Trace a pitot-static fault

Why does airspeed need two pressure sources?

Trace the connections before memorising a blockage mnemonic.

One measurement compares two pressures

A static source samples atmospheric pressure as accurately as its installation permits. A pitot opening decelerates the approaching flow and senses total pressure. At low Mach number, total pressure minus static pressure is approximately dynamic pressure, q = ½ρV². An airspeed indicator compares those sources; the altimeter uses static pressure, while a conventional vertical-speed indicator responds to the rate of static-pressure change.

Position error arises when the local pressure at a source differs from the undisturbed pressure needed by the instrument. Configuration, angle of attack, sideslip and manoeuvre can change that local field. Instrument error belongs to the measuring mechanism itself. Corrections may be made by calibration tables, sensor placement or air-data processing; a digital display still depends on the physical quality of its pressure inputs.

Architecture defines independence

Pitot and static heating reduces icing risk but requires a functioning power supply, correct operation and an unobstructed system. Drains remove moisture; a pitot inlet and its drain are separate openings, so a blockage description must specify both. An alternate static source may sample a different pressure environment. In an unpressurised cabin it often causes predictable errors, but the actual correction comes from the aircraft documentation.

Large aircraft may have separate probes, static sources, air-data modules and computers feeding different displays. Redundancy is useful only when common causes are understood. Two displays using the same blocked pressure input can agree and both be wrong. Before assuming a display has failed, map the input source and compare other evidence such as attitude, thrust and inertial motion according to the published unreliable-air-data procedure.

What changes when pressure becomes trapped?

Hold the trapped pressure fixed and let the aircraft move.

Specify the blocked opening

If the pitot inlet is blocked while the drain remains open, the total-pressure line can vent toward ambient and the airspeed indication falls toward zero in the conventional arrangement. If both inlet and drain are blocked, the line traps total pressure. With the static source still open, a climb reduces case pressure while trapped total pressure remains fixed, making indicated airspeed rise even without a real increase in speed.

A descent with the same trapped total pressure increases static pressure and reduces the indicated pressure difference. This is the reason the ASI can behave like an altimeter; it does not mean every pitot blockage makes speed indication increase. Partial blockage, leaks and a modern computer’s validity logic can alter the response. The model assumes a sealed line and states which source is fixed.

Static blockage is a different experiment

With static pressure trapped and total pressure still live, an altimeter remains at the pressure level of blockage, and a conventional VSI returns toward zero after any transient. Above the blockage level the trapped static pressure is too high relative to ambient, so the ASI underreads for the otherwise comparable flight condition. Below it, the trapped static pressure is too low and the ASI overreads.

These predictions identify a pattern, not an operational permission to trust a remaining speed value. Groundspeed includes wind and is not a direct substitute for airspeed. Apply the aircraft’s pitch, power, configuration and source-selection procedure, with independent cross-checks. In the laboratory change altitude while keeping the actual speed fixed, then compare a pitot trap with a static trap. The opposite ASI trends follow from the two different fixed references.

Can you explain and apply the mechanism?

Four original questions. Work from the stated facts, then compare your reasoning with the explanation.