The pitot-static system is the network of air-pressure ports, tubing, and instruments that measures the air moving around your airplane and turns it into three cockpit readings: airspeed, altitude, and vertical speed. Almost every airplane you will fly, from your first trainer to a jet, uses the same basic pitot-static system, so knowing how it senses pressure (and how it fails) is one of the highest-value systems lessons in your training.
Part of our How Airplanes Work guide.
What the pitot-static system is and its parts
The pitot-static system works by comparing two kinds of air pressure: the ram pressure of the air your airplane is flying into, and the still (ambient) pressure of the air around it. The difference between those two pressures gives you airspeed, and the ambient pressure alone gives you altitude and rate of climb or descent. To do that job the system needs four things.
- The pitot tube. A small forward-facing tube, usually mounted under a wing or on the nose, that captures ram air (also called impact or dynamic pressure) as the airplane moves forward. The faster you go, the higher the ram pressure. Each pitot tube has a tiny drain hole at the bottom so moisture can escape, which matters when we talk about blockages below.
- The static port (or ports). One or more small flush holes on the side of the fuselage that sense the undisturbed ambient pressure of the surrounding air. Many airplanes have a static port on each side of the fuselage, plumbed together so that a slip or skid does not skew the reading.
- The alternate static source. A backup valve, standard on most instrument-capable airplanes, that lets the static instruments draw pressure from another location (usually inside the cabin) if the primary static port becomes blocked.
- The plumbing. Separate lines carry ram pressure from the pitot tube and static pressure from the static ports to the instruments. The airspeed indicator receives both lines. The altimeter and vertical speed indicator receive only the static line.
The three instruments it drives
The pitot-static system feeds exactly three flight instruments, and each one derives its reading a little differently.
Airspeed indicator (ASI). The ASI is the only instrument that uses both pressures. Inside it, ram pressure from the pitot tube pushes against a diaphragm while static pressure surrounds that diaphragm. The instrument displays the difference between them (the dynamic pressure), which is your indicated airspeed. When ram and static pressure are equal, as they are when the airplane is parked, the ASI reads zero.
Altimeter. The altimeter uses static pressure only. It contains sealed aneroid wafers that expand as outside pressure drops with altitude and compress as pressure rises, driving the altitude hands. You set the current sea-level pressure in the Kollsman window so the instrument reads height above the correct datum.
Vertical speed indicator (VSI). The VSI also uses static pressure only, but it measures the rate of change rather than the value. Static pressure reaches a diaphragm directly and reaches the case around it through a calibrated leak. When you climb or descend, the pressure difference across that leak shows how fast you are changing altitude, which is why the VSI lags a few seconds behind an abrupt pitch change.
What each instrument needs
Here is the whole system on one line per instrument. If you memorize this table you can reason through any pitot-static failure on your checkride.
| Instrument | Pressure source | What it shows |
|---|---|---|
| Airspeed indicator (ASI) | Pitot (ram) and static | Indicated airspeed |
| Altimeter | Static only | Altitude above a set pressure datum |
| Vertical speed indicator (VSI) | Static only | Rate of climb or descent |
Notice the pattern: only the airspeed indicator touches the pitot line, so a pitot problem affects only the ASI, while a static problem affects all three instruments.
When a pitot-static port blocks
Blockages from ice, insects, or a left-on pitot cover are a favorite oral-exam topic because the instrument behavior is predictable once you know which line is affected. Work through the three classic cases.
Pitot tube blocked, drain hole open
The airspeed indicator drops toward zero. With the ram inlet plugged but the drain still open, the trapped ram pressure simply bleeds out the bottom, so the diaphragm relaxes and airspeed falls off even though you are still flying. The altimeter and VSI are unaffected.
Pitot tube and drain both blocked
The airspeed indicator starts behaving like an altimeter. With ram pressure sealed inside the line, the ASI no longer responds to speed. Instead it responds to changes in the static pressure around its diaphragm, so it over-reads as you climb and under-reads as you descend, which can dangerously mask a decaying airspeed in a climb.
Static port blocked
A blocked static port disables the whole static side. The altimeter freezes at the altitude where the port clogged, the VSI sticks at zero and shows no vertical movement, and the airspeed indicator becomes inaccurate: it reads low when you climb above the trapped altitude and high when you descend below it. This is exactly why the alternate static source exists.
The alternate static source and its errors
The alternate static source restores usable readings to the altimeter, VSI, and airspeed indicator when the primary static port is blocked, usually by opening the instruments to unpressurized cabin air. It keeps you flying, but it does not read exactly the same. Because cabin pressure is normally a little lower than the outside static pressure (air flowing over and through the airframe lowers it), the altimeter and airspeed indicator read slightly high and the VSI may show a brief false climb the moment you select it. Your Pilot’s Operating Handbook lists the specific correction for your airplane, if any, so check it during systems study rather than in the air. If your airplane has no alternate static valve, the emergency technique in many light airplanes is to break the glass face of the VSI, which vents the static instruments to the cabin at the cost of that instrument.
Pitot heat and icing
Pitot heat is an electric heating element inside the pitot tube that prevents and removes ice so the ram inlet stays open. In visible moisture near or below freezing, ice can plug the pitot inlet and its drain within seconds, which is the setup for the dangerous pitot-and-drain-blocked case above, so pitot heat is a standard item before entering clouds or precipitation. Watch your ammeter or electrical load when you switch it on, since the element draws significant current, and confirm it is heating during preflight (carefully, it gets hot fast). Some airplanes also heat the static ports. Because reliable pressure instruments are essential when you cannot see the horizon, the pitot-static system and its anti-ice equipment become even more important as you move toward an instrument rating and flight in clouds. The electrical side of all this ties back to the airplane’s charging system, covered in our aircraft engine and systems guide, and the airspeeds the ASI displays are the same ones that define your stall margins.
Build your systems knowledge
These two FAA handbooks explain the pitot-static system in full, with the diagrams and the exact failure language an examiner wants to hear.
What you'll need
Frequently asked questions
What is the pitot-static system?
The pitot-static system is the group of ports, lines, and instruments that measure ram and static air pressure and turn them into three cockpit readings: airspeed, altitude, and vertical speed. It has a pitot tube for ram pressure, one or more static ports for ambient pressure, an alternate static source as backup, and the plumbing that carries those pressures to the instruments.
Which instruments use the pitot-static system?
Three instruments use it: the airspeed indicator, the altimeter, and the vertical speed indicator. The airspeed indicator uses both pitot (ram) and static pressure, while the altimeter and the vertical speed indicator use static pressure only.
What happens if the pitot tube is blocked?
If only the ram inlet is blocked and the drain hole is open, the airspeed indicator drops toward zero. If the drain is blocked as well, the airspeed indicator behaves like an altimeter, over-reading in a climb and under-reading in a descent. The altimeter and vertical speed indicator are not affected by a pitot blockage.
What happens if the static port is blocked?
If the static port is blocked, the altimeter freezes at the altitude where it clogged, the vertical speed indicator sticks at zero, and the airspeed indicator becomes inaccurate, reading low above the trapped altitude and high below it. Selecting the alternate static source restores usable readings.
What is the alternate static source for?
The alternate static source restores static pressure to the altimeter, vertical speed indicator, and airspeed indicator when the primary static port is blocked, usually by drawing cabin air. Because cabin pressure is a bit lower than outside static pressure, the altitude and airspeed then read slightly high, so consult your POH for the correction.