Hydraulic transmission systems are occasionally used in aircraft to connect the aircraft engine to the generator or other accessory components. These systems provide mechanical power transfer through hydraulic fluid instead of a direct mechanical linkage. The hydraulic transmission system typically includes a hydraulic motor and a hydraulic pump to transmit power from the engine to the generator. Here's how it generally works:

1. Hydraulic Pump: The hydraulic pump is typically driven by the aircraft engine's mechanical output, such as a driveshaft or gearbox. The pump converts mechanical energy from the engine into hydraulic energy by pressurizing hydraulic fluid (usually hydraulic oil).

2. Hydraulic Fluid: The hydraulic fluid is circulated through the hydraulic system under pressure. It serves as the medium for power transmission. Hydraulic fluids are selected for their lubricating properties, temperature resistance, and overall performance characteristics.

3. Hydraulic Motor: The hydraulic motor is connected to the generator or other accessory being driven. When hydraulic fluid under pressure is supplied to the hydraulic motor, it converts hydraulic energy back into mechanical energy by rotating a shaft. This rotation powers the generator.

4. Control Valves: The hydraulic transmission system includes control valves that regulate the flow of hydraulic fluid to the hydraulic motor. By controlling the flow rate and pressure of the hydraulic fluid, the system can regulate the speed and torque delivered to the generator.

5. Hydraulic Lines: A network of hydraulic lines carries the pressurized hydraulic fluid from the pump to the hydraulic motor and back to the reservoir for recirculation. These lines are carefully designed and maintained to ensure proper flow and pressure.

6. Reservoir: The hydraulic system typically includes a reservoir or tank to store and supply hydraulic fluid. It also helps in cooling the hydraulic fluid and removing air and contaminants.

Benefits of using hydraulic transmission between the aircraft engine and the generator may include:

- Variable Speed: Hydraulic transmission allows for variable speed control of the generator, which can be advantageous for different operating conditions or to match the generator's output to specific requirements.

- Redundancy: Some aircraft designs use hydraulic transmissions to provide redundancy and backup power generation systems. If the primary mechanical drive fails, the hydraulic system can still operate the generator.

- Vibration Isolation: Hydraulic systems can help isolate vibrations and shocks between the engine and the generator, contributing to overall system reliability and reduced wear and tear.

It's important to note that hydraulic transmission systems in aircraft are not as common as mechanical drives or electrical systems for power generation. Their use depends on specific aircraft design requirements and the intended application. Maintenance and monitoring of hydraulic systems are critical to ensure their proper function and reliability in aviation applications.