The Pitot-Static System: Blocked-Pitot and Blocked-Static Failures, Alternate Static, and Airspeed Types
Updated: Sep 26
Among the most important systems in any airplane is the pitot-static system. While it operates quietly in the background, this system provides the data needed for some of the most critical flight instruments — airspeed, altitude, and vertical speed. Without it, a pilot loses reliable information about three pillars of safe aircraft control. But the system's real test comes when it FAILS: a blocked pitot tube and a blocked static port produce very different — and very specific — false indications, and knowing exactly how each behaves in a climb or descent is the difference between recognizing the failure and being fooled by it. Understanding the pitot-static system, and precisely how it lies when blocked, is essential knowledge for every pilot.
This post covers the pitot-static system in practical depth: what it is and how it works, the three instruments and which pressures they use, exactly how blocked-pitot and blocked-static failures behave (including in climbs and descents), the alternate static source and its correction, the airspeed types (IAS/CAS/EAS/TAS), and preflight and inspection requirements.
Study this full length lesson (video, podcast, flashcards, and quiz) here: Full Length Lesson >
What the Pitot-Static System Is
The pitot-static system is a pressure-based system that measures two pressures and routes them to cockpit instruments.
The two pressures:
Dynamic pressure: the pressure from the aircraft's motion through the air (ram air)
Static pressure: the ambient air pressure surrounding the aircraft
These are sensed and routed through plumbing to the instruments
The instruments interpret them into flight information
The two sources:
Pitot pressure: captured by the pitot tube (facing into the airflow) — this is TOTAL pressure (dynamic + static)
Static pressure: captured by the static port(s) (flush with the airflow) — ambient pressure
Together, they enable sensing speed, altitude, and vertical movement
The system's inputs
The pressure relationship:
Total pressure (pitot) = static pressure + dynamic pressure
Dynamic pressure = total (pitot) − static
This relationship is how airspeed is derived
The core equation
Pitot Pressure: Measuring Airspeed
The pitot tube:
Mounted on the wing, nose, or a mast extending into the airflow
Faces directly into the relative wind
Captures RAM AIR pressure (increases with airspeed)
This is pitot (total) pressure
The pitot tube details:
Has a small drain hole (to drain moisture)
May have pitot heat (to prevent icing)
Positioned in undisturbed airflow
The airspeed sensor
The airspeed indicator (ASI):
The ONLY instrument using BOTH pitot AND static pressure
Pitot pressure enters the instrument case (into a diaphragm)
Static pressure surrounds the diaphragm
The difference (pitot − static) = dynamic pressure = indicated airspeed
The speed instrument
How the ASI works:
Pitot (total) pressure inflates the diaphragm
Static pressure is the reference (around the diaphragm)
The diaphragm expands/contracts with the pressure difference
Mechanical linkage shows airspeed
Dynamic pressure displayed as speed
The behavior:
Pitot pressure increases → airspeed increases
Pitot pressure decreases → airspeed decreases
(Assuming static is constant)
Responsive to ram air
Static Pressure: Measuring Altitude and Vertical Speed
The static ports:
One or more ports, usually on the side of the fuselage
Positioned in undisturbed airflow (to sense ambient pressure accurately)
Static pressure decreases with altitude, increases descending
The ambient-pressure sensor
The altimeter:
Uses static pressure ONLY
Sealed aneroid wafers inside
As static pressure decreases (climbing), the wafers expand
Mechanical linkage shows altitude
Height above the set pressure reference
How the altimeter works:
The aneroid wafers respond to static pressure
Lower static pressure (higher altitude) → wafers expand → higher altitude reading
The Kollsman window sets the reference pressure (altimeter setting)
Shows altitude (MSL with the correct setting)
The vertical speed indicator (VSI):
Uses static pressure, but through a calibrated restriction
Static pressure enters a diaphragm directly AND through a restricted orifice (into the case)
The DIFFERENCE in pressure change over time = rate of climb/descent
Shows the trend (rate)
How the VSI works:
The calibrated leak creates a lag between the diaphragm and the case pressure
When climbing, the case pressure lags the (falling) diaphragm pressure → shows a climb
When level, they equalize → shows zero
The rate of change is displayed
Trend, with lag
The VSI lag:
The VSI shows TRENDS, not instantaneous changes
It LAGS behind actual aircraft movement (a few seconds)
Because of the calibrated restriction
Not immediate
The Three Instruments Summarized
Instrument | Pitot? | Static? |
Airspeed Indicator (ASI) | YES | YES |
Altimeter | No | YES |
Vertical Speed Indicator (VSI) | No | YES |
The ASI uses both (pitot AND static)
The altimeter and VSI use static only
These are the "pitot-static instruments" or "pressure instruments"
Know which uses what (tested)
Blocked Pitot Tube: The Specific Failures
Here's the depth the basic treatment gestures at but doesn't fully detail: a blocked pitot tube behaves differently depending on WHAT'S blocked.
