Which statement best describes how sampling static pressure from within the boundary layer affects readings?

• A. Sampling from within the boundary layer is ideal and yields the most accurate reading.
• B. Static pressure sampled from outside the boundary layer is always wrong.
• C. Sampling from within the boundary layer can cause inaccurate static pressure readings.
• D. The boundary layer has no effect on static pressure readings.

Answer: (C)

Static pressure should come from undisturbed air, not from the viscous, velocity-changing region next to the surface. The boundary layer is where air slows to zero at the surface and speeds up to the outer flow, with pressure and velocity varying across that thin layer. A static port sitting inside that region samples a pressure influenced by local flow conditions, turbulence, and viscous effects, so the value it reads isn’t the true freestream static pressure. Because the pitot-static system depends on accurate static pressure to determine airspeed, taking the sample from within the boundary layer can lead to readings that are off from the actual ambient pressure. Placing the static port outside the boundary layer yields a more representative measurement of the ambient static pressure.