Describe how an APU fuel control would respond to a lean fuel condition.

• A. It would adjust metering to maintain proper combustion; a fault could cause flameout or abnormal EGT and RPM.
• B. It would shut down immediately to protect the engine.
• C. It would ignore the condition and continue operation.
• D. It would increase fuel to maximum to correct the lean condition.

Answer: (A)

When the mixture is lean, there is too much air for the amount of fuel. The APU fuel control actively meters fuel to match the air flow and keep combustion stable. It senses operating conditions such as speed and exhaust temperature and increases fuel flow to restore the proper fuel–air ratio, helping maintain steady RPM and safe temperatures.

If a fault affects the fuel-control system or its sensors, the control may fail to deliver the needed increase in fuel, which can lead to flameout or abnormal exhaust-gas temperature and RPM readings. This is why the correct response is to adjust metering to achieve proper combustion, with faults posing a risk to stable operation.