Study guide
12.1 Pitot-Static Instruments and Blockage Errors
FAA Private Pilot — Airplane PAR
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1 Introduction & Exam Overview
1.1 Current Exam Facts 1.2 ACS Codes, the AKTR, and How to Use This Guide
2 Pilot Qualifications & Medicals
2.1 Eligibility, Training, and Knowledge-Test Authorization 2.2 Private Pilot Privileges and Limitations 2.3 Medical Certificates, Duration, and Disqualification 2.4 BasicMed (14 CFR 61.113(i) and Part 68)
3 Airworthiness, Documents & Currency
3.1 Required Pilot and Aircraft Documents 3.2 Inspections, Airworthiness Directives, and Preventive Maintenance 3.3 Inoperative Equipment and 91.213 3.4 Recent Flight Experience and the Flight Review
4 14 CFR Part 91 Operating Rules
4.1 Right-of-Way, Minimum Safe Altitudes, and Speed Limits 4.2 VFR Cruising Altitudes, Altimeter Settings, and Oxygen 4.3 Fuel Requirements, Transponder, ADS-B, and ELT 4.4 NOTAMs, TFRs, and Accident/Incident Duties
5 National Airspace System
5.1 Class A, B, and C Airspace 5.2 Class D, E, and G Airspace and VFR Weather Minimums 5.3 Special Use Airspace 5.4 Other Airspace, TFRs, TRSA, ADIZ, and SFRA
6 Aviation Weather Theory
6.1 Atmosphere, Pressure, Temperature, and Density 6.2 Stability, Clouds, and Moisture 6.3 Air Masses and Fronts 6.4 Wind, Jet Stream, and Local Wind Systems
7 Weather Hazards
7.1 Thunderstorms and Convective Weather 7.2 Structural Icing, Frost, and Carburetor Ice 7.3 Fog, Low Ceilings, and Visibility 7.4 Wind Shear, Microbursts, Turbulence, and Mountain Waves
8 Weather Products & Briefings
8.1 METAR Decoding 8.2 TAF and Graphical Forecast Products 8.3 PIREPs, AIRMET, SIGMET, Convective SIGMET, and Winds Aloft 8.4 Briefings and Go/No-Go Decisions
9 Principles of Flight
9.1 Four Forces and Lift Production 9.2 Angle of Attack, Critical AoA, and Stalls 9.3 Drag, L/D, and Ground Effect 9.4 Axes, Stability, and Flight Controls
10 Maneuvering Flight, Stalls & Spins
10.1 Load Factor, Bank Angle, and Stall Speed 10.2 Left-Turning Tendencies 10.3 Slow Flight, Power-On/Off Stalls, and Spin Awareness 10.4 Steep Turns and Ground-Reference Maneuvers
11 Powerplant, Fuel & Electrical Systems
11.1 Reciprocating Engines, Mixture, Detonation, and Preignition 11.2 Ignition, Magnetos, Carburetor vs Fuel Injection 11.3 Fuel Grades, Contamination, and Fuel-System Management 11.4 Electrical System and Alternator Failure
12 Flight Instruments & Avionics
12.1 Pitot-Static Instruments and Blockage Errors 12.2 Gyroscopic Instruments, Vacuum Failure, and Magnetic Compass Errors 12.3 Engine Instruments, Transponder Modes, and Basic Avionics
13 Aircraft Performance
13.1 Pressure Altitude, Density Altitude, and Performance Effects 13.2 Takeoff and Climb Performance 13.3 Cruise, Descent, and Landing Performance 13.4 Wind, Gradient, Surface, and Crosswind Component
14 Weight and Balance
14.1 Weight-and-Balance Principles 14.2 Moment, CG, and Sample Computations 14.3 Forward and Aft CG Effects 14.4 V-Speeds, Markings, and Limitation Placards
15 Charts, Pilotage & Dead Reckoning
15.1 Sectional Chart Symbology and Airspace Depiction 15.2 Pilotage and Dead Reckoning 15.3 Variation, Deviation, Wind Triangle, and E6B 15.4 VFR Cross-Country and Fuel Planning
16 Radio Navigation, Lost Procedures & Airport Operations
16.1 VOR Navigation 16.2 GPS, RAIM, and Radar Services 16.3 Lost Procedures and Diversion 16.4 Communications, Light-Gun Signals, Lighting, and Traffic Patterns
17 Aeromedical Factors
17.1 Hypoxia, Hyperventilation, and Carbon Monoxide 17.2 Spatial Disorientation, Illusions, and Vision 17.3 Scuba-to-Flight, Alcohol/Drugs, and IMSAFE
18 ADM, Night Operations & Emergencies
18.1 ADM, Hazardous Attitudes, PAVE, and DECIDE 18.2 Night Operations and Night Illusions 18.3 Emergency Descent, Engine Failure, and System Malfunctions 18.4 Survival Gear, ELT, and Forced-Landing Considerations
19 Preflight Procedures & Airport Ground Operations
19.1 Preflight Assessment, Airworthiness Determination, and the Preflight Inspection 19.2 Flight Deck Management, Passenger Briefings, and Engine Starting 19.3 Taxi Operations: Signs, Markings, Lighting, and Runway Incursion Avoidance 19.4 Before-Takeoff Check, After-Landing, and Securing the Airplane
20 Class-Specific Knowledge: Seaplane & Multiengine
20.1 Seaplane and Water Operations Knowledge 20.2 Multiengine Knowledge: VMC, VYSE, and Engine-Out Fundamentals
12.1 Pitot-Static Instruments and Blockage Errors
Key Takeaways Explain This Topic AI
The pitot tube supplies ram (total) pressure to the airspeed indicator only; one or more static ports supply static pressure to the ASI, altimeter, and VSI (PHAK Chapter 8).
