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gauges

alternate static source

lower air pressure inside

Due to the venturi effect of the air flowing around the fuselage, the air pressure inside the flight deck is lower than the exterior pressure.

—IFH pg 5-3 PDF 98

  1. The altimeter indicates a slightly higher altitude than actual.
  2. The ASI indicates an airspeed greater than the actual airspeed.
  3. The VSI shows a momentary climb and then stabilizes if the altitude is held constant.