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).