Common causes:
Ice (the most common — hence pitot heat)
Insects (bugs building nests in the tube)
Debris
Protective covers left installed (a preflight catch)
Obstruction of the tube
Which instrument is affected:
ONLY the airspeed indicator (the only instrument using pitot pressure)
The altimeter and VSI are UNAFFECTED (they use static only)
So a blocked pitot affects ONLY airspeed
Isolated to the ASI
Scenario 1 — Pitot RAM inlet blocked, DRAIN hole open:
The ram air inlet is blocked, but the drain hole is open
Trapped pitot pressure escapes through the drain
Pitot pressure drops to static (ambient) pressure
The airspeed indicator reads ZERO (or drops toward zero)
Airspeed reads zero
Scenario 2 — Pitot inlet AND drain hole BOTH blocked:
Both the ram inlet and the drain hole are blocked
The pitot pressure is TRAPPED (can't change)
Now the ASI acts like an ALTIMETER
As you CLIMB: static pressure drops, so the (constant) trapped pitot pressure looks relatively higher → airspeed INCREASES (falsely)
As you DESCEND: static pressure rises → airspeed DECREASES (falsely)
The ASI responds to altitude changes, not actual speed
The "acts like an altimeter" detail:
With both blocked, the trapped pitot pressure is constant
The ASI compares this constant pitot to the CHANGING static
Climbing (static drops): the difference grows → airspeed reads higher
Descending (static rises): the difference shrinks → airspeed reads lower
It mimics an altimeter (increasing with altitude)
Why this is dangerous:
In a climb with both blocked, the airspeed reads INCREASING (falsely)
A pilot might lower the nose to "slow down" — and actually accelerate toward an overspeed or dive
Recognizing the false indication is critical
The trap
The pitot heat mitigation:
Most airplanes have pitot heat (to prevent ice blockage)
Use it in visible moisture or cold conditions
Prevents the ice-blockage scenario
A key preventive
Blocked Static Port: The Specific Failures
The static blockage, detailed — it affects all three instruments (via static).
Which instruments are affected:
The altimeter, VSI, AND airspeed indicator (all use static)
Static is common to all three
So a static blockage affects all three
Widespread effect
The altimeter (static blocked):
The altimeter FREEZES at the altitude where the blockage occurred
(The trapped static pressure doesn't change)
Climbing or descending, the altimeter reads the SAME (frozen)
Stuck reading
The VSI (static blocked):
The VSI reads ZERO (no pressure change reaches it)
Even while climbing/descending
Stuck at zero
Falsely shows level
The airspeed indicator (static blocked):
The ASI becomes unreliable (uses static as a reference)
With static trapped:
CLIMBING: the ASI reads LOWER than actual (the trapped static is higher than actual ambient, reducing the indicated difference)
DESCENDING: the ASI reads HIGHER than actual
The ASI errors are OPPOSITE to the blocked-pitot-both scenario
The static-blockage airspeed detail:
With static blocked at a certain altitude, then climbing:
Actual static (outside) drops, but the trapped static is higher
The ASI uses the (higher) trapped static as reference → smaller pitot-static difference → LOWER airspeed reading
Underreads climbing
Descending: overreads
The reverse of the both-pitot-blocked case
Why it's confusing:
All three instruments are affected (frozen altimeter, zero VSI, erroneous ASI)
The pattern (frozen alt, zero VSI) is a clue to static blockage
Recognize the combination
The diagnostic
The Alternate Static Source
The backup for a blocked static, with the correction detail.
