What action should be taken if the ambient temperature is below -40°F (-40°C)?

When the ambient temperature is below -40°F (-40°C), the correct action to take is to perform an APU emergency start. This is because extremely low temperatures can significantly affect the starting capability of the Auxiliary Power Unit (APU). An emergency start procedure is designed to adapt to harsh conditions that may impede the regular start process. It accounts for the potential challenges posed by cold weather, such as increased viscosity in fluids and decreased battery efficiency, ensuring that the APU can operate effectively in critical situations.

The other actions, such as conducting a regular start or just monitoring the APU, may not adequately address the unique challenges presented by such low temperatures. Delaying the start until conditions improve might also lead to operational delays if immediate power is needed, particularly in mission-critical scenarios. Thus, utilizing the emergency start procedure is essential when facing extreme temperature conditions to ensure reliability and functionality of the APU.