What is the impact of high ambient temperatures on APU operation?

High ambient temperatures negatively impact APU efficiency primarily due to the reduction in the differential temperature that drives the thermodynamic cycle. As ambient temperatures rise, the APU's ability to cool its components diminishes. This leads to a decrease in performance because the APU is designed to operate optimally within a specific temperature range.

Under high heat conditions, the airflow through the APU may be reduced or less efficient, leading to higher operational loads and potential overheating. Consequently, this scenario decreases the overall efficiency of the APU, making it harder for the unit to produce the necessary power and pneumatic pressure for aircraft systems.

The other choices suggest scenarios that either positively impact performance, indicate no effect, or imply a drastic shutdown, which do not align with the operational realities and physical limitations of an APU in high temperature conditions.