What is the minimum N2 required for starter assist at an altitude from 21,000 to sea level?

The correct answer highlights that the minimum N2 required for starter assist at altitudes from 21,000 feet to sea level is 7.2%. Understanding this limitation is crucial for safe aircraft operation, as it ensures that the engine has adequate rotational speed in the N2 compressor section to engage the starter and initiate the combustion process effectively.

At lower altitudes, the atmospheric pressure is higher, which can affect engine performance and efficiency differently compared to higher altitudes. Therefore, the specific percentage is established to prevent issues such as an unsuccessful engine start, which could result in critical operational hazards. The selection of 7.2% N2 as the minimum is based on performance data and safety margins outlined by the manufacturer, ensuring the engine is adequately prepared to start under varying conditions.

Recognizing the significance of these thresholds aids flight crews in operating within safe parameters and making informed decisions during engine start procedures.