Clogged Pitot Tube: The Two Failure Modes, Unreliable Airspeed Response, and the Accidents That Prove It
Updated: Sep 26
The airspeed indicator (ASI) is one of the most critical instruments in an airplane cockpit. Pilots rely on it to avoid stalls, prevent overspeeding, and fly precise approaches. Because the ASI depends directly on the pitot-static system, any disruption — especially a clogged pitot tube — can produce misleading and potentially dangerous indications. The truly insidious part is that one blockage scenario makes the instrument read zero (obviously wrong, easy to catch) while the other makes it behave believably and smoothly — while being completely false. That second scenario has killed hundreds of people, and understanding it is one of the most important instrument-failure lessons in aviation.
This post covers pitot blockage in practical depth: the two scenarios and exactly how each behaves, partial blockages, how to recognize the failure, the unreliable-airspeed response (pitch and power), the accident history that defines the hazard, pitot heat and its limits, and the glass-cockpit picture.
Study this full length lesson (video, podcast, flashcards, and quiz) here: Full Length Lesson >
A Quick Review: How the ASI Works
The ASI compares two pressures to derive airspeed.
The mechanism:
Pitot pressure (TOTAL pressure) enters a DIAPHRAGM
Static pressure surrounds the diaphragm (in the case)
The DIFFERENCE (dynamic pressure) moves the needle
Pitot − static = airspeed
Why blockage matters:
If pitot pressure can't reach the instrument correctly, the difference is wrong
The ASI displays a false airspeed
The only instrument affected by a pitot blockage (the altimeter and VSI use static only)
Isolated but critical
(For the full ASI mechanism and V-speeds, see our airspeed indicator post.)
Common Causes of Pitot Blockage
The causes, expanded.
Ice formation:
The most common cause in flight
Forms in visible moisture at cold temperatures
Can block the tube (and the drain hole) quickly
Why pitot heat exists
Insects:
Mud daubers and other insects nest in pitot tubes (especially parked aircraft)
Can completely block the tube
A real hazard for aircraft parked outside
Preflight catch
Debris and dirt:
Accumulated contamination
Washing/painting residue
Foreign objects
Preflight inspection
Pitot covers left installed:
The classic preflight miss
A "remove before flight" flag
Causes an immediate blockage on takeoff
Preventable
The vulnerability:
The pitot tube faces directly into the airflow
Exposed to ice, insects, and debris
Especially vulnerable
By design (it must face the airflow)
Scenario 1: Pitot Blocked, Drain Hole OPEN
The first (and more obvious) failure mode.
What happens:
The main pitot (ram) opening is blocked
But the DRAIN HOLE (for moisture) remains open
The trapped pitot pressure LEAKS OUT through the drain
The pressure in the diaphragm drops to STATIC pressure
The ASI behavior:
With pitot pressure = static pressure, the difference (dynamic pressure) is ZERO
The ASI gradually indicates ZERO airspeed
Regardless of actual speed
Reads zero
What the pilot sees:
The airspeed DECREASES (toward zero)
Reads zero even in flight (obviously false)
The altimeter and VSI continue working normally
An obviously incorrect indication
Why it's easier to diagnose:
An airspeed of zero in cruise flight is obviously wrong
The pilot recognizes it immediately
Cross-check confirms (attitude and power say you're flying)
Obvious failure
The danger is lower:
Because it's obvious, the pilot recognizes it
Less likely to be trusted
Still a serious failure (no airspeed), but not deceptive
Recognizable
Scenario 2: Pitot AND Drain Hole BOTH Blocked
The dangerous one — and the reason this topic matters.
What happens:
Both the ram opening AND the drain hole are blocked
Pitot pressure is TRAPPED in the line (can't change)
Static pressure continues changing normally (with altitude)
The ASI compares TRAPPED pitot to CHANGING static
The ASI acts like an ALTIMETER:
CLIMB: static pressure decreases → the (constant) trapped pitot looks relatively HIGHER → the difference GROWS → the ASI indicates INCREASING airspeed
DESCENT: static pressure increases → the difference SHRINKS → the ASI indicates DECREASING airspeed
The ASI responds to ALTITUDE, not speed
An altimeter in disguise
The behavior in detail:
The aircraft may be at a constant actual airspeed
But climbing, the ASI shows the airspeed increasing
Descending, it shows the airspeed decreasing
Completely false, but smooth and believable
Why this is so dangerous:
The ASI appears "ALIVE" (the needle moves)
The indications change SMOOTHLY and BELIEVABLY
Nothing looks obviously broken
But the readings are completely wrong
Deceptive
The deadly scenario:
Climbing with both blocked, the ASI shows increasing airspeed
The pilot thinks they're accelerating (maybe toward an overspeed)
They LOWER the nose to "slow down"
But lowering the nose means descending — and the ASI then shows DECREASING airspeed (the opposite of the real effect)
Confusion, and potentially a dive or a stall
The trap
The reverse trap:
Descending with both blocked, the ASI shows decreasing airspeed
The pilot thinks they're slowing (toward a stall)
They lower the nose further (to "gain speed") — accelerating the descent
Or they add power and pitch up (toward an actual stall)
Either way, wrong inputs
The confusion compounds
Partial Blockage: The Subtle Case
A scenario the basic treatment doesn't cover: a partial blockage.
