Honeywell 131-9A FCU Instability and Automatic APU Shutdown on Boeing 737NG
A Boeing 737-800 Next Generation experienced an automatic APU shutdown during overnight ground operations after repeated exhaust gas temperature fluctuations and unstable acceleration characteristics were observed on the Honeywell 131-9A Auxiliary Power Unit. Maintenance troubleshooting identified progressive varnish contamination and restricted movement within the Fuel Control Unit metering assembly, resulting in unstable fuel scheduling during transient load conditions.
Event Summary
During overnight ground preparation at a European maintenance station, a Boeing 737-800 Next Generation experienced an automatic shutdown of the Honeywell 131-9A Auxiliary Power Unit approximately four minutes after startup. Flight crew initially observed unstable APU exhaust gas temperature indications accompanied by short-duration RPM oscillations during electrical load transfer from external ground power.
Operational Symptoms
Maintenance personnel documented several abnormal operating characteristics during troubleshooting, including elevated and fluctuating EGT indications, delayed acceleration response, intermittent RPM instability and transient fuel scheduling deviations during load transitions. The FADEC system subsequently initiated an automatic protective shutdown to prevent potential thermal exceedance within the APU turbine section.
Component Description
The Honeywell 131-9A Auxiliary Power Unit installed on Boeing 737 Next Generation aircraft supplies pneumatic bleed air and electrical power during ground operations and backup electrical scenarios. The Fuel Control Unit regulates fuel delivery throughout startup, acceleration and load transfer phases to maintain stable combustion and controlled turbine speed. Accurate fuel metering is critical for preventing combustion instability, hot starts and automatic shutdown events.
Maintenance Investigation
Initial troubleshooting included Built-In Test Equipment (BITE) data retrieval together with inspection of the ignition and fuel systems. Downloaded maintenance data revealed repeated transient fuel scheduling deviations during acceleration phases. The Fuel Control Unit was subsequently removed for bench inspection. Detailed examination identified varnish contamination within the FCU metering section, restricted movement of the servo valve assembly, abnormal wear patterns on internal actuator surfaces and carbon accumulation on two fuel spray nozzles. No faults were identified within the electronic control circuitry.
Root Cause Analysis
Engineering evaluation determined that progressive varnish buildup and intermittent sticking within the FCU servo metering assembly produced unstable fuel flow characteristics during transient operating conditions. Temporary restriction of the metering valve generated lean combustion periods followed by exhaust gas temperature fluctuations and unstable turbine acceleration. The resulting combustion instability activated the FADEC automatic shutdown logic designed to protect the APU turbine section from thermal damage.
Corrective Actions
Maintenance actions completed before aircraft release included replacement of Fuel Control Unit P/N 3506582-5, inspection and cleaning of the APU fuel nozzles, replacement of the APU fuel filter element, fuel contamination inspection and full operational testing under electrical and pneumatic load conditions. Maintenance personnel also performed EGT trend verification following engine runs and reviewed recent deferred maintenance records associated with the APU system.
Engineering Lessons
This event demonstrated how relatively small degradation within precision fuel metering assemblies can progressively destabilize APU combustion performance before complete operational failure occurs. Trend monitoring of EGT behavior, transient acceleration response and repeated start anomalies provides valuable early indication of Fuel Control Unit deterioration. The incident also reinforced the importance of contamination control and fuel cleanliness management within aircraft fuel systems operating under repeated thermal cycling conditions.