The alternate static pressure valve switches from which static pressure ports to which pressure in the event of icing or an obstruction?

• A. External static ports to Internal Cabin pressure
• B. Internal to External pressure
• C. External to External pressure
• D. Internal to Internal pressure

Answer: (A)

When icing or an obstruction blocks the normal static ports, the system uses an alternate static source to keep the air data system referencing a pressure. The valve is designed to switch from the external static ports to the internal cabin pressure, so the instruments still receive a static pressure reference even though the outside ports aren’t usable. Because cabin pressure is higher than outside atmospheric pressure at altitude, the altimeter and airspeed indicators can behave differently once the switch is made (altimeter may read lower than actual, and indicated airspeed can shift). This switch to the cabin pressure is the intended method to maintain usable air data when external static sources are compromised.