PHAK · Chapter 8

Pitot-Static System

Master the pitot-static system: how the airspeed indicator, altimeter, and VSI work, blockage failures, alternate static source, and preflight checks.

Source reviewReviewed by GroundScholar Editorial ReviewLast reviewed: 2026-08-04Primary FAA source

CFI's Whiteboard Explanation

Picture two air inputs on your airplane. The pitot tube points forward and catches ram air—it only feeds the airspeed indicator. The static ports sit flush on the fuselage and sense ambient pressure—they feed the airspeed indicator, altimeter, and VSI.

Quick failure cheat sheet:

Pitot blocked (ice, drain plugged): ASI now acts like an altimeter—reads high in a climb, low in a descent.
Static blocked: Altimeter freezes, VSI reads zero, ASI reads low climbing and high descending. Pull the alternate static.

In visible moisture near freezing, turn pitot heat on.

Handbook Reference

PHAK Ch 8

7.pitot-static-system. Pitot-Static System

The pitot-static system is a combined system that uses the static air pressure and the dynamic pressure due to the motion of the aircraft through the air to drive three of the most important flight instruments: the airspeed indicator (ASI), the altimeter, and the vertical speed indicator (VSI). Understanding how this system works—and how to recognize when it fails—is fundamental to safe flight, especially in instrument conditions.

Components

The system has two distinct pressure inputs:

Pitot tube (impact/ram pressure): Mounted on the leading edge of the wing, nose, or vertical stabilizer, the pitot tube faces directly into the relative wind. It captures ram air pressure (also called impact or total pressure), which is the sum of static pressure plus the dynamic pressure produced by the aircraft's forward motion. The pitot tube feeds only the airspeed indicator.
Static port(s): Small, flush-mounted holes located on the side of the fuselage (or on the pitot mast itself) in undisturbed airflow. They sense ambient static pressure only. Static pressure is fed to all three instruments: ASI, altimeter, and VSI.

Most certificated airplanes have an alternate static source that the pilot can select if the primary static port becomes blocked (commonly by ice). On unpressurized airplanes, this alternate source typically vents to the cabin. Because cabin pressure is slightly lower than outside static pressure (due to the venturi effect of air flowing past the fuselage), using the alternate source generally causes the altimeter to read slightly higher, the airspeed indicator to read slightly faster, and the VSI to momentarily show a climb.

Most pitot tubes have an electrically heated pitot heat element to prevent ice from blocking the ram-air inlet or the small drain hole. Pitot heat should be turned on any time visible moisture is present near or below freezing, and it should be checked during preflight and runup (ammeter deflection or warm pitot tube).

How Each Instrument Works

Airspeed Indicator: A sealed, sensitive differential pressure gauge. The pitot pressure enters a diaphragm; static pressure surrounds it inside the case. The difference (dynamic pressure, q = ½ρV²) deflects the diaphragm and drives the needle. Indicated airspeed (IAS) is therefore a direct measurement of dynamic pressure, not true speed through the air mass.
Altimeter: A stack of sealed aneroid wafers expands and contracts as the surrounding static pressure changes. A linkage converts that motion into altitude in feet. Setting the Kollsman window to the local altimeter setting corrects the instrument for non-standard surface pressure.
Vertical Speed Indicator: Also a diaphragm referenced to static pressure, but the case is vented to static through a calibrated leak. When altitude changes, the pressure inside the case lags the diaphragm, and the resulting differential is displayed as a rate of climb or descent in feet per minute. Because of the calibrated leak, the VSI has a 6–9 second lag.

Blockage Failures

Knowing what each instrument does when a line is blocked is a frequent oral-exam and IFR scenario:

Pitot inlet blocked, drain hole open: Ram air bleeds out through the drain. Airspeed drops to zero. Altimeter and VSI unaffected.
Pitot inlet AND drain hole blocked (e.g., ice): The pitot line is sealed and the ASI behaves like an altimeter—it reads higher as the airplane climbs and lower as it descends, regardless of actual airspeed. Pitch and power become the primary speed reference.
Static port blocked (pitot clear): The altimeter freezes at the altitude where the blockage occurred. The VSI freezes at zero. The ASI still works but is inaccurate—it reads low in a climb (because trapped static is higher than current static) and high in a descent. Switch to the alternate static source.
Both pitot and static blocked: All three instruments are unreliable.

