INSTRUMENT RATING 06 / LEARN · EXPLORE · CHECK
Pitot-static instruments
Trace pressure sources, interpret airspeed, altitude and vertical speed, and distinguish the indications caused by each blockage.
Your learning goals
- Trace pitot and static pressure to the instruments that use each source.
- Predict indications for specific blockage cases and distinguish those cases.
- Explain alternate-static and pitot-heat use under the aircraft procedures.
Trace pressure before interpreting the number
The airspeed indicator, altimeter, and vertical speed indicator are pressure instruments. Their faces can be round dials or electronic tapes, but the source information matters more than the display shape. The pitot inlet faces the relative airflow and senses total pressure: ambient static pressure plus the pressure produced by bringing the airflow to rest. Static ports sample ambient pressure at tested locations on the aircraft.
A conventional airspeed indicator compares pitot total pressure with static pressure. The altimeter and VSI use static pressure only. An air data computer (ADC) can make the same pressure measurements and distribute the results to several displays and other systems. Two airspeed tapes supplied by one ADC and the same plumbing are not two independent pressure measurements.
The pitot tube's drain opening lets collected moisture escape. It is part of the pitot system, not the aircraft's static port. Keep the inlet, drain, and static openings distinct when predicting a failure. Dirt, insects, covers, ice, leaks, or damaged plumbing can make the pressure sent to an instrument differ from the pressure outside.
What airspeed actually tells you
Indicated airspeed (IAS) is the displayed speed before correction for instrument and position errors. Calibrated airspeed (CAS) corrects those errors using the aircraft's data. True airspeed (TAS) describes speed through the surrounding air after the relevant pressure, temperature, and compressibility corrections. Groundspeed describes travel over the ground and includes wind effects.
The static port location is tested, but airflow around it can still cause position error, particularly with changes in angle of attack or configuration. Use the applicable AFM/POH correction tables. The fact that a tape is electronic does not remove the error in its pressure source.
For cockpit control, compare airspeed with attitude, power, configuration, and the expected trend. A decrease after reducing power is plausible; a decrease to almost zero with unchanged power and corroborated flight performance is a reason to suspect the indication. A GPS groundspeed cannot replace indicated airspeed: a headwind, tailwind, or wind change can make the two differ greatly.
IFH 5-9: airspeed indicator and types of airspeed · PHAK Chapter 8: airspeed indicator, position error and blockage
The altimeter measures pressure, then applies a setting
A sensitive altimeter responds to static pressure with evacuated aneroid capsules or an electronic sensor. Lower static pressure corresponds to a higher indicated altitude. The barometric setting establishes the pressure reference used for the displayed altitude; it does not measure distance above the ground.
On the ground, use the current appropriate setting and check the indication against known field elevation within the applicable instrument-check guidance and aircraft procedures. In flight, compare both the setting and the altitude trend before diagnosing a discrepancy. Different barometric settings can make two serviceable altimeters disagree.
Indicated altitude is not necessarily true altitude. Uncorrected changes in pressure and nonstandard temperature can create error even with a clear static system. Flying toward lower pressure without updating the setting, or into colder air than the reference atmosphere, can place the aircraft lower than the indication suggests. Detailed altitude-setting and cold-temperature operating procedures belong in the IFR planning and procedures lessons.
IFH 5-3–5-7: sensitive altimeter, errors and altitude references
Read a VSI trend before a stabilized rate
A conventional VSI compares current static pressure with pressure that changes more slowly through a calibrated leak. That difference shows the direction and rate of pressure change, typically in feet per minute. After a pitch change, the trend becomes useful before the rate has fully settled. The instrument has lag; turbulence or abrupt control inputs can produce distracting swings.
After initiating a climb, cross-check attitude, power, airspeed, and increasing altitude. Allow the VSI to settle before making another correction based on its rate. Chasing each movement can create an oscillation. Some instruments use acceleration sensing or electronic processing to reduce lag, but their operating behavior and source dependencies still need to be learned for the installed system.
