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Unreliable Airspeed

This oversimplified diagram of a typical pitot-static system shows the routing of pitot and static lines.
Gemini Sparkle

Key Takeaways:

  • Modern "glass cockpits" utilize Air Data Computers (ADCs) with solid-state piezoresistive sensors to derive airspeed from dynamic pressure (pitot vs. static) and altitude from static pressure, a shift from older mechanical instruments.
  • Blockages in the pitot tube (e.g., water, ice) primarily affect airspeed, which can read zero or incorrectly increase with climb, while static port blockages (e.g., tape) cause altimeter/VSI to freeze and airspeed to inaccurately decrease during a climb.
  • In today's digitally integrated cockpits, pitot/static failures are more critical, potentially triggering erroneous envelope protection systems and causing "illogical failures" of primary flight displays that can affect attitude and heading information.
  • Pilots encountering these failures must possess strong system knowledge, cross-reference all available flight information (including pitch/power settings, wind noise, and visual cues), utilize alternate static sources, and prioritize reaching visual meteorological conditions (VMC).
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With “steam gauge” round dials, the airspeed reading was mechanically determined inside the airspeed indicator. Likewise, the altimeter had its own independent mechanical movement, and the vertical speed indicator had a similar arrangement.

In today’s digitally integrated cockpits, the altitude, airspeed, and vertical speed are developed from solid state sensors inside an air data computer (ADC) or air data module (ADU). This article will discuss how the air data information is generated in today’s “glass cockpit” and what to expect if the pitot tube or static port is obstructed.

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