One frozen sensor can turn three of your primary flight instruments into confident liars.
On the EASA ATPL Instrumentation paper (subject 022), the pitot-static system is a guaranteed source of questions and a guaranteed source of lost marks. The catch is not the theory. It is that students memorise "blocked pitot" and "blocked static" as slogans without tracing the pressure, so the moment a question adds a climb or a descent, the answer flips and they get it wrong.
Trace the air, and every failure case falls out on its own.
The pitot-static system in one minute
Three primary instruments run on air pressure: the airspeed indicator (ASI), the altimeter, and the vertical speed indicator (VSI). They are fed by two pressure sources.
- The pitot tube faces into the airflow and senses total pressure (also called pitot, ram or stagnation pressure) which is static pressure plus dynamic pressure.
- The static port sits flush with the airflow and senses static pressure only.
Here is the rule that unlocks everything:
- The ASI measures the difference between total and static pressure. That difference is dynamic pressure, which drives the airspeed reading.
- The altimeter measures static pressure against a sealed reference, converted to altitude.
- The VSI measures the rate of change of static pressure.
Notice the asymmetry. Total pressure reaches only the ASI. Static pressure reaches all three. That single fact predicts which instruments fail in each blockage, as the schematic below makes clear.
Blocked pitot tube: only the ASI reacts
If the pitot ram inlet blocks, whether from ice, insects or a forgotten cover, the altimeter and VSI carry on normally because they never used total pressure. The ASI is the only casualty. What it does next depends on the drain hole.
Ram inlet and drain hole both blocked
Total pressure is now trapped at a fixed value. Fly level and the ASI simply holds whatever number it had. Now climb: static pressure falls, so the trapped total minus the falling static grows, and the ASI over-reads. In a descent, static rises and the ASI under-reads. The airspeed indicator has quietly turned into a crude altimeter, responding to altitude rather than speed.
Ram inlet blocked, drain hole open
Trapped pressure now leaks out through the drain hole toward static, the differential collapses, and the ASI falls toward zero. Same blockage, opposite-looking symptom, which is exactly why "a blocked pitot sends the ASI to zero" is only half the story.
Blocked static port: all three instruments lie
Block the static source and you lose the input shared by every pressure instrument. Static is trapped at the pressure where the blockage occurred.
- Altimeter: freezes at the blockage altitude and refuses to move.
- VSI: reads zero. With no rate of change of static, it sees no vertical speed, even in a genuine climb or descent.
- ASI: still gets live total pressure from the pitot, but now compares it against a stale static value. In a climb the ASI under-reads; in a descent it over-reads, the mirror image of the blocked-pitot case.
This is where the alternate static source earns its place. In an unpressurised aircraft it admits cabin air, which sits at slightly lower pressure than the outside static. Lower sensed static makes the altimeter and ASI read slightly high, and gives a brief VSI climb indication when selected. Examiners love this detail because most students guess the wrong direction.
Quick reference
- Pitot blocked, drain blocked: ASI over-reads in the climb, under-reads in the descent; altimeter and VSI normal.
- Pitot blocked, drain open: ASI falls toward zero; altimeter and VSI normal.
- Static blocked: ASI under-reads in the climb, over-reads in the descent; altimeter frozen at the blockage altitude; VSI reads zero.
- Alternate static, unpressurised aircraft: altimeter and ASI read slightly high; brief VSI climb indication when selected.
The misconceptions that cost marks
- "A blocked pitot upsets the altimeter." It does not. The pitot feeds only the ASI; the altimeter and VSI live entirely on static pressure.
- "A normal-looking ASI means the pitot is clear." Not in level flight. A blocked, drain-sealed pitot holds a steady, plausible reading until the altitude changes, then it moves the wrong way.
- "A blocked pitot always drops the ASI to zero." Only if the drain hole is open. With the drain sealed, it over-reads in the climb instead.
- "Blocked static just freezes the altimeter." It hits all three: altimeter frozen, VSI at zero, ASI mis-reading with altitude.
- "The alternate static source makes everything read low." In an unpressurised aircraft the opposite is true. Lower cabin static makes the altimeter and ASI read high.
How EASA tests this in 022 Instrumentation
ECQB items rarely ask "what does the pitot sense?" outright. They stage it. An aircraft in a steady climb reports a rising ASI and you must identify the failure; or a descent shows a frozen altimeter and a zero VSI and you pick the cause. The examinable skill is direction, over-read or under-read, climb or descent, not the definition. Trace the pressure every time and you never have to memorise the outcomes.
Two habits pay off across the whole 022 syllabus: always ask which pressure source feeds the instrument, then add which way the aircraft is moving before you commit to an answer.
Frequently asked questions
Does a blocked pitot tube affect the altimeter?
No. The altimeter runs on static pressure only. A blocked pitot affects just the airspeed indicator; the altimeter and VSI keep working normally.
What does the ASI do if the static port blocks during a climb?
It under-reads. The pitot still supplies live total pressure, but the static value is trapped at the blockage altitude, so the sensed difference is smaller than the true dynamic pressure and indicated airspeed reads low.
Why does the VSI read zero with a blocked static port?
The VSI measures the rate of change of static pressure. If static is trapped there is no change to measure, so it indicates zero vertical speed even in a real climb or descent.
Is the pitot-static system tested in ECQB 2026?
Yes. Pitot-static theory sits in EASA ATPL subject 022, Instrumentation, and remains a core, high-frequency topic in the ECQB 2026 question bank used from August 2026.
Pressure logic like this rewards repetition until it becomes reflex. Drill EASA ATPL Instrumentation questions with worked explanations at atpltraining.io and turn the pitot-static system into free marks.
Photo by Tim Dennert on Unsplash
