During which condition is APU efficiency most likely impacted?

The efficiency of the Auxiliary Power Unit (APU) is most likely impacted during severe weather conditions due to various factors that can affect its performance. Severe weather, such as heavy rain, snow, or icing conditions, can create challenges for the APU. For instance, extreme temperatures can affect fuel and oil viscosity, leading to compromised combustion efficiency and potential mechanical issues.

In addition, precipitation can lead to moisture ingress in the APU's components, potentially causing electrical short circuits or other malfunctions. Furthermore, strong winds can affect the airflow necessary for cooling the APU, again impacting its efficiency. Therefore, while other situations may impose various operational stresses, severe weather conditions uniquely create direct threats to APU performance and reliability.