Before starting the C-5 APU, what is crucial to check?

Before starting the C-5 APU, checking the ambient temperature is crucial because the performance and operational efficiency of the APU can significantly vary with temperature conditions. Extreme temperatures can affect the APU's ability to generate power, start properly, and maintain optimal operating conditions. For example, starting an APU in very cold conditions may necessitate additional warm-up time or specific operational procedures to ensure that it functions correctly and harmoniously with the aircraft's systems.

While other factors, such as fuel quality and oil pressure, are also important for the overall operation and longevity of the APU, the ambient temperature's role in starting and initial operation makes it a critical check before beginning the APU sequence. Ensuring that the APU is within the appropriate temperature range is essential to avoid complications that could arise during startup or subsequent operations.