Which statement best describes a boundary layer effect at a static port?

• A. A stable, accurate static pressure reading.
• B. Discrepancies between altimeter/VSI and outside air conditions or erratic static pressure readings.
• C. Pitot pressure equals static pressure.
• D. No impact on any instrument.

Answer: (B)

The important idea here is that the static port must sample undisturbed outside air pressure. The boundary layer is that thin region of air hugging the aircraft surface where viscous forces slow the flow and pressure can be altered by the surface and nearby geometry. If a static port sits in or near this layer, it doesn’t read the true ambient static pressure. Instead, it picks up a pressure influenced by the local flow, turbulence, or separation. That faulty pressure input causes the altimeter to show one value while the vertical speed indicator and actual outside air pressure suggest another, leading to discrepancies or erratic readings. In short, boundary layer effects at a static port produce inconsistent or incorrect static pressure readings, which is why altimeter and VSI readings can disagree with real outside conditions. The other options don’t describe this mismatch caused by sampling the wrong part of the flow—static pressure readings would not be reliably accurate, and the idea that pitot pressure equals static pressure isn’t how the systems operate in normal flight, and there is indeed an impact on instruments when the port is affected.