What is an alternate static source?

• A. It senses ambient atmospheric pressure for several instruments
• B. It is a secondary static pressure source used if the primary static port is blocked
• C. It provides total pressure
• D. It measures dynamic pressure

Answer: (B)

Alternate static source is a backup static pressure supply used if the primary static port becomes blocked. The static pressure reference feeds instruments like the altimeter and vertical speed indicator; when the main port is blocked, the backup line taps into another source of ambient pressure so those instruments can still function. Because this alternate source isn’t exactly the outside ambient pressure, readings on the altimeter and VSI may be biased or momentarily off when switching to or relying on it, until the system stabilizes. This source only handles static pressure and does not provide total pressure or dynamic pressure—the pitot tube handles total pressure, and dynamic pressure is derived from the difference between total and static.