If no alternate static source is installed, breaking the face of the VSI is the emergency last resort—it vents the static system to the cabin. Because the VSI then reads backward and is unusable, this is only done when no other option exists.

Errors and Corrections

Indicated airspeed must be corrected to obtain true airspeed:

IAS → CAS (calibrated airspeed): corrects for installation and instrument error; from the POH airspeed calibration table.
CAS → EAS (equivalent airspeed): corrects for compressibility, significant above ~200 KIAS and ~10,000 ft.
EAS → TAS (true airspeed): corrects for non-standard density. A practical rule of thumb is that TAS increases about 2% per 1,000 ft of density altitude above sea level.

Preflight

During preflight, verify the pitot tube cover is removed, the pitot inlet and drain hole are clear, the static ports are unobstructed, and pitot heat operates. After engine start and during taxi, the airspeed indicator should read zero, the altimeter should be within ±75 feet of field elevation when set to the local altimeter setting, and the VSI should read zero (or be noted as a known offset).

FAA PHAK Chapter 8 decoder

One pitot-static instrument needs both pressures, and one of them no rule requires at all

Source checked: FAA PHAK Chapter 8: Flight Instruments and 14 CFR §91.411; Last verified: 2026-08-04; date_retrieved: 2026-08-04.

As verified 2026-08-04, the airspeed indicator is the only pitot-static instrument that needs both pitot and static pressure, and the vertical speed indicator is the only one 14 CFR §91.205 requires for neither VFR nor IFR flight. The static system that feeds the altimeter must be tested every 24 calendar months before it is legal for IFR in controlled airspace (§91.411). The first table below adds that regulatory layer to the handbook explanation; the second reads every blockage case across all three instruments at once.

Pressure input → required equipment → test cycle

Instrument	Pressure input	Required by §91.205?	§91.411 24-month test
Airspeed indicator	Pitot (ram) pressure inside the diaphragm, static pressure around it	Yes — 14 CFR §91.205(b)(1) for VFR day, carried into IFR by §91.205(d)(1). The only instrument that needs both inputs.	Not tested as an instrument by §91.411, but the static system feeding it is, every 24 calendar months.
Altimeter	Static pressure only, against sealed aneroid wafers	Yes — §91.205(b)(2) for VFR day; §91.205(d)(5) upgrades it to a sensitive altimeter adjustable for barometric pressure for IFR.	Yes — §91.411(a)(1): each altimeter instrument, within the preceding 24 calendar months, to part 43 appendices E and F.
Vertical speed indicator	Static pressure only, with the case vented through a calibrated leak	No — §91.205 lists no vertical speed or rate-of-climb indicator for VFR day, VFR night, or IFR.	Not named by §91.411, but it shares the static pressure system the rule does test.
Automatic pressure altitude reporting (Mode C encoder)	Static pressure, reported to ATC through the transponder	Not in §91.205; §91.215 governs when a transponder with altitude reporting is required.	Yes — §91.411(a)(1) names it alongside the altimeter and static system on the same 24-calendar-month cycle.

Blockage and alternate-source matrix

Condition	Airspeed indicator	Altimeter	VSI	First action
Pitot inlet blocked, drain hole open	Falls to zero — ram air bleeds out of the drain.	Normal.	Normal.	Fly pitch and power; the static instruments are still trustworthy.
Pitot inlet and drain hole both blocked (typically ice)	Behaves like an altimeter — indication rises in a climb and falls in a descent, whatever the real airspeed.	Normal.	Normal.	Pitot heat on; set a known pitch and power and disregard the ASI.
Static ports blocked, pitot clear	Still moves but reads low in a climb and high in a descent.	Frozen at the altitude where the blockage occurred.	Frozen at zero.	Select the alternate static source.
Alternate static source selected, unpressurized airplane	Reads faster than actual — cabin static is lower than ambient.	Reads higher than actual.	Momentarily shows a climb, then settles.	Apply the POH or AFM alternate-static correction table.
Pitot and static both blocked	Unreliable.	Unreliable.	Unreliable.	With no alternate static installed, breaking the VSI face vents static to the cabin — a last resort that makes the VSI itself unusable.

