What Does a Pitot Tube Do on an Airplane?

The pitot tube, which is part of the pitot-static system, is one of the main systems that makes those flight instruments work.

Here’s a detailed look at the pitot, the static, and the system, including what’s connected to what and why.

What Is the Pitot-Static System?

The pitot-static system (pronounced “PEE-toe,” named for the 18^th-century French physicist Henri Pitot) is one of the two systems that keeps your flight instruments working.

The airspeed indicator (ASI), altimeter, and vertical speed indicator (VSI) all rely on the pitot-static system to operate correctly.

In glass-cockpit aircraft, the ADC (Air Data Computer) completes the pitot-static system’s functions.

This magic black box completes all the same tasks that the analog system does but uses digital sensors instead of mechanical gauges to make measurements.

In addition to the instruments, the pitot-static system consists of two primary components: the static port and the pitot tube. 

Static Port

The static port is an opening on the side of the aircraft that measures ambient air pressure.

The air pressure outside the aircraft changes with the weather (as the barometric pressure rises and falls), and the aircraft climbs or descends. 

In most aircraft, the static port is a small hole on the side of the fuselage (most Cessnas).

Some planes use a combination pitot-static tube, which combines both ports on a single appendage mounted under the wing (most Pipers).

What if the Static Port is Blocked?

Because the static port is not protected from icing, many manufacturers also include an alternate static port that can be opened inside the cockpit for emergencies.

This alternate measures pressure inside the cabin instead of outside, so some errors are introduced by using it. A table is located in the POH/AFM for making corrections. 

If your plane doesn’t have an alternate static port and the port becomes blocked, it is often advised to break out the glass face of the vertical speed indicator.

This will vent the whole system to the cockpit, enabling you to continue using your altimeter and airspeed indicators.

Why sacrifice the VSI? It’s the least important instrument, and you can safely operate without it working. 

Pitot Tube

The purpose of the pitot tube is to measure impact air pressure.

It is a small opening that faces forward, usually mounted to the underside of the wing. As the plane moves through the air faster and faster, more and more air presses its way into the pitot tube. 

More air coming in means you’re flying at a faster airspeed.

In addition to the main hole where the air enters, there’s also a small drain hole at the bottom of the pitot tube to drain off any rain or moisture that might get in. 

The hole on the pitot tube is very small, so the air must enter the tube at a perfect right angle to accurately measure airspeed. Since the plane’s angle of attack changes during different phases of flight, the pitot tube is not perfectly accurate all the time.

Aircraft manufacturers mount the tube to be accurate most of the time, which means it’s most reliable during flight at cruise speed.

There’s a correction table included in the POH/AFM to fix the errors at other times.

This is the basis for the difference between indicated airspeed and calibrated airspeed (KIAS vs KCAS). 

Pitot tubes are especially susceptible to icing since they are mounted directly in the airflow. As a result, pitot tubes have a built-in electric heater to melt any possible ice accumulation.  

The Pitot-Static System: Making It All Work to Keep You Flying

The airspeed indicator, vertical speed, and altimeter all use air pressure measurements, so they are the reason that the pitot-static system exists at all.

If you know what the atmospheric pressure is at sea level, and your static port takes a measurement of the pressure where you are, then your altimeter can tell you how high above sea level you are. 

Additionally, the VSI (vertical speed indicator) is designed to measure changes that occur in pressure. If pressure is dropping rapidly, it means you must be climbing fast. 

The airspeed indicator uses the ram or dynamic air pressure measured by the pitot tube to measure how fast you’re flying.

The more air that flows into the pitot tube, the faster you must be flying.

However, the airspeed indicator is also attached to the static port. It tells the indicator what “zero knots” looks like.

In other words, as your altitude and outside air pressure change, the indicator measures the ram air pressure against the static air pressure. So, if your static port becomes blocked, your airspeed indicator will continue to work, but it won’t be accurate.  

So, how do they all work together? The static port is connected to all three instruments: airspeed, VSI, and altimeter.

The pitot tube, however, is only connected to the airspeed indicator. 

The pitot-static system is discussed in detail in Chapter 8: Flight Instruments of the FAA’s Pilots Handbook of Aeronautical Knowledge.