022 Instrumentation topic guide
Pitot-Static Blockages and Errors
Three instruments share the same two pressure sources, and almost every pitot-static question is really asking whether you remember which source feeds which gauge. The airspeed indicator needs both dynamic pressure from the pitot head and static pressure from the static vents, because indicated airspeed is built from the difference between the two. The altimeter and the vertical speed indicator need only static pressure, so a fault confined purely to the pitot line leaves them completely unaffected.
The exam turns this wiring diagram into a small set of blockage scenarios, and each one produces a distinct, learnable signature rather than a vague 'instrument goes wrong'. Once you can predict what a blocked pitot, a blocked pitot with its drain also blocked, and a blocked static source each do in a climb and in a descent, most stems in this area become recognition rather than calculation.
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.
Worked example
Worked example: descending with a blocked static source
An aircraft is in a steady descent when the static source becomes completely blocked; the pitot system continues to operate normally. Which set of indications follows?
- AThe altimeter keeps unwinding normally, the ASI under-reads, and the VSI shows the correct rate of descent.
- BThe altimeter freezes at the blockage altitude, the ASI over-reads, and the VSI indicates zero.
- CThe altimeter freezes, the ASI under-reads, and the VSI indicates a climb.
- DThe altimeter freezes, the ASI reads normally because the pitot head is unaffected, and the VSI indicates zero.
Show the answer and walkthrough
Correct answer: B
- A. This assumes the altimeter is unaffected by a static blockage, when in fact the altimeter depends on static pressure alone and must freeze.
- B. Correct: the altimeter holds the reading from the moment of blockage, the ASI over-reads because its frozen static reference belongs to a higher, lower-pressure altitude, and the VSI has no pressure change reaching it at all.
- C. This takes the correct climb-case direction for the ASI and VSI and applies it to a descent, reversing both errors.
- D. This forgets that the ASI needs the static side as well as the pitot side; a fault confined to static pressure still corrupts the ASI's reading.
Step by step
- Identify what the static blockage touches: the altimeter, the VSI, and the static side of the ASI's differential, all at once.
- The altimeter has nothing new to compare against, so it freezes at the altitude it showed the instant the blockage occurred.
- The VSI compares current static pressure to static pressure moments earlier; with no new pressure arriving, there is nothing to compare, so it settles on zero.
- The ASI still receives live pitot pressure, but subtracts a static value frozen at a higher, lower-pressure altitude than the aircraft's true position during the descent, which inflates the differential and over-reads the speed.
Common mistakes
Assuming a pitot fault always leaves the ASI alone
The ASI needs both pressure sources. A stem that blocks the static system, not the pitot head, still corrupts the ASI, and treating the ASI as pitot-only loses the mark on every static-blockage question.
Learning climb and descent directions as a fixed pair without the reasoning behind them
If the exam swaps which phase of flight the aircraft is in, a memorised pair collapses. Working from the frozen-reference idea gets the direction right every time, including on variants you have not seen before.
Treating a pressurised-hull static break as identical to an external static blockage
Sensing cabin pressure instead of a frozen outside value produces a different, generally larger error in a different direction, since cabin pressure corresponds to a much lower altitude than cruise; conflating the two scenarios gives the wrong magnitude and sometimes the wrong sign.
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Last reviewed July 2026