How might you identify a boundary layer effect at a static port?

• A. Discrepancies between altimeter/VSI and outside air conditions.
• B. Erratic engine instruments readings.
• C. Consistent altimeter readings with outside air.
• D. No effect on any instrument.

Answer: (B)

This question tests how a boundary layer condition at the static port affects the readings fed to the airplane’s instruments. The boundary layer is the thin layer of air hugging the aircraft surface where the flow is slowed and can be turbulent. If a static port samples air from within that layer, the pressure it senses is not the true freestream ambient static pressure and can vary with speed, angle of attack, and local disturbances. That contaminated or fluctuating static pressure propagates to any instrument that relies on static pressure as a reference or input. Engine instruments, in particular, can display erratic readings when their signals depend on this unstable pressure source, making the readings jump or vary unexpectedly. In contrast, if the static port were sampling clean freestream air, altimeter and VSI readings would stay consistent with the outside air, and there would be no pervasive instrument instability.