Which sensors provide critical data for APU control and what do they monitor?

• A. Bleed air pressure sensors (air supply) and EGT sensors
• B. N2 sensor (speed), EGT sensor (exhaust temperature), oil pressure/temperature sensors (lubrication), and bleed air pressure sensors (air supply)
• C. Oil pressure sensor only
• D. N1 sensor only

Answer: (B)

The main concept here is what data the APU control system relies on to operate safely and effectively. To regulate fuel flow and maintain the desired speed, the control unit needs a reliable speed reference from the N2 sensor, which tracks the core turbine speed. It also needs to watch turbine health and prevent overheating, which is where the exhaust gas temperature (EGT) sensor comes in. Keeping the lubrication system healthy is vital for lasting operation, so oil pressure and oil temperature sensors are essential to detect lubrication problems early. Finally, the bleed air pressure sensors monitor the air supply available for starting, pneumatic systems, and loading requirements, ensuring there’s enough pressure to meet the APU’s demands.

Together, these sensors give a complete picture of how the APU is performing and whether any protective actions are needed. Relying only on a subset, like just bleed air and temperature, could leave the system blind to speed regulation or lubrication issues. Conversely, focusing on oil alone or on a single speed sensor misses other critical protections and control inputs.

That’s why the combination of N2 speed, EGT, oil pressure/temperature, and bleed air pressure sensors best fits the needs of accurate, safe APU control.