If the Pitot drain hole is open but the static hole is blocked, which readings are affected?

• A. ASI = inaccurate reading, Altimeter = freezes, VSI = 0
• B. ASI = 0, Altimeter = freezes, VSI = 0
• C. ASI = inaccurate reading, Altimeter = 0, VSI = 0
• D. ASI = inaccurate reading, Altimeter = freezes, VSI = 2

Answer: (A)

The pitot–static system relies on a correct static pressure reference to interpret ram (pitot) pressure for airspeed and to sense ambient pressure for altitude and vertical speed. When the static hole is blocked, the static pressure inside the system can’t follow ambient changes as you climb or descend. That means the altimeter and the vertical speed indicator lose their correct reference: the altimeter can’t register real altitude changes and will appear to freeze, while the VSI shows zero because there’s no change in static pressure inside the system.

The airspeed indicator uses the difference between pitot (total) pressure and static pressure. With the static port blocked, this reference is no longer ambient, so the IAS indication becomes unreliable or inaccurate, even though the pitot line itself isn’t obstructed. The open pitot drain hole doesn’t fix the blocked static port, so it won’t restore normal readings for all instruments.