The Pitot-Static System and Its Failures
How three key flight instruments get their pressure information — and the telltale errors when ports block.
Three primary instruments — the airspeed indicator (ASI), altimeter and vertical speed indicator (VSI) — run on air pressure supplied by the [pitot-static system](/glossary/pitot-static-system). Understanding its plumbing turns confusing failures into recognisable patterns.
Two pressures
- The pitot tube faces into the airflow and senses ram (total) pressure. Only the ASI uses it.
- One or more static ports sit flush with the fuselage and sense static (ambient) pressure. All three instruments use it.
The ASI measures the *difference* between ram and static; the altimeter and VSI measure static alone.
Failure signatures
Blocked pitot, static clear: the ASI behaves like an altimeter — it reads too low in a descent and too high in a climb (because trapped pressure is compared against changing static). The altimeter and VSI are normal.
Blocked static, pitot clear: the altimeter freezes at the altitude where it blocked, the VSI reads zero, and the ASI reads inaccurately (too low in a climb, too high in a descent). The remedy in many aircraft is the alternate static source, which draws static pressure from inside the cabin — usually slightly lower than outside, so the altimeter reads a little high and the ASI a little fast when selected.
Blocked drain hole with blocked ram: the ASI can drop toward zero.
Practical airmanship
Check the pitot heat before flight in any chance of icing, and confirm the static ports are clear of dirt, wax and insects. If instrument indications disagree with pitch and power, suspect the system, cross-check the attitude indicator and engine instruments, and fly known pitch-and-power settings.
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*Reference and training only — not for navigation. Always consult the current AIM, your aircraft’s POH/AFM and a certificated flight instructor.*