How Pitot-Static Failures Affect Your Indicated Airspeed and Altitude

The pitot-static system is essential for accurate airspeed, altitude, and vertical speed readings in an aircraft. However, failures in this system can lead to erroneous instrument indications, creating serious risks during flight. Understanding how pitot-static failures affect your indicated airspeed and altitude is critical for safe decision-making and effective troubleshooting.

The Pitot-Static System Overview

The pitot-static system consists of:

• Pitot Tube: Measures dynamic pressure (caused by the aircraft's motion through the air) to determine airspeed.
• Static Port: Measures ambient atmospheric pressure, used to calculate altitude and vertical speed.
• Instruments: Includes the airspeed indicator (ASI), altimeter, and vertical speed indicator (VSI).

Common Pitot-Static Failures and Their Effects

Failures in the pitot or static systems can lead to specific instrument errors:

Pitot Tube Blockages

A blockage in the pitot tube can severely affect the airspeed indicator:

• Completely Blocked Pitot Tube (with an open drain hole): The airspeed indicator will read zero because it cannot measure dynamic pressure.
• Completely Blocked Pitot Tube (with a blocked drain hole):
  • As altitude changes, the trapped pressure in the pitot tube causes the airspeed indicator to behave like an altimeter, showing increasing or decreasing airspeed inaccurately.

Static Port Blockages

A blocked static port affects all instruments relying on static pressure:

• Airspeed Indicator: Becomes inaccurate.
  • At higher altitudes, the ASI underreads because the static pressure is trapped at a lower altitude value.
  • At lower altitudes, the ASI overreads for the same reason.
• Altimeter: Freezes at the altitude where the static port was blocked.
• Vertical Speed Indicator: Reads zero because it cannot sense pressure changes to calculate rate of climb or descent.

Both Pitot and Static Systems Blocked

A simultaneous blockage of both systems can render the ASI, altimeter, and VSI completely unreliable:

• The airspeed indicator freezes or provides erratic readings.
• The altimeter is stuck at a fixed value.
• The VSI indicates no change in vertical speed, regardless of actual climb or descent.

Recognizing Pitot-Static Failures

Pilots must recognize abnormal instrument behavior to identify pitot-static failures. Warning signs include:

• Discrepancies between airspeed and power settings (e.g., unusually high or low airspeed for the given engine power).
• Altimeter or VSI readings inconsistent with aircraft behavior or external references.
• Erratic or stuck instrument needles during maneuvers or changes in flight conditions.

Mitigation and Recovery

In case of a pitot-static failure:

• Use Backup Instruments: If available, rely on secondary systems such as standby instruments or an alternate static source.
• Trust Attitude and Power: Fly using attitude indicators, power settings, and external visual references to maintain control.
• Activate Pitot Heat: If icing is suspected, turn on pitot heat to clear ice blockages in the pitot tube.
• Report to ATC: Notify air traffic control of the failure and request assistance as needed.