Understanding the pitot static system is important for pilots and student pilots to ensure a safe flight. Here is what you should know.
If you have ever looked at an airplane and seen quite a few tiny little tubes pointing forward, you have already seen an element of the pitot static system. Those little tubes are actually quite important and are vital for a safe flight. In this article, we will take a look at the pitot static system, talk through how it works, and tell you which instruments rely on the data they provide.
What is a Pitot Static System?
The pitot static system is a network of pipes, sensors, and instruments that measure air pressure in different forms. These various measurements are displayed on calibrated instruments to provide meaningful data to the pilot. This data will include things such as:
- Altitude
- Rates of Climb and Descent
- Airspeed
This is all important information to a pilot. The pitot static system consists of:
1. One or more forward-facing pitot-static tubes
This is normally an ‘L’ shaped tube with an opening at the front. The pitot tube is used to measure airspeed. We will explain more about how it works in a moment.
2. Static ports
These are normally flush mounted inlets that are designed to measure ambient air pressure. They are normally located in areas with relatively undisturbed airflow (which can be difficult to find on a moving airplane).
3. Pitot static lines
Both the pitot and static inlets are connected to their respective instruments using ‘lines’. This is a fancy way of saying ‘pipes’. In most training aircraft, this is literally a hollow tube that air can pass through. In bigger and more advanced aircraft, the pressure is fed through to an air data computer, which converts the associated pressure to electronic signals.
What 3 Instruments Work on the Pitot Static System?
As we have seen, various parameters are measured by different instruments. Here are the main instruments that receive air pressure data from the pitot static system. We will include the port from which the instrument receives information.
1. The Altimeter
This is used to measure vertical above a given datum. The altimeter is literally a calibrated barometer set to read zero at ground level due to ambient air pressure.
Source: Static System
2. The Vertical Speed Indicator
This is used to monitor the aircraft’s rate of climb and descent
Source: Static System
3. The Airspeed Indicator
This is used to assess the aircraft’s speed through the air
Source: Static System and Pitot System
Note that all of the three pressure instruments receive pressure information from the static system. It is only the airspeed indicator that uses air pressure from the pitot tube.
How Does a Pitot Work?
Have you ever placed your hand out of a car window when the car is moving? What happened to your hand? You will have undoubtedly felt the force of the air on your palm. This is, in principle, how a pitot tube works.
If you could accurately measure the force exerted on your hand as you hold it out of the window, you would be able to assess what speed you were traveling at.
Obviously, pilots do not go flying around with their hands out of the window. A tube placed facing forward, and connected to an airspeed indicator, is a much more convenient (and hand-friendly) solution!
But you may have noticed that the airspeed indicator has two inputs. The pitot tubes and the static ports. Why is this?
The airspeed indicator actually measures something called dynamic pressure.
If it sounds technical, don’t worry, it is really easy to understand.
Dynamic pressure is simply the difference between the ambient air pressure and the force of the air caused by the aircraft’s forward movement. To measure this difference, we need to know what the ambient air pressure actually is… And the best place to get this is from the static port.
Why Do Static Ports Need Undisturbed Airflow?
When moving air is brought to rest, it exerts a force. We call this force pressure. Remember how we talked about dynamic pressure and how we measure it against ambient air?
If the static ports (that measure ambient air) were subject to air pressure, they would create a false baseline that would not represent the real story. The other instruments that read static pressure would be affected too.
The way around this is to place the static ports at a point on the aircraft where there is minimum air pressure created by the airplane’s movement.
What Will Happen if the Pitot Static System is Blocked?
Both the pitot and the Static inlets have to be free of obstructions. Air pressure instruments are calibrated according to the size and shape of their respective inlets. In the simplest terms, if you change the size of the inlet or block it completely, the instruments will not read correctly.
This is a dangerous situation that affects small aircraft… And big ones. Air France 447 stalled as a result of ice on their probes and sensors.
Why would the pitot-static system become blocked?
There are a whole host of reasons, let us talk through a few and how they can be dealt with.
1. Failure to remove covers
You will often see pitot tubes covered with a red flag. It normally says “remove before flight”. If a pilot forgets to do this, air can’t get into the lines and instruments.
A professional flight training college will go through the importance of a proper ‘walkaround’ in the early stages of your flight training.
2. Blockages from bugs or dust
There are certain types of bugs that love to shelter in a nice thin tube. They are not always easy to see. Again, it will be a part of your flight training that you are taught how to thoroughly inspect your sensors as part of your daily aircraft checks.
3. Ice Accumulation
This one is particularly dangerous, as it often only happens once airborne. Fortunately, many modern training aircraft have an electrical element installed inside the pitot tube that can be switched on to heat and melt the ice.
The risk is obviously greatly reduced if you are flying in warmer, drier climates.
While simple in principle, the pitot static system can be a little complex to understand, especially when discussing the implications of blockages and failures. The instruments that it feeds are crucially important to safe flight.
California Aeronautical University offers expert tuition in aircraft instrumentation, procedures, and flight safety as a standard part of their pilot training courses.
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Mr. Matthew A. Johnston has over 23 years of experience serving various roles in education and is currently serving as the President of California Aeronautical University. He maintains memberships and is a supporting participant with several aviation promoting and advocacy associations including University Aviation Association (UAA), Regional Airline Association (RAA), AOPA, NBAA, and EAA with the Young Eagles program. He is proud of his collaboration with airlines, aviation businesses and individual aviation professionals who are working with him to develop California Aeronautical University as a leader in educating aviation professionals.
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