Pitot ram blocked with the drain open drives the ASI to zero. Pitot ram and drain both blocked freezes the ASI in level flight and makes it overread in a climb and underread in a descent.
A blocked static system freezes the altimeter at the blockage altitude, drives the VSI to zero, and makes the ASI underread in a climb and overread in a descent.
An unpressurized alternate-static source usually sits in a slightly lower-pressure cabin, so the ASI and altimeter overread slightly and the VSI shows a momentary climb (PHAK).
Altitude names are indicated, calibrated, pressure, density, true, and absolute. Airspeed names are IAS, CAS, EAS, TAS, and groundspeed.
Last updated: August 2026
ACS PA.I.G.K1h (pitot-static, vacuum/pressure, and associated flight instruments) and the knowledge under PA.VIII basic instrument maneuvers are one plumbing problem. The Private Pilot Airplane knowledge test does not ask you to design a Kollsman window. It does ask which instrument dies when ice covers which hole, whether a climb with a blocked pitot makes the airspeed needle rise or fall, and whether density altitude is a height above the ground.
Two holes, three instruments
PHAK Chapter 8 is blunt about the sources.
- Source What it measures Who drinks it
- Pitot tube (ram / total pressure) Impact pressure of the relative wind Airspeed indicator only
- Pitot drain hole Lets water and, if the ram inlet ices, trapped pressure out of the pitot line ASI system only
- Static port(s) Ambient still-air pressure ASI, altimeter, and VSI
The airspeed indicator is a differential-pressure instrument. It compares pitot (total) pressure with static pressure . The difference is dynamic pressure, the q in the lift equation. The altimeter is a static-only instrument: sealed aneroid wafers expand as static pressure falls (climb) and contract as it rises (descent). The Kollsman window adds a local altimeter setting so those wafers can be referenced to a sea-level pressure rather than to a raw vacuum. The vertical speed indicator is also static-only: a diaphragm sees the rate of change of static pressure through a calibrated leak . Instantaneous VSIs add accelerometers; trainers usually have the simple leak design.
If you remember nothing else: pitot blockage cannot freeze the altimeter or the VSI , because those two never see ram air. Static blockage can lie to all three.
The blockage table PAR actually writes
Ice, tape left over a static port after painting, a bug in the pitot, or a forgotten pitot cover are the same exam. Work each failure from the plumbing, not from a memorized cartoon.
Pitot ram inlet blocked, drain hole OPEN. Remaining pressure in the pitot line vents overboard through the drain. Pitot pressure falls to essentially static. Dynamic pressure goes to zero. The ASI drops to zero . Altimeter and VSI are untouched. This is the “ASI to zero” row.
Pitot ram inlet AND drain hole both blocked. Pressure in the pitot line is trapped at the value that existed when the ice sealed both holes. The ASI now compares a constant total pressure with a changing static pressure, so it acts like an altimeter .
Level flight: ASI is frozen at the indicated speed when the blockage occurred.
Climb: outside static pressure falls, the trapped-minus-static difference grows, and the ASI overreads (shows faster than you really are).
Descent: static pressure rises, the difference shrinks, and the ASI underreads .
Altimeter and VSI still work. The trap is treating every pitot failure as “needle to zero.” Zero is only the open-drain case.
Static port(s) blocked, pitot open. Static pressure in all three instruments is trapped at the blockage altitude.
