Which accessory is directly coupled to the compressor drive shaft?

The accessory that is directly coupled to the compressor drive shaft is the APU AC generator. This generator is responsible for providing electrical power when the Auxiliary Power Unit (APU) is running, and it operates by being mechanically linked to the compressor drive shaft. This coupling allows the generator to spin at the same speed as the compressor, ensuring it produces the appropriate frequency of electrical output.

In an aircraft's power setup, the APU serves as a vital source of electrical power, and its integration with the compressor drive shaft is essential for efficient operation. This design allows for a straightforward transfer of mechanical energy, turning it into electrical energy that can be utilized for various aircraft systems when the main engines are not running.

The other components listed, while important in their own right, do not share this direct coupling with the compressor drive shaft. For example, the oil cooler fan is driven by airflow and operates to cool engine oil but is not directly linked to the compressor function. Similarly, the starter motor is involved in initiating the engine's operation but is not permanently coupled in the same way as the APU generator. The tachometer generator is used for providing engine speed information but also operates independently and is not directly linked to the compressor drive shaft.