguished again. During this sequence of events, the crew reduced the aircraft speed from Mach 0.76 to Mach 0.73 in accordance with the appropriate Quick Reference Handbook (QRH) procedure. Shortly after, the `Stab Out Of Trim' warning light illuminated. The `Stab Out Of Trim' warning extinguished after descent was commenced.
The descent was initiated using the autopilot and was normal until approximately FL250 when the aircraft pitched down steeply and the airspeed increased rapidly. The autopilot was disconnected, and the crew flew the aircraft manually, noticing that a considerable amount of back trim was required to stabilise the descent at 300 kts. The aircraft was flown manually for the remainder of the descent, and the aircraft was landed safely without further incident.
After landing, the pilot in command notified maintenance support of the in-flight problems that had been encountered. However, he did not lodge an incident report with the operator until 2 days after the occurrence. Upon receipt of the incident report, the operator took action to recover the aircraft's digital flight data recorder (DFDR). However, the DFDR was found to be faulty, and information from the flight was not available. The operator tested the DFDR and reported that the recorder would intermittently return the recording head to a random section of tape, resulting in existing data being overwritten. In order to obtain data about the aircraft flight path, air traffic control radar information was examined during the investigation.
The operator also reported that during the subsequent maintenance investigation, the K11 relay in the autopilot accessory unit was found to have an intermittent fault when it was subjected to initial testing. However, further testing was unable to reproduce the fault that was experienced in flight, and both the K11 and K6 relays were replaced as a precaution. The "Mach Trim Actuator" was also tested by the operator. The tests were conducted under various temperature conditions but they revealed no indication of defect. The manufacturer was also unable to fault the actuator. The aircraft manufacturer suggested that the operator verify sections of the aircraft wiring and perform the Mach trim and stabiliser rigging checks specified in the Boeing 737-300 Maintenance Manual.
The automatic flight system (AFS) of Boeing 737-376 aircraft comprises the autopilot flight director system (AFDS) and the autothrottle (A/T). Management of the vertical and lateral navigation of the aircraft is controlled by pilot selection of appropriate AFDS MCP settings in conjunction with the flight management computer (FMC). Normally, once an aircraft is airborne and the AFS engaged by the crew, the AFDS and A/T are controlled automatically by the FMC to fly an optimised lateral and vertical flight path throughout the climb, cruise and descent phases of a flight.
Data entry into the FMC is made by the crew on the flight management system control display unit (CDU) fitted to the cockpit centre stand. The CDU displays various "pages" of information that are required for the management of the flight. The crew will enter relevant data into these pages, including the intended cruise level.
Once an aircraft reaches the planned cruise level, it accelerates from the climb speed to the cruise speed. During acceleration, rearward movement of the wing aerodynamic centre of pressure results in a tendency for the aircraft to pitch nose down in a phenomenon known as "Mach Tuck". The Boeing 737-376 is equipped with a Mach Trim System (MTS) to provide improved stability at the high altitudes and airspeeds typically encountered during the cruise phase of flight. The MTS functions automatically at speeds in excess of Mach 0.615 and causes elevator adjustment as speed increases. Failure of the MTS is indicated by the illumination (amber) of the Mach Trim Fail light, and the crew response is to limit the aircraft speed to below Mach 0.74.
During cruise and with the autopilot engaged, the AFS causes the horizontal stabiliser to be trimmed to a position which will ensure that the pitch of the aircraft maintains the required cruise level. The Stabiliser Out Of Trim (Stab Out Of Trim) light illuminates amber to alert the crew if the autopilot is not trimming the stabiliser properly and that the elevator position exceeds a certain value in relation to the stabiliser and/or that the elevator is positioned too far away from the neutral position. The crew response for a "Stab Out Of Trim" amber warning is to disengage the autopilot and re-trim the stabiliser. The autopilot may then be re-engaged as required.