What it is:
The pitot tube is PARTIALLY blocked (not completely)
Some pitot pressure gets through (but restricted)
The ASI reads low (but not zero) or responds sluggishly
A subtle failure
The behavior:
Airspeed may read LOWER than actual (restricted pressure)
Or respond slowly to changes (lagging)
Not an obvious failure
Harder to detect
Why it's tricky:
The ASI "works" (the needle moves, shows a plausible speed)
But the reading is wrong
Subtle and hard to catch
Requires cross-checking
The detection:
Cross-check: does the airspeed match the expected pitch and power?
Compare with GPS groundspeed (accounting for wind)
A discrepancy suggests a problem
Diligent cross-checking
The progression:
A partial blockage (ice building) can become complete
Watch for a developing problem
Use pitot heat early
Prevention
How to Recognize a Clogged Pitot Tube
The recognition cues, expanded.
The cross-check cues:
Airspeed changes WITHOUT corresponding pitch or power changes
Airspeed INCREASES during a climb at constant power (the classic both-blocked cue)
Airspeed DECREASES during a descent without deceleration
Disagreement between airspeed, attitude, and engine performance
The "airspeed increases in a climb" cue:
At constant power, climbing normally REDUCES airspeed (energy goes to altitude)
If the ASI shows airspeed INCREASING in a climb, that's backward
A strong indicator of the both-blocked scenario
The key recognition
The attitude-power-performance check:
Attitude + power should produce a predictable airspeed
If the ASI disagrees with what attitude and power should produce, suspect it
The fundamental cross-check
Your defense
The GPS groundspeed cross-check:
Compare the ASI to GPS groundspeed (accounting for wind)
A large discrepancy (beyond the wind) suggests an ASI problem
A useful but imperfect check (wind matters)
A supplementary cue
The GPS limitation:
GPS shows GROUNDSPEED (not airspeed)
The difference is the wind (which you may not know precisely)
At altitude, TAS is higher than IAS (another difference)
So GPS is a rough cross-check, not a precise one
Useful but imperfect
The takeoff catch:
"Airspeed alive" on the takeoff roll
If the airspeed doesn't increase during the roll, abort
Catches a blocked pitot before flight
The best catch
The disciplined scan:
A regular instrument scan catches disagreements
Attitude, airspeed, altimeter, VSI, power
The scan is the defense
Discipline
The Unreliable Airspeed Response: Pitch and Power
Here's the operational depth the basic treatment gestures at: the actual response to unreliable airspeed.
The core principle:
ATTITUDE + POWER = PERFORMANCE
If you set a known pitch attitude and a known power setting, you get a known airspeed
Even without a working ASI
The fundamental relationship
The response steps:
Recognize the airspeed is unreliable (cross-check disagreement)
Do NOT chase the airspeed indicator (its indications are false)
Set a known pitch attitude and power setting (from memory/the POH)
Cross-check with the altimeter, VSI, and GPS groundspeed
Fly the aircraft using attitude and power
Follow the abnormal/emergency procedure (if published)
Declare an emergency if situational awareness is compromised
The known pitch-and-power combinations:
Every aircraft has known combinations (e.g., level cruise: X° pitch, Y% power → approximately Z knots)
Know your aircraft's numbers (level flight, climb, descent, approach)
Use them when the airspeed fails
The backup performance data
The GA examples (typical, know YOUR aircraft):
Level flight: cruise power, level pitch attitude
Climb: climb power, climb attitude (per the POH)
Descent: reduced power, descent attitude
Approach: approach power, approach attitude, landing configuration
Memorize your aircraft's numbers
Why this works:
Attitude determines the pitch (and thus the speed relationship)
Power determines the energy
Together, they produce a predictable airspeed
Physics, not the instrument
The transport-aircraft parallel:
Airliners have "unreliable airspeed" memory items (specific pitch/power for the phase of flight)
The same principle, formalized
GA pilots should know their aircraft's equivalents
A universal technique
In IMC:
Without visual references, the ASI failure is more dangerous
Rely on the attitude indicator (pitch) and power
The altimeter and VSI still work (pitot blockage doesn't affect them)
Use them to confirm the profile
Declare if needed
The altimeter/VSI advantage:
A pitot blockage affects ONLY the ASI
The altimeter and VSI still work correctly
So you have altitude and vertical speed (valid)
Use them (level flight = zero VSI, constant altitude)
Valid references
The Accidents That Define the Hazard
The case history the basic treatment alludes to — worth knowing specifically.
