Unreliable Airspeed

BTeter

Monday, November 14, 2022 at 03:35 PM ET

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

Key Takeaways:

Modern "glass cockpits" utilize an Air Data Computer (ADC) and solid-state sensors to electronically derive airspeed from pitot-static pressure differential and altitude from static pressure, a shift from older mechanical "steam gauge" systems.
Obstructions in the pitot tube or static port lead to distinct and potentially confusing errors in airspeed and altitude indications, requiring pilots to understand specific failure modes (e.g., increasing airspeed with pitot blockage during climb, decreasing airspeed with static blockage during climb).
In today's digitally integrated cockpits, pitot/static failures can cause "illogical" and widespread malfunctions beyond simple instrument errors, impacting systems like envelope protection and primary flight display information, making the situation more complex and dangerous.
When encountering such a failure, pilots must rely on comprehensive system knowledge, cross-check all available information (pitch/power settings, visual cues), utilize alternate static sources, and prioritize seeking visual meteorological conditions (VMC) as ATC cannot assist with erroneous altitude readings.

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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.