What it is:
Many aircraft have an ALTERNATE STATIC SOURCE
A valve that opens the static system to CABIN air pressure (inside the aircraft)
Substitutes cabin static for the (blocked) outside static
The backup
When to use it:
If the static port is blocked (frozen altimeter, zero VSI)
Open the alternate static source
Restores static pressure input (from the cabin)
The instruments work again (with a correction)
The correction (important):
Cabin static pressure is usually LOWER than outside static pressure
(Due to the venturi effect of air moving over/around the fuselage)
So with the alternate static source:
The altimeter reads HIGHER than actual (lower reference pressure = higher indicated altitude)
The airspeed reads HIGHER than actual
The VSI momentarily shows a climb (when opened)
Account for the errors
Why it reads high:
Lower cabin static pressure "looks like" a higher altitude to the altimeter
And makes the pitot-static difference larger for the ASI (higher airspeed)
The corrections are: altitude and airspeed read HIGH on the alternate source
Know the direction
The unpressurized-cabin technique:
In an unpressurized aircraft without an alternate static source, you can BREAK THE GLASS of the VSI
This vents the static system to the cabin (an emergency alternate static)
The VSI is then inoperative, but the altimeter and ASI work (with the cabin-static errors)
A last-resort technique
The practical application:
If static blocks: use the alternate static source
Remember the altimeter and airspeed read slightly HIGH
Fly accordingly (you're actually slightly lower and slower than indicated)
The correction matters
The Airspeed Types: IAS, CAS, EAS, TAS
A concept the basic treatment doesn't cover but that's fundamental: airspeed has several forms.
Indicated Airspeed (IAS):
What the airspeed indicator shows directly
Raw pitot-static reading
Uncorrected
What you read
Calibrated Airspeed (CAS):
IAS corrected for INSTRUMENT and POSITION error
Position error: the static port doesn't always sense true ambient pressure (varies with angle of attack, configuration)
The POH provides the correction (an airspeed calibration table)
IAS ± position/instrument error = CAS
Equivalent Airspeed (EAS):
CAS corrected for COMPRESSIBILITY (at high speed/altitude, air compresses)
Matters at high speeds (jets)
For most GA, CAS ≈ EAS (compressibility is negligible at low speed)
A high-speed correction
True Airspeed (TAS):
The actual speed through the air mass
CAS (or EAS) corrected for AIR DENSITY (altitude and temperature)
At altitude, air is less dense, so TAS > IAS
Rule of thumb: TAS increases about 2% per 1,000 feet above IAS
The real speed
The progression:
IAS → (instrument/position error) → CAS → (compressibility) → EAS → (density) → TAS
Each correction refines the speed
IAS is what you read; TAS is what you're actually doing
The chain
Why it matters:
IAS is what you fly by (stall speeds, etc. are in IAS/CAS)
TAS is for navigation (groundspeed = TAS ± wind)
At altitude, your TAS is much higher than your IAS
Different uses
The position error:
The static port location causes small errors (position error)
Greatest at high angle of attack (slow flight, where the airflow over the port changes)
The POH calibration table corrects it
A real error source
Preflight and Pilot Responsibilities
Verifying the system, expanded.
The preflight checks:
Inspect the pitot tube for obstructions (ice, insects, debris)
Ensure the pitot cover is REMOVED (a common oversight)
Check the static ports for blockage (bugs, dirt, tape)
Test the pitot heat (if required — feel it warm up)
Verify the drain holes are clear
The pitot cover catch:
A pitot cover left on causes a blocked pitot (zero airspeed on takeoff)
A "remove before flight" flag
Check it's removed
A classic preflight item
The static port check:
Ensure the static ports are clear (not taped over from washing/painting)
Bugs and dirt can block them
Verify undisturbed
Often overlooked
The after-takeoff cross-check:
After takeoff, cross-check the instruments (airspeed alive, altimeter increasing, VSI showing climb)
Catch a blockage early
"Airspeed alive" call on takeoff
Early detection
The IFR pitot-static inspection:
For IFR, the pitot-static system and altimeter must be inspected every 24 CALENDAR MONTHS (91.411)
Tests the system integrity and altimeter accuracy
Required for IFR
(Covered in the required-inspections post)
The Pitot-Static System in Modern Aircraft
The glass-cockpit reality, expanded.
