What is the vacuum gauge narrow green arc range for altitudes between 15,000 to 35,000 feet?

The narrow green arc range on the vacuum gauge for altitudes between 15,000 to 35,000 feet indicates the optimal vacuum pressure required for the proper operation of the aircraft's gyroscopic instruments. For the King Air 350, the correct specified range is 2.8 to 4.3 inches of mercury (inHg). This range is crucial as it ensures that the gyros spin at the appropriate rate to maintain stability and accuracy in flight instruments during those altitudes. Operating within this range is essential for reliable instrument performance and overall flight safety, as below or above these values can lead to instrument errors that may affect navigation and control. It is important for pilots to recognize this range, as it corresponds to the altitude operational envelope and ensures that they maintain adequate conditions for the aircraft's systems.