What can an APU's pneumatic power be used for?

An APU, or Auxiliary Power Unit, provides pneumatic power primarily to start engines and support air conditioning systems on aircraft. The pneumatic functions it supports are critical, especially during ground operations when the main engines are not running.

When it comes to engine starting, the APU supplies compressed air to start the main engines, especially in larger aircraft where engine start cycles demand high amounts of airflow. Additionally, the APU's pneumatic power is essential for providing bleed air to the environmental control system, which manages interior temperature and air quality by supplying conditioned air for the cabin.

While other options listed involve important systems and functions within an aircraft, they do not utilize the APU's pneumatic power in the same way. Increasing engine thrust involves fuel and combustion processes rather than pneumatic support. Flight control operations primarily rely on hydraulic systems for actuation, and navigation enhancements typically depend on electrical systems and avionics rather than the pneumatic capabilities of the APU. Thus, the correct answer highlights the key roles of starting engines and supporting air conditioning as primary uses for APU pneumatic power.