Pitot-Static Blockages and Errors

What each instrument actually senses

The pitot head faces into the airflow and captures total pressure, which is dynamic pressure plus static pressure. The static vents, usually flush with the fuselage skin, capture static pressure alone. The airspeed indicator subtracts static pressure from total pressure inside its mechanism, so its reading depends on both sources moving correctly and independently of each other.

The altimeter compares static pressure to a reference set on its subscale and displays the result as a height. The vertical speed indicator compares current static pressure to static pressure a few seconds earlier, using a calibrated leak, so it displays a rate of pressure change rather than a pressure value itself. Neither of them cares what the pitot head is doing.

The blockage matrix

Four scenarios cover almost everything the exam asks about this system, and the difference between the first two is entirely about whether trapped air can escape.

A blocked static source is the one case that touches all three instruments at once, because all three depend on static pressure somewhere in their mechanism, and a pressurised hull turns the same fault into a different, and arguably more dangerous, version of itself.

• Pitot blocked, drain hole clear: trapped air can vent through the drain, so the pressure differential the ASI measures collapses towards zero and the indication reads low, then effectively stays near zero regardless of any real speed change.
• Pitot and drain both blocked: the pressure captured at the moment of blockage is sealed in. As static pressure falls in a climb, the fixed trapped pressure now looks larger by comparison, so the ASI over-reads; in a descent the opposite happens and the ASI under-reads, so the instrument starts behaving like a second altimeter.
• Static source blocked, pitot normal: the ASI over-reads in a descent and under-reads in a climb, the altimeter freezes and holds the reading from the moment of blockage, and the VSI settles on zero because no pressure change can reach it at all.
• Static line fractures inside a pressurised hull: the instruments start sensing cabin pressure instead of true outside static pressure. Because the cabin is held at a pressure equivalent to a far lower altitude than the aircraft is actually flying, the altimeter under-reads, often by a large margin, the ASI under-reads too, and the VSI typically shows a brief false indication as the sudden jump to cabin pressure is momentarily read as a rate of change.

Reasoning out the direction, rather than memorising it

Every over-read or under-read in this topic comes from the same idea: one side of a comparison keeps moving with the real world while the other side is frozen. In the blocked-static case, the altimeter's static reference is frozen at the blockage altitude while the aircraft keeps changing height, so the comparison drifts further wrong the longer the flight continues.

Apply that logic to the ASI in a descent with static blocked: the frozen static value belongs to a higher altitude, which means a lower pressure, than the true static pressure now surrounding the aircraft. Subtracting a value that is too low from the live pitot pressure produces a differential that is too large, and an ASI reading that is too high. Working it through this way, rather than recalling a table, also tells you what happens if the exam swaps climb for descent.