How might ambient temperature affect APU starting and operation?

How might ambient temperature affect APU starting and operation?

• A. Ambient temperature has no effect on APU performance.
• B. Cold conditions can increase start time and fuel needs; hot conditions can raise EGT and cooling demands.
• C. Cold conditions can increase start time and fuel needs; hot conditions can raise EGT and cooling demands; extreme temps affect lubricant and performance.
• D. Hot conditions always reduce cooling requirements.

Answer: (C)

Ambient temperature changes how the APU starts and runs by altering lubrication, fuel behavior, and heat management. In cold conditions, oil and other lubricants stiffen, making it harder for moving parts to turn and reach light-off. The starter may have to work longer, which increases fuel use, and battery performance can drop in the cold, making cranking less vigorous. In hot conditions, air density falls, reducing the mass flow through the compressor and making it easier for the turbine to run hotter, so exhaust gas temperatures rise during operation. The cooling system then has to remove more heat to keep temperatures in range, and lubricants can behave differently at elevated temperatures, potentially reducing lubrication effectiveness. Extreme temperatures amplify these effects, so lubrication and overall performance can be pushed toward limits in both directions. This combination explains why cold increases start time and fuel need, hot raises EGT and cooling demands, and extreme temps impact lubrication and performance.

Ambient temperature changes how the APU starts and runs by altering lubrication, fuel behavior, and heat management. In cold conditions, oil and other lubricants stiffen, making it harder for moving parts to turn and reach light-off. The starter may have to work longer, which increases fuel use, and battery performance can drop in the cold, making cranking less vigorous. In hot conditions, air density falls, reducing the mass flow through the compressor and making it easier for the turbine to run hotter, so exhaust gas temperatures rise during operation. The cooling system then has to remove more heat to keep temperatures in range, and lubricants can behave differently at elevated temperatures, potentially reducing lubrication effectiveness. Extreme temperatures amplify these effects, so lubrication and overall performance can be pushed toward limits in both directions. This combination explains why cold increases start time and fuel need, hot raises EGT and cooling demands, and extreme temps impact lubrication and performance.