Which component converts air pressure into mechanical energy in an aircraft?

The correct answer is the pneumatic starter, which is designed to convert compressed air (air pressure) into mechanical energy. In an aircraft, pneumatic starters are often used to initiate the operation of gas turbine engines. By using compressed air, which is harnessed from auxiliary power units (APUs) or ground sources, the pneumatic starter spins the engine's turbines and allows it to reach the speeds necessary for combustion to occur.

This method of starting the engine is effective because it utilizes the high energy content of compressed air to create the initial mechanical motion required for engine ignition and startup. This mechanism is particularly useful in situations where electrical power may not be readily available or reliable, thus providing an efficient means to start the engines in various operational environments.

On the other hand, while fluid couplings do transfer power between rotating shafts via hydraulic fluid, they do not specifically convert air pressure into mechanical energy. Electric motors convert electrical energy into mechanical energy, which is a different principle altogether. Gearbox assemblies, meanwhile, are used to change the speed, torque, or direction of mechanical energy but do not initiate engine operations. Understanding these distinctions helps clarify why the pneumatic starter is the answer to the question.