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.
Effects on the Airspeed Indicator:
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.
Cause of Zero Reading: A blocked pitot tube with an open drain port can cause the airspeed indicator to register zero or the last known speed. However, if both the pitot tube and the drain port are blocked, the instrument could display incorrect or erratic airspeed readings.
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.
Effects on the Altimeter and Vertical Speed Indicator:
Minimal or No Effect: The blockage of the pitot tube does not directly affect the altimeter or the vertical speed indicator (VSI), as these instruments primarily rely on static pressure, not dynamic pressure. Therefore, the aircraft's altitude and rate of climb/descent should still be accurately displayed.
Static Port Blockage:
The static port measures the surrounding atmospheric pressure and is used by several instruments, including the altimeter, airspeed indicator, and vertical speed indicator. A blockage of the static port can cause significant issues with all of these instruments.
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.
Effects on the Airspeed Indicator:
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.
Blocked Static Port: The airspeed indicator relies on both static and dynamic pressure. If the static port is blocked, the airspeed indicator may behave erratically because it cannot reference the correct surrounding atmospheric pressure. The result can be a false or incorrect reading of airspeed. For instance, the airspeed could appear to increase or decrease inappropriately, even when the aircraft’s speed has remained constant.
Effects on the Altimeter:
Blocked Static Port: The altimeter measures the pressure difference between the static port and a sealed reference inside the instrument. A blockage in the static port causes the altimeter to either freeze at the last known altitude or display incorrect values, depending on the altitude changes that occur. In some cases, the altimeter may continue to show an incorrect altitude that does not correspond to the aircraft’s actual height.
Effects on the Vertical Speed Indicator:
Blocked Static Port: The vertical speed indicator measures changes in static pressure over time to determine the rate of climb or descent. If the static port is blocked, the VSI will either freeze or give erroneous readings, as it cannot detect changes in atmospheric pressure correctly.
Summary of Effects:
Pitot Tube Blockage:
Affects the airspeed indicator by causing it to freeze or show a zero reading.
No significant effect on the altimeter or vertical speed indicator.
Static Port Blockage:
Affects the airspeed indicator by causing incorrect or fluctuating readings.
Affects the altimeter by freezing or showing incorrect altitude.
Affects the vertical speed indicator by freezing or displaying inaccurate rate-of-climb data.
In the event of these blockages, pilots must rely on alternative instruments and procedures to manage flight, as the accuracy of key flight data may be compromised.
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.
Effects on the Airspeed Indicator:
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.
Cause of Zero Reading: A blocked pitot tube with an open drain port can cause the airspeed indicator to register zero or the last known speed. However, if both the pitot tube and the drain port are blocked, the instrument could display incorrect or erratic airspeed readings.
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.
Effects on the Altimeter and Vertical Speed Indicator:
Minimal or No Effect: The blockage of the pitot tube does not directly affect the altimeter or the vertical speed indicator (VSI), as these instruments primarily rely on static pressure, not dynamic pressure. Therefore, the aircraft's altitude and rate of climb/descent should still be accurately displayed.
Static Port Blockage:
The static port measures the surrounding atmospheric pressure and is used by several instruments, including the altimeter, airspeed indicator, and vertical speed indicator. A blockage of the static port can cause significant issues with all of these instruments.
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.
Effects on the Airspeed Indicator:
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.
Blocked Static Port: The airspeed indicator relies on both static and dynamic pressure. If the static port is blocked, the airspeed indicator may behave erratically because it cannot reference the correct surrounding atmospheric pressure. The result can be a false or incorrect reading of airspeed. For instance, the airspeed could appear to increase or decrease inappropriately, even when the aircraft’s speed has remained constant.
Effects on the Altimeter:
Blocked Static Port: The altimeter measures the pressure difference between the static port and a sealed reference inside the instrument. A blockage in the static port causes the altimeter to either freeze at the last known altitude or display incorrect values, depending on the altitude changes that occur. In some cases, the altimeter may continue to show an incorrect altitude that does not correspond to the aircraft’s actual height.
Effects on the Vertical Speed Indicator:
Blocked Static Port: The vertical speed indicator measures changes in static pressure over time to determine the rate of climb or descent. If the static port is blocked, the VSI will either freeze or give erroneous readings, as it cannot detect changes in atmospheric pressure correctly.
Summary of Effects:
Pitot Tube Blockage:
Affects the airspeed indicator by causing it to freeze or show a zero reading.
No significant effect on the altimeter or vertical speed indicator.
Static Port Blockage:
Affects the airspeed indicator by causing incorrect or fluctuating readings.
Affects the altimeter by freezing or showing incorrect altitude.
Affects the vertical speed indicator by freezing or displaying inaccurate rate-of-climb data.
In the event of these blockages, pilots must rely on alternative instruments and procedures to manage flight, as the accuracy of key flight data may be compromised.