A zero VSI can mean level flight, a blocked static source, or an instrument problem. Compare it with the altimeter and other evidence; the needle alone does not establish that the aircraft has stopped climbing.
Predict the specific blockage, not a generic pitot failure
The following cases assume a conventional system, no leak, a complete blockage where stated, and otherwise serviceable instruments. Electronic installations can flag invalid data or add processing, so the table predicts the pressure effect, not every manufacturer's annunciation. For the altitude comparisons, hold actual airspeed approximately constant.
| Case | Airspeed indication | Altimeter and VSI |
|---|---|---|
| Pitot inlet blocked; drain clear; static clear | Pitot pressure escapes through the drain. IAS tends toward zero. | Remain pressure-responsive because their static source is clear. |
| Drain blocked; inlet and static clear | The drain blockage alone does not trap total pressure while the inlet is clear. The ASI may still respond normally, but retained moisture raises the risk of further obstruction or freezing. | No direct effect from that drain blockage. |
| Pitot inlet and drain blocked; static clear | Total pressure is trapped. IAS rises in a climb and falls in a descent as static pressure changes, regardless of the intended speed change. At the blockage altitude it can remain plausible. | Remain pressure-responsive. |
| Static blocked; pitot inlet clear | Uses trapped static pressure. Above the blockage altitude, IAS is lower than it would be with a clear static source; below it, IAS is higher. | Altimeter freezes near blockage altitude; VSI settles to zero. |
| Both total-pressure path and static path sealed | Both pressures are trapped, so the ASI can remain fixed despite changing flight conditions. | Altimeter is fixed; VSI settles to zero. |
The reasoning is always the same: ask which pressure can still change. With total pressure trapped but static clear, climbing reduces static pressure, increasing the difference measured by the ASI. With static trapped but total pressure still changing, a climb gives the ASI too high a static reference for that altitude and makes it underread relative to a clear system. A partial blockage or leak can produce slower, intermittent, or less tidy behavior.
Common error: remembering “a blocked pitot makes the ASI an altimeter” without specifying the drain. That behavior requires trapped total pressure. A clear drain produces a different result.
IFH 5-2–5-3: blocked pitot and blocked static systems · PHAK Chapter 8: blockage of the pitot-static system
Alternate static changes the reference pressure
If the aircraft has an alternate static source, learn its location, operating procedure, configuration requirements, and corrections before flight. Selecting it replaces the suspect primary static pressure with another source; it does not repair a blocked pitot inlet or a defective airspeed instrument.
In a typical unpressurized cabin installation, cabin pressure is slightly lower than undisturbed outside pressure. Selecting that source may make the altimeter read higher, the ASI read higher, and the VSI show a momentary climb before stabilizing if altitude remains constant. Treat those as common tendencies, not universal corrections. Cabin airflow, vents, windows, heater settings, configuration, and the installation affect the error. Use the AFM/POH instructions and tables for the actual aircraft.
A brief VSI jump after changing the source is a pressure-reference change; it does not by itself prove an actual climb. Cross-check before changing attitude. Do not invent a cabin configuration or break an instrument as a routine substitute for an installed alternate source. Any emergency action requires the applicable aircraft procedure.
IFH 5-3: alternate static source and effects of flight conditions · PHAK Chapter 8: alternate static source and AFM/POH corrections
Prevent blockage and prepare to recognize it
During preflight, remove the pitot cover and inspect the specified openings for contamination and damage. Do not probe openings or blow into the system as an improvised check. Follow the aircraft's inspection and maintenance guidance. Check required pitot-heat operation using the published method; a heater can cause burns and its ground use may have limitations.
Use pitot heat according to the aircraft checklist and conditions. Do not assume that selecting pitot heat heats the static ports, clears every blockage immediately, or authorizes flight in icing. Ice protection capability is an aircraft limitation, not a property of the airspeed display.