Birgenair Flight 301 (1996):
A Boeing 757 departing Puerto Plata, Dominican Republic
The aircraft had been parked for weeks; a pitot tube was likely blocked (suspected mud dauber wasp nest)
On takeoff, the captain's airspeed indicator read erroneously HIGH (the blocked-pitot climb behavior)
The crew received conflicting indications (overspeed warning and stall warning)
Confusion; the aircraft stalled and crashed; 189 died
The classic blocked-pitot accident
The Birgenair lessons:
A blocked pitot (from an insect nest) produced a believable but false high airspeed
Conflicting warnings confused the crew
Cross-checking the other ASI (the first officer's, which was reading correctly) could have resolved it
The danger of trusting a convincing false indication
Aeroperú Flight 603 (1996):
A Boeing 757 departing Lima, Peru
The STATIC ports had been covered with tape during cleaning and not removed
Blocked static → erroneous altitude and airspeed
The crew received conflicting/false indications (including a false "too low" warning while at altitude, and false overspeed)
Confused, the aircraft descended into the ocean; 70 died
The blocked-static counterpart
The Aeroperú lessons:
A blocked static (from maintenance tape) corrupted the altimeter, VSI, AND airspeed
All the pressure instruments lied
A preflight catch (checking the static ports) would have prevented it
The importance of the preflight inspection
Air France Flight 447 (2009):
An Airbus A330 over the Atlantic
Pitot tubes ICED OVER (in a convective area), causing unreliable airspeed
The autopilot disconnected; the crew faced unreliable airspeed
The pilot flying made sustained nose-up inputs; the aircraft stalled and descended into the ocean; 228 died
(The stall response was the fatal error, but pitot icing initiated it)
The AF447 lessons:
Pitot icing causes unreliable airspeed (even in modern aircraft)
The response (pitch and power, don't stall) is critical
Led to improved pitot tube designs and unreliable-airspeed training
Modern relevance
The common thread:
A pressure-system blockage produces FALSE but BELIEVABLE indications
Crews trusted the false indications (or were confused by conflicts)
The response (attitude + power) was the correct answer
Understanding the failure modes saves lives
Pitot Heat and Its Limits
Pitot heat, with the important caveats.
What pitot heat does:
Electrically heats the pitot tube
Prevents ice formation
May clear light ice if activated EARLY
Ice prevention
When to use it:
Visible moisture (clouds, rain)
Cold temperatures (near or below freezing)
Anytime icing is possible
Proactively (before ice forms)
The "use it early" principle:
Pitot heat PREVENTS ice better than it CLEARS it
Turn it on BEFORE entering icing conditions
Once the tube is blocked with ice, heat may take time (or fail) to clear it
Prevention over cure
The limits (important):
Pitot heat does NOT clear an insect nest
It does NOT remove a pitot cover
It does NOT clear debris
It only addresses ICE
Not a cure-all
The preflight connection:
Pitot heat won't help with a bug nest or a cover
The PREFLIGHT inspection catches those
Both are needed (preflight + pitot heat)
Complementary defenses
Verifying pitot heat:
Check it during preflight (carefully feel the tube warm, or check the ammeter load)
Confirm it works before you need it
Required for IFR/known icing
Verification
Pitot Blockage in Glass Cockpits
The modern picture, expanded.
The technology:
Pitot pressure is sensed electronically (by the Air Data Computer)
The ADC calculates airspeed
Displayed on the PFD (tape)
Digital processing
The failure is the same:
A blocked pitot still produces erroneous airspeed (the ADC gets bad pressure)
The physics are unchanged
The display is digital, but the error is the same
Same underlying failure
The advantages:
Comparator warnings: if there are two air data sources, the system may flag a disagreement ("AIRSPEED MISCOMPARE")
Flags: the airspeed may be flagged as invalid (red X)
Alerts: the system may annunciate the problem
Better detection
The dual-source advantage:
Aircraft with two pitot systems (pilot and copilot) can compare
A disagreement triggers an alert
Helps identify which is wrong
(Birgenair had this — the first officer's ASI was correct)
The caution:
Don't assume the glass cockpit will catch it
Single-source GA systems may not have comparators
Understand the mechanical principles
Still need pilot recognition
Why This Knowledge Matters
The stakes, expanded.
