At what altitude is the APU restricted from providing electrical power?

The correct answer reflects that the Auxiliary Power Unit (APU) is restricted from providing electrical power below 30,000 feet. This limitation is in place due to the operating parameters and safety regulations governing APUs. At altitudes below 30,000 feet, the aircraft typically relies on the engines to provide electrical power, which ensures that the systems are operating under the most efficient conditions and reducing the load on the APU, which is more vulnerable to performance issues in this range.

Typically, APUs are designed to operate best within certain altitude ranges, and their capability to generate power decreases at lower altitudes due to various atmospheric and engineering factors. It is essential for flight safety and aircraft management to adhere to these limitations, ensuring that the electrical systems are adequately powered during various phases of flight. This reinforces operational efficiency and reliability in the aircraft's systems.