Leslie Caubble, CFI/IGI
Two of the most important pieces of information that every pilot needs to know while flying is how high you are and how fast you’re going. Pilots rely on the airspeed indicator and altimeter instruments to measure and display this vital information. Both instruments are part of the pitot-static system, which every pilot needs to understand in order to fly safely.
Overview of the Pitot-Static System
The pitot-static system is a network of ports, tubes and lines that utilizes ambient (static) and dynamic pressure for the operation of the airspeed indicator (ASI), altimeter, and vertical speed indicator (VSI).
The pitot tube is a peculiar looking probe on aircraft to our non-flying friends. On the Cessna 172, one of the most popular training aircraft, the pitot tube is an L-shaped tube mounted under one of the wings, where it is exposed to an undisturbed airflow. Look closely on the underneath side of the tube and you’ll see a small drain hole where moisture drains out. The pitot tube aids in measuring total pressure, or ram pressure. This total pressure is the combination of dynamic pressure as the aircraft moves through the air and static pressure. The pitot tube is only connected to the ASI.
The static port is located on the side of the aircraft where undisturbed air can be found. As the aircraft moves through the air and between various altitudes, the atmospheric pressure changes, however the static pressure is unaffected by the movement of the aircraft itself. The static port is connected to the ASI, altimeter, and VSI.
Static port lines are tubes that are interconnected between the pitot tube, static port, and the three pitot-static driven instruments. These lines transport air pressure from the static port and pitot tube to the instruments. In more complex aircraft with glass panels, the pressures are read instead by a data computer which sends electronic signals to the instruments to be measured.
How does the pitot-static system operate the airspeed indicator?
The airspeed indicator (ASI) is the only instrument that utilizes both the pitot tube and static port to gather atmospheric pressure readings. Air pressure is exerted into a small hole in the front of the pitot tube as the plane flies through the air. This is called dynamic pressure. There is also static pressure that is input into the pitot tube. The dynamic pressure and the static pressure equal the total pressure. However, the static pressure around the pitot tube is constantly changing due to altitude and weather.
The ASI takes the total pressure input from the pitot tube, less the actual static pressure input from the static port, giving an airspeed indication. Inside the airspeed indicator is a diaphragm which moves with the difference in pressure. As the diaphragm moves, the needle of the ASI moves.
How does the pitot-static system operate the altimeter?
The altimeter is the only instrument that indicates the altitude of the aircraft, so this is a vital instrument for pilots. The altimeter utilizes the static port to receive inputs of the outside static pressure. Inside the altimeter is a stