What can excessive APU operation time lead to?

Excessive operation time of an Auxiliary Power Unit (APU) can lead to potential system failures and overheating due to several factors related to its design and function. The APU is designed to provide power and pneumatic air for starting the main engines and to support various electrical and pneumatic loads when the aircraft is on the ground. Prolonged use beyond recommended limits can result in overheating, which occurs as the APU components may exceed their thermal limits.

When an APU runs for extended periods, the increased temperatures can degrade lubricants and other internal components, leading to a higher risk of mechanical failure or malfunction. Furthermore, excessive operation can result in carbon buildup and other contaminants which can affect the efficiency and reliability of the unit. These issues underscore the importance of adhering to operational guidelines to ensure the longevity and reliability of the APU. Reducing operation time when unnecessary helps avoid these risks, ensuring that the APU remains functional for necessary operations.