EASA Part-66 · Module 13 — 13.4 Instrument Systems · Cat B2

Pitot-Static System & Blockage Failures

Objective: explain how the airspeed indicator, altimeter and vertical speed indicator derive their readings from total and static pressure, and predict the indication errors produced by a blocked pitot tube, a blocked static port and use of the alternate static source.

Probe, ports & pressure pathsdrag to rotate

Instrument indications

Aircraft altitude 5000 ft

True airspeed 200 kt

Indicated airspeed

0kt

Indicated altitude

0ft

Vertical speed

0fpm

Condition

Normal

p_total = p_static + ½ρV² · ASI senses (p_total − p_static) · Altimeter senses p_static only

System normal

Both the pitot probe and the static port are clear, so every instrument reads correctly.

Failure summary, memorise this table

Condition	ASI in climb	ASI in descent	Altimeter	VSI
Normal	Correct	Correct	Correct	Correct
Pitot blocked, drain open	Drops to zero	Drops to zero	Correct	Correct
Pitot + drain blocked	Over-reads	Under-reads	Correct	Correct
Static blocked	Under-reads	Over-reads	Frozen	Zero
Alternate static (unpressurised)	Over-reads slightly	Over-reads slightly	Over-reads	Momentary climb

The pitot probe faces the airflow and senses total (pitot) pressure, the sum of static pressure and the dynamic pressure ½ρV². The static port sits flush with the fuselage where the air is undisturbed and senses static pressure alone. The airspeed indicator is a differential instrument: it measures total minus static, so it displays dynamic pressure as an indicated airspeed. The altimeter and the vertical speed indicator are fed by static pressure only.

That is why the failures behave as they do. Block the pitot with the drain open and the trapped pressure leaks away, so the differential collapses and the ASI falls toward zero. Block the pitot completely and the total pressure is trapped at its value at the moment of blockage; as the aircraft climbs the static pressure falls, the differential grows, and the ASI behaves like an altimeter, over-reading in the climb and under-reading in the descent. Block the static port and the altimeter freezes at the blockage altitude and the VSI reads zero, while the ASI errs in the opposite sense to a blocked pitot.

Try this: Select Pitot + drain blocked, then climb from 5,000 ft to 20,000 ft. Why does the indicated airspeed rise when the aircraft has not accelerated?
Try this: Select Static blocked and change altitude. What do the altimeter and VSI do, and what does that tell you about which instruments share the static line?
Try this: Select Alternate static. The cabin pressure is slightly below ambient static, so which way do the altimeter and ASI err, and why does that matter on an instrument approach?