What is the limitation for left engine start on the ground using APU bleed air?

The limitation for starting the left engine on the ground using APU bleed air is 9,000 feet. This restriction is in place due to the decreasing efficiency of the APU's bleed air system at higher altitudes. At elevations beyond 9,000 feet, the atmospheric pressure becomes too low, impacting the APU’s ability to provide sufficient air pressure and flow to successfully start the engine. Thus, ensuring a reliable start is crucial for safety, leading to this specific altitude limitation. It is essential for pilots and ground operations to adhere to this restriction to maintain operational integrity and avoid potential engine start failures when using APU bleed air under less than ideal atmospheric conditions.