Even in glass cockpits:
Pitot and static pressures STILL feed the system
An Air Data Computer (ADC) processes them digitally
The data drives the electronic displays (PFD)
The physics is the same
The Air Data Computer (ADC):
Receives pitot and static pressure
Computes airspeed, altitude, vertical speed digitally
Feeds the glass displays
The digital brain
The failures still matter:
A blocked pitot or static still causes errors (the ADC gets bad data)
Glass cockpits may have monitoring/alerting (a benefit)
But the underlying failures are the same
Understand the fundamentals
The AHRS/ADC distinction:
The ADC handles air data (pitot-static)
The AHRS handles attitude/heading (gyros/magnetometer)
Together they drive the glass cockpit
Air data is still pitot-static
Conclusion
The pitot-static system is a deceptively simple but vital component of airplane operation. By measuring pitot and static pressure, it provides airspeed, altitude, and vertical speed — core elements of safe flight. But its most important lessons come from understanding how it FAILS: a blocked pitot (with both holes blocked) makes the airspeed indicator act like an altimeter — increasing in a climb, decreasing in a descent — while a blocked static freezes the altimeter, zeros the VSI, and reverses the airspeed errors. The alternate static source restores the static instruments but makes altitude and airspeed read slightly high.
A pilot who understands how the pitot-static system works — and precisely how it lies when blocked — is far better prepared to detect instrument failures, interpret abnormal indications, and maintain control when systems malfunction. In aviation, systems knowledge isn't optional — it's a safety tool.
On the Written Test and Checkride
The pitot-static system appears on tests and checkride orals. The most commonly tested topics:
Which instruments use pitot vs. static (ASI both; altimeter and VSI static only)
Blocked pitot behavior (zero airspeed, or acts like an altimeter if both blocked)
Blocked static behavior (frozen altimeter, zero VSI, reversed airspeed errors)
The alternate static source (reads high on altitude and airspeed)
The airspeed types (IAS, CAS, EAS, TAS)
The IFR pitot-static inspection (24 months)
Quick Reference
The System:
Pitot pressure: total (dynamic + static) — from the pitot tube
Static pressure: ambient — from the static ports
Dynamic pressure = pitot − static
The Three Instruments:
Instrument | Pitot | Static |
Airspeed (ASI) | YES | YES |
Altimeter | No | YES |
VSI | No | YES |
Blocked Pitot (affects ASI only):
Ram blocked, drain OPEN → airspeed reads ZERO
Ram AND drain blocked → ASI acts like an ALTIMETER:
Climb: airspeed reads HIGHER (falsely)
Descent: airspeed reads LOWER (falsely)
Causes: ice, insects, debris, cover left on
Mitigation: pitot heat
Blocked Static (affects all three):
Altimeter: FREEZES (stuck at blockage altitude)
VSI: reads ZERO
Airspeed: erroneous —
Climb: reads LOWER than actual
Descent: reads HIGHER than actual
(Opposite of the both-pitot-blocked case)
Alternate Static Source:
Opens static to CABIN air (backup for blocked static)
Cabin static is LOWER than outside →
Altimeter reads HIGHER than actual
Airspeed reads HIGHER than actual
Unpressurized backup: break the VSI glass
Airspeed Types:
IAS: indicated (what you read)
CAS: IAS corrected for instrument/position error
EAS: CAS corrected for compressibility (high speed)
TAS: corrected for density (≈ +2%/1,000 ft above IAS)
Progression: IAS → CAS → EAS → TAS
Preflight:
Pitot tube clear, COVER REMOVED
Static ports clear (no tape/bugs)
Test pitot heat
Cross-check after takeoff ("airspeed alive")
IFR Inspection:
Pitot-static/altimeter: every 24 calendar months (91.411)
Modern Aircraft:
Air Data Computer (ADC) processes pitot-static digitally
Feeds the glass displays
Failures still matter
Key Principle:
The pitot-static system derives airspeed (ASI, using both pitot and static), altitude, and vertical speed (altimeter and VSI, static only). When blocked, it lies in specific ways: a fully blocked pitot makes the ASI act like an altimeter (increasing in a climb), while a blocked static freezes the altimeter, zeros the VSI, and reverses the airspeed errors. The alternate static source restores the static instruments but reads altitude and airspeed slightly HIGH.
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Author: Nathan Hodell
CFI, CFII, MEI, ATP, Creator and CEO
Nathan is an aviation enthusiast with thousands of hours of flying and dual instruction over the past 15+ years. Through his aviation career he has been able to earn his ATP, fly as an airline pilot, own/operate flight schools, and create and host wifiCFI.