A clogged pitot turns a vital instrument into misinformation:
The ASI becomes a source of false data
Trusting it leads to wrong control inputs
Loss of control is possible
The instrument becomes the hazard
Pilots who understand pitot failures can:
Recognize abnormal indications immediately
Avoid incorrect control inputs (not chasing the false airspeed)
Maintain safe flight using fundamentals (attitude + power)
Prevent loss-of-control accidents
Survive the failure
The accident-investigation reality:
Many investigations reveal that MISINTERPRETING airspeed — not the mechanical failure alone — was the critical factor
The blockage was survivable; the response wasn't
Correct response = survival
The human factor
Conclusion
When the pitot tube becomes clogged, the airspeed indicator either drops to zero (if the drain hole is open) or behaves like an altimeter (if both are blocked) — increasing in a climb, decreasing in a descent. Both are dangerous, but the second is especially deceptive because the instrument appears to function normally, showing smooth, believable, and completely false readings.
The response is the same regardless: don't chase the airspeed indicator, set a known pitch attitude and power setting, and cross-check with the altimeter and VSI (which a pitot blockage doesn't affect). Birgenair 301, Aeroperú 603, and Air France 447 are permanent reminders that a pressure-system blockage can produce convincing false data — and that the pilots who survive are the ones who recognize it and fly attitude and power.
In aviation, knowing how instruments fail is just as important as knowing how they work — and the pitot tube is a perfect example of why that knowledge saves lives.
On the Written Test and Checkride
Pitot blockage appears on tests and checkride orals. The most commonly tested topics:
Scenario 1 (drain open): the ASI reads zero
Scenario 2 (both blocked): the ASI acts like an altimeter
That the altimeter and VSI are unaffected by a pitot blockage
Recognizing the failure (airspeed increases in a climb)
The response (attitude + power, don't chase)
Pitot heat (prevention)
Quick Reference
How a Pitot Blockage Affects the ASI:
The ASI is the ONLY instrument affected (altimeter and VSI use static only)
Two scenarios, depending on the drain hole
Scenario 1 — Ram Blocked, Drain OPEN:
Pitot pressure leaks out through the drain
Drops to static pressure → difference = zero
ASI reads ZERO
Obviously wrong (easier to diagnose)
Scenario 2 — Ram AND Drain BOTH Blocked:
Pitot pressure is TRAPPED (constant)
Compared to CHANGING static
ASI acts like an ALTIMETER:
Climb: airspeed reads INCREASING (falsely)
Descent: airspeed reads DECREASING (falsely)
Smooth, believable, completely wrong (the dangerous one)
Partial Blockage:
Reads low or responds sluggishly
Subtle, hard to detect
Cross-check to catch
Causes:
Ice (most common in flight — use pitot heat)
Insects (mud daubers — preflight)
Debris/dirt
Pitot cover left on
Recognition Cues:
Airspeed changes without pitch/power changes
Airspeed INCREASES in a climb at constant power (classic)
Airspeed decreases in a descent without decelerating
Disagreement with attitude and power
"Airspeed alive" check on takeoff
The Response (Unreliable Airspeed):
Recognize (cross-check disagreement)
DON'T chase the ASI
Set known PITCH + POWER
Cross-check altimeter, VSI, GPS groundspeed
Fly attitude and power
Follow the abnormal procedure
Declare if needed
Attitude + Power = Performance
Valid References (pitot blockage):
Altimeter: WORKS (static only)
VSI: WORKS (static only)
Attitude indicator: WORKS
GPS groundspeed: useful (but it's groundspeed, not airspeed)
Pitot Heat:
Prevents ICE (use early, before entering icing)
Does NOT clear insects, covers, or debris
Verify during preflight
Prevention over cure
The Accidents:
Accident | Cause | Lesson |
Birgenair 301 (1996) | Blocked pitot (insect nest) | False high airspeed; cross-check the other ASI |
Aeroperú 603 (1996) | Blocked static (tape) | All pressure instruments lied; preflight catches it |
Air France 447 (2009) | Pitot icing | Unreliable airspeed; fly pitch and power, don't stall |
Glass Cockpits:
Same failure (ADC gets bad pressure)
May have comparator warnings/flags (dual sources)
Don't assume it'll catch it
Understand the principles
Key Principle:
A clogged pitot makes the ASI read ZERO (drain open) or act like an ALTIMETER (both blocked — increasing in a climb, decreasing in a descent). The second is deceptive: smooth, believable, and completely false. The response is always the same — don't chase the airspeed, set a known pitch attitude and power setting, and cross-check the altimeter and VSI (which still work). Attitude + Power = Performance.
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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.