What is used to keep critical parts of the aircraft ice-free?

The use of bleed air is crucial for maintaining critical parts of an aircraft ice-free. Bleed air is hot compressed air that is extracted from the engine's compressor section during operation. This heated air can be routed to various parts of the aircraft, such as the wings and empennage, to prevent ice formation, especially during flight in cold and moist conditions. Ice accumulation on surfaces can significantly affect the aircraft's aerodynamics and control, making the role of bleed air essential for safety and performance.

While other options such as compressed gas or hydraulic fluid may have applications in aircraft systems, they do not specifically serve the purpose of preventing ice formation in the critical areas of the aircraft. Nitrogen, typically utilized for pressurizing systems or as an inert gas, lacks the properties required to effectively melt or prevent ice accumulation. Therefore, bleed air stands out as the appropriate method for keeping vital aircraft components free from ice, enhancing operational safety and efficiency.