PITOT TUBE BLOCKAGE
A pitot tube blockage can be caused by dirt, moisture, ice or even bugs. And, of course, aborted takeoffs after a failure to note “airspeed alive” on the takeoff roll, only to later find the pitot-tube cover still on, almost mockingly. Assuming a proper preflight inspection, ensuring the pitot tube is free and clear, obstructions can still unfortunately occur during flight. There are a few possible scenarios.
Blocked pitot tube with unblocked drain hole: This would result in the airspeed indicator reading zero. Because the pitot tube would not be able to sense any airflow, and the drain hole would let any residual air out, there would be no pressure differential for the airspeed indicator to measure.
Blocked pitot tube with blocked drain hole: With trapped dynamic pressure, the airspeed will indicate whatever speed was showing when the blockages occurred. If the aircraft’s static port remains unblocked, pressure will increase as the aircraft descends and decrease as the aircraft climbs.
If static pressure decreases during a climb, the trapped ambient pressure with allow the diaphragm to expand, showing an increase in airspeed. Conversely, descending into denser air will force the diaphragm to close, showing a decrease in airspeed. In short, the airspeed will act in a manner similar to an altimeter.
STATIC PORT BLOCKAGE
Blockages of the static port will affect all pitot/static instruments.
Airspeed Indicator: If the static port is blocked but the pitot tube remains clear, the ASI will function but not with accuracy. If speed remains constant, and the aircraft climbs or descends, the static pressure will result in changes to your airspeed indication. It’s the same principle as a blocked pitot tube but reversed.
A climb will result in a decrease in atmospheric pressure entering the pitot tube, which will result in the ASI indicating decreasing airspeed, causing the ASI to act as a “reverse altimeter.” As long as the pitot tube is not blocked, changes in airspeed will reflect on the ASI, but not accurately.
Vertical Speed Indicator: With no changes to static air, there can be no differential pressure for the VSI to work with. With no differential pressure, the VSI will be stuck at zero, and you’ll marvel at how well you’re maintaining altitude.
Altimeter: The altimeter will freeze at whatever altitude was indicated when the static port was blocked.
PITOT TUBE BLOCKAGE
A pitot tube blockage can be caused by dirt, moisture, ice or even bugs. And, of course, aborted takeoffs after a failure to note “airspeed alive” on the takeoff roll, only to later find the pitot-tube cover still on, almost mockingly. Assuming a proper preflight inspection, ensuring the pitot tube is free and clear, obstructions can still unfortunately occur during flight. There are a few possible scenarios.
Blocked pitot tube with unblocked drain hole: This would result in the airspeed indicator reading zero. Because the pitot tube would not be able to sense any airflow, and the drain hole would let any residual air out, there would be no pressure differential for the airspeed indicator to measure.
Blocked pitot tube with blocked drain hole: With trapped dynamic pressure, the airspeed will indicate whatever speed was showing when the blockages occurred. If the aircraft’s static port remains unblocked, pressure will increase as the aircraft descends and decrease as the aircraft climbs.
If static pressure decreases during a climb, the trapped ambient pressure with allow the diaphragm to expand, showing an increase in airspeed. Conversely, descending into denser air will force the diaphragm to close, showing a decrease in airspeed. In short, the airspeed will act in a manner similar to an altimeter.
STATIC PORT BLOCKAGE
Blockages of the static port will affect all pitot/static instruments.
Airspeed Indicator: If the static port is blocked but the pitot tube remains clear, the ASI will function but not with accuracy. If speed remains constant, and the aircraft climbs or descends, the static pressure will result in changes to your airspeed indication. It’s the same principle as a blocked pitot tube but reversed.
A climb will result in a decrease in atmospheric pressure entering the pitot tube, which will result in the ASI indicating decreasing airspeed, causing the ASI to act as a “reverse altimeter.” As long as the pitot tube is not blocked, changes in airspeed will reflect on the ASI, but not accurately.
Vertical Speed Indicator: With no changes to static air, there can be no differential pressure for the VSI to work with. With no differential pressure, the VSI will be stuck at zero, and you’ll marvel at how well you’re maintaining altitude.
Altimeter: The altimeter will freeze at whatever altitude was indicated when the static port was blocked.