Components in the engine-mounted HMU box include an electrohydraulic servo valve (EHSV) driving a fuel-metering valve (FMV). Fuel flow to the engine is computed by the full-authority digital engine controller's (Fadec) electronic engine control unit, which sends a command to the HMU and receives feedback from the EHSV actuator, closing the control loop on desired fuel flow. Boeing says the events are “characterized” by a momentary oscillation of the FMV, which may indicate a control-loop malfunction caused by a contaminated fuel valve.

CFM has issued a change to the Fadec software through a service bulletin that Boeing says “reduces the duration and degree of thrust-instability events” by cycling the FMV and EHSV during engine shutdown to clean the EHSV spool. Boeing says the software change also slows the FMV response when the engine control logic detects a “poorly responding” FMV. The company says that no thrust instabilities have occurred since the new software was uploaded to the engines, though it does not see the fix as the final solution.

The airframer in January also issued a Flight Operations Bulletin to airlines advising pilots to use the existing quick-reference handbook procedures for engine surge or compressor stall events. The handbook instructs pilots to disengage the autothrottles and decrease the thrust levers until abnormal engine noises or engine performance indications (rpm and exhaust gas temperatures) return to normal levels. If the problem persists, they are instructed to shut down the engine and land “at the nearest suitable airport” using the single-engine checklist.

Problems with the HMU are not new to the CFM56. Installed on the aft-left side of the engine's accessory gearbox, the HMU controls the fuel sent to the combustor but also houses other control valves that operate the variable stator vanes, variable intercompressor bleed valve, turbine active-clearance-control system and fuel-nozzle staging.

Issues first cropped up early in 1998, shortly after the initial versions of the 737NG entered service. In at least two cases, the problems led to uncommanded acceleration to full power in flight and flame-outs on the ground. One of the uncommanded accelerations was due to contamination in the HMU and the other was triggered by a problem with the FMV resolver. The issues were corrected by a software change and manufacturing improvements.

Problems encountered by early operators, such as launch customer Southwest Airlines, which has the largest 737NG fleet in the U.S., included excessive fuel feed to the engines at start-up and associated low-frequency fluctuations in fuel pressure to the combustor.

With Guy Norris in Los Angeles.