Which of the following is a safety precaution during APU maintenance?

Wearing appropriate personal protective equipment (PPE) is a fundamental safety precaution during APU maintenance. PPE is essential in protecting maintenance personnel from various hazards that may be present when working on or around the APU. This includes risks associated with moving parts, electrical systems, fuel vapors, and high temperatures. Proper PPE—such as gloves, safety goggles, hearing protection, and flame-resistant clothing—ensures that workers can perform their tasks safely while minimizing the risk of injury.

In contrast to this, the other choices demonstrate practices that wouldn’t enhance safety. For example, wearing flip-flops is impractical and unsafe in a maintenance environment where the risk of dropping heavy objects or exposure to hazardous materials is present. Ensuring that the APU is operational does not relate to maintenance safety; rather, maintenance should occur when the APU is deactivated and properly secured. Ignoring the manufacturer's guidelines can lead to unsafe conditions, as those guidelines include critical safety protocols and operational procedures necessary for maintenance tasks.