How many PSI signifies the start of the red arc on the cabin differential pressure gauge?

The red arc on the cabin differential pressure gauge indicates the maximum allowable cabin differential pressure limit during operation. In the King Air 350, the start of this red arc at 6.6 PSI signifies that any value above this threshold could lead to structural issues or an unsafe operating environment for the aircraft.

Maintaining cabin differential pressure below the red arc is critical in ensuring passenger and crew safety, as exceeding this limit can compromise the integrity of the aircraft's cabin and systems. Understanding this limitation is part of operating the King Air 350 effectively and ensuring compliance with safety protocols. The other values provided, while related to differential pressure, do not accurately represent the threshold at which caution must be exercised, making 6.6 PSI the correct choice.