Before an IFR flight, identify the alternate-static control, pressure-fed instruments and ADCs, and any separate standby plumbing. Review expected pitch, power, and performance with your instructor so an implausible indication does not tempt you to chase it. This lesson teaches recognition and source reasoning; instructor-led partial-panel control and aircraft emergency procedures require separate practice.
IFH 5-2–5-3: blockage prevention · IFH 11-4 and 11-8: system inspection and pitot/static failure
FLIGHT SCENARIO
What would change your plan?
In a fictional training airplane, the pitot inlet and drain are sealed but the static ports remain clear. During a corroborated climb at roughly constant actual airspeed, indicated airspeed rises. In a second case the static source is sealed, the altimeter remains fixed and VSI settles to zero. Explain the pressure difference in each case. Would selecting a cabin alternate-static source solve both problems?
- NoticeWhat does this situation require?
- VerifyWhat evidence is still missing?
- DecideWhat keeps an option open?
Compare your reasoning
Trapped pitot total pressure with a clear static source makes IAS rise in a climb as outside static pressure falls. Trapped static pressure instead fixes altitude and vertical speed and makes IAS underread above the blockage altitude. Alternate static may restore a usable static reference in the second case, with aircraft-specific errors and corrections; it does not clear the first case’s blocked pitot. Maintain control using corroborated information and the actual checklist.
SUMMARY
Find which pressure can still change before trusting the indication.
Trapped pitot total pressure with a clear static source makes IAS rise in a climb as outside static pressure falls. Trapped static pressure instead fixes altitude and vertical speed and makes IAS underread above the blockage altitude. Alternate static may restore a usable static reference in the second case, with aircraft-specific errors and corrections; it does not clear the first case’s blocked pitot. Maintain control using corroborated information and the actual checklist.
Before moving on, explain the decision in your own words: what would you verify, and what would make you change the plan?
PRACTICE
Flashcards and knowledge check
10 flashcards, then 8 questions with explanations.
Enable JavaScript for flashcards and the knowledge check. The lesson and scenario remain available without it.
Sources & lesson notes
Sources & lesson notes
- IFH FAA-H-8083-15B: printed 5-2–5-9; 11-4 and 11-8Pressure-system operation, airspeed types, altimeter and VSI behavior, specific blockage cases and source changes. Current FAA handbook register checked October 8, 2026; use its listed errata and addenda.
- PHAK FAA-H-8083-25C, Chapter 8: Flight InstrumentsPitot inlet/drain/static blockage cases, alternate static and instrument errors; current FAA-listed chapter checked October 8, 2026.
- FAA-S-ACS-8C: II.B (printed 6–7), IV.A (printed 10)Lesson association with pressure-system knowledge and failure-mode risk. No flight skill or proficiency is assessed.
- FAA handbook publication registerEdition and errata/addenda listing verified October 8, 2026. Use the actual AFM/POH for inspection, heat, alternate-static configuration and corrections; no aircraft manual has been supplied.
For U.S. single-engine airplane instrument study. Official sources checked October 8, 2026; eCFR displayed Title 14 current through October 6, 2026. Use current publications and the applicable aircraft AFM/POH and avionics supplements. These lessons support ground study and do not replace required instruction, endorsements, experience, or tests.
ACS study associations
These associations identify the concepts taught here. Skill elements describe preparation for instructor-led flight training; reading or completing this lesson does not demonstrate flight proficiency.
- IR.II.B.K1 — Explains the pressure-instrument portion of the aircraft flight-instrument systems element.
- IR.II.B.K1a — Traces pitot/static inputs, distinct blockages and alternate-static qualifications.
- IR.IV.A.K3 — Distinguishes normal lag and reference changes from abnormal pressure indications.
- IR.II.B.R3 — Addresses misdiagnosis, pressure-source sharing and misuse of alternate static or heat.