If the primary static port is blocked, which source is commonly used to obtain static pressure?

• A. Cabin differential pressure sensor
• B. Pitot tube dynamic pressure
• C. Alternate static source
• D. Flight deck pressure relief valve

Answer: (C)

Static pressure is what the altimeter and vertical speed indicator rely on to show altitude and rate of climb or descent. When the primary static port is blocked, an alternate static source is used to feed the same instruments with ambient pressure, ensuring they continue to operate. This alternate source is typically connected from a cabin or dedicated port and routed into the static system through a valve, so the instruments still have a reference pressure even with the blocked port.

Using this source means the readings won’t match true ambient conditions exactly, so you’ll see some instrument indication errors: the altimeter and VSI may diverge from real altitude and rate, and the airspeed indicator can also show slight deviations due to the different static pressure compared to the outside atmosphere. The other options don’t provide a source of static pressure to the instruments: the cabin differential sensor is part of the pressurization system, the pitot tube supplies dynamic pressure for airspeed (not static pressure), and the flight deck pressure relief valve isn’t used to feed static pressure to the instruments.