What is boundary layer and why does it matter for static ports?

• A. The boundary layer is the layer of air near surfaces; sampling from within it can cause inaccurate static pressure readings.
• B. The boundary layer is the fuel-air interface; sampling affects engine efficiency.
• C. The boundary layer is a region of high velocity air far from surfaces.
• D. The boundary layer is only relevant to pitot tubes.

Answer: (A)

The boundary layer is the thin region of air that clings to and moves with a surface, where viscosity slows the flow and the velocity changes from zero at the surface to the free-stream value just outside. Static ports need to sense ambient, undisturbed pressure, so they should read from air outside this slowed, layered region. If a port encounters air still within the boundary layer, the pressure it measures can be shifted by the local flow gradient, leading to readings that are higher or lower than true ambient. That pressure error translates into incorrect altitude and vertical speed indications, since the static pressure feeds the altimeter and VSI in the pitot–static system. Hence boundary layer matters for static ports: placement must avoid sampling inside it to obtain accurate, untainted static pressure. The other ideas don’t fit because the boundary layer isn’t the fuel–air interface, isn’t a region of high velocity far from surfaces, and isn’t exclusive to pitot tubes.