Altimeter freezes at the altitude where the port iced or was taped.
VSI goes to zero and stays there; the calibrated leak no longer sees a changing pressure.
ASI is wrong in the opposite sense from the both-holes-blocked pitot case. In a climb , trapped static is higher than the real outside static, so the indicated dynamic pressure is too small and the ASI underreads . In a descent , trapped static is lower than real static and the ASI overreads .
Pitot and static both blocked. All three instruments freeze at the values they held when the last hole closed. You are now flying a clock and a compass.
- Failure ASI Altimeter VSI
- Pitot ram blocked, drain open Drops to zero Normal Normal
Pitot ram and drain blocked Frozen in level flight; overreads in a climb ; underreads in a descent Normal Normal
Static blocked, pitot open Underreads in a climb ; overreads in a descent Frozen at blockage altitude Goes to zero
Pitot and static blocked Frozen Frozen Frozen
Alternate static selected (typical unpressurized cabin) Reads slightly high Reads slightly high Momentary climb , then a usable rate
That table is the chapter. If a stem says “both the pitot tube and the drain hole ice over, then you climb,” the needle rises . If it says “only the ram inlet ices and the drain is still open,” the needle dies . If it says “static port ices and you descend,” the ASI overreads and the altimeter does not move .
Alternate static — PHAK’s direction, not a vibe
Most trainers have an alternate static source under the panel that opens the static lines to cabin air . In an unpressurized airplane the cabin is usually at a slightly lower pressure than the true outside static (venturi effect of the slipstream, leaks, the heater). PHAK’s direction is therefore fixed:
Altimeter indicates a slightly higher altitude than true.
ASI indicates a slightly higher airspeed than true.
VSI shows a momentary climb , then settles to a usable rate.
Do not invert that for a 172. A pressurized cabin is a different machine; PAR is asking about the unpressurized trainer. After you pull the alternate-static knob, expect the three needles to jump, then fly the new slightly-high picture. Opening a window or breaking the VSI glass is the old emergency static trick some orals still mention; the certificated alternate-static valve is what the AFM tells you to use.
- Six altitudes, five airspeeds
- The test recycles vocabulary until you mix “true” with “absolute.”
- Name What it is
- Indicated altitude What the altimeter reads with the current Kollsman setting
- Calibrated altitude Indicated altitude corrected for instrument error
- Pressure altitude What the altimeter reads with 29.92 inHg set — height above the standard datum plane
Density altitude Pressure altitude corrected for nonstandard temperature — the altitude the airplane performs at
- True altitude Actual height above mean sea level
- Absolute altitude Actual height above the terrain (AGL)
Pressure altitude is a setting, not a weather product. Density altitude is a performance number, not a height AGL. Absolute is the only one that cares about the dirt under you. True is MSL. If the stem says “height above the surface,” the word they want is absolute .
Airspeed is the same kind of ladder:
- Name What it is
- IAS What the ASI reads, uncorrected
- CAS IAS corrected for position / installation error
EAS CAS corrected for compressibility (a PAR vocabulary word; trainers live well below the regime where it matters)
- TAS CAS (or EAS) corrected for density — how fast you are really moving through the air
- Groundspeed (GS) TAS corrected for wind — how fast the ground is sliding by
IAS is what you fly. TAS rises with altitude at a given IAS because density falls. Groundspeed is a navigation number, not a pitot-static gauge. The ASI never “knows” the wind.
Pitot heat is the blockage you can prevent
The pitot mast has an electric heating element . It is anti-ice for the ram inlet and, on many masts, the drain. Turn it on in visible moisture when the air is near freezing, and anytime the AFM/POH or icing checklist says so. It is not a 14 CFR 91.205 day-VFR required instrument. A failed pitot-heat breaker plus a winter climb through a cloud is how you get the “both holes blocked, ASI acts like an altimeter” row in real life. Glass or steam, the plumbing is the same: the air-data computer still drinks the same two holes.
Scenario: Priya’s taped static port
Priya launches after a fresh paint job. Nobody pulled the tape off the static port. Pitot is clear. At 3,000 feet indicated she starts a climb to 5,500. The altimeter stops . The VSI dies at zero . The ASI, which had been an honest 80 knots, winds down as she climbs — she is not slowing; trapped static is too high and the ASI is underreading . She does not chase the decaying needle with pitch. She levels, pulls alternate static , watches the altimeter and ASI jump slightly high , and flies the PHAK picture home. If the ice had been on the pitot ram and the drain instead, the altimeter would still have climbed and the ASI would have risen as she climbed. Same airplane. Different hole.