Vocabulary

Pitot-static head (or pitot-static mast) is a single probe carrying the pitot inlet and the static source in one unit, rather than a pitot tube plus flush fuselage static ports. It is the same two pressure inputs in one package — which is why a single iced or obstructed head can degrade the airspeed indicator, altimeter, and VSI together, while a fuselage-port installation usually fails one input at a time.

Aircraft-specific limit: alternate static source corrections, airspeed calibration tables, and abnormal procedures come from the aircraft POH or AFM. A handbook and regulation summary explains the system; it does not replace those limitations.

Pitot-static system FAQ

Which instrument requires both pitot and static pressure?

Only the airspeed indicator. Ram pressure from the pitot tube enters its diaphragm while static pressure fills the case around it, and the needle displays the difference. The altimeter and the vertical speed indicator run on static pressure alone, which is why a pitot blockage leaves them working normally.

If the pitot tube becomes blocked by an insect in flight, which instruments fail?

It depends on the drain hole. If the inlet is blocked but the drain is open, trapped ram air escapes and the airspeed indicator drops toward zero; the altimeter and VSI are unaffected. If the inlet and drain are both sealed, the airspeed indicator behaves like an altimeter — reading higher in a climb and lower in a descent — and the altimeter and VSI still read correctly. Either way, only the airspeed indicator is affected.

If you are using an alternate static source, what does the airspeed indicator show?

In an unpressurized airplane the alternate source usually vents to the cabin, where pressure is slightly lower than ambient static. The airspeed indicator therefore reads faster than actual, the altimeter reads higher than actual, and the VSI momentarily shows a climb. The POH or AFM gives the specific correction for the aircraft.

Is a pitot-static head the same thing as a pitot tube?

Not quite. A pitot-static head — also called a pitot-static mast — is a single probe that houses the pitot inlet and the static source in one unit, instead of putting the static ports on the fuselage. The two pressure inputs are unchanged; they are just packaged together, which means one blocked or iced head can degrade the airspeed indicator, altimeter, and VSI at the same time.

How often must the static system and altimeter be tested?

Under 14 CFR §91.411(a)(1), an airplane or helicopter may not be operated in controlled airspace under IFR unless each static pressure system, each altimeter instrument, and each automatic pressure altitude reporting system has been tested and inspected within the preceding 24 calendar months and found to comply with appendices E and F of part 43. Opening and closing the static system — other than using the drain and alternate static valves — triggers a fresh appendix E check.

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Oral Exam Questions a DPE Might Ask

Q1Which instruments use the pitot tube and which use the static ports?

The pitot tube feeds only the airspeed indicator with ram (impact) pressure. The static ports feed all three pitot-static instruments: the airspeed indicator, altimeter, and vertical speed indicator.

Q2What happens to the airspeed indicator if the pitot tube and its drain hole both become blocked in a climb?

With the pitot line completely sealed, the trapped pressure can no longer escape, so the ASI behaves like an altimeter—indicated airspeed increases as you climb and decreases as you descend, regardless of actual airspeed.

Q3What errors should you expect when using an alternate static source in an unpressurized airplane?

Cabin pressure is typically lower than ambient static, so the altimeter usually reads higher than actual, the airspeed indicator reads faster than actual, and the VSI momentarily shows a climb. Pilots should consult the POH for specific corrections.

Related FAR References

FAR § 91.205

Plain-English + oral questions

FAR § 91.411

Plain-English + oral questions