During the APU operation, which component measures the operating time?

The correct choice relates to the usage of an hour meter, which is specifically designed to measure the operating time of machinery and equipment, including the Auxiliary Power Unit (APU). This device accumulates operational hours, allowing maintenance personnel and operators to track how long the unit has been in use.

Hour meters play a crucial role in maintenance scheduling, as they provide essential data regarding the runtime of the APU, which helps in determining when maintenance or inspections are due based on manufacturer recommendations or operational requirements. This tracking is critical for ensuring reliability and efficiency in both ground and flight operations.

In contrast, other options serve different functions. For instance, a fuel gauge measures the amount of fuel remaining, a voltmeter gauges electrical voltage, and a tachometer indicates the engine speed expressed in revolutions per minute (RPM). Each of these instruments provides important data, but none specifically tracks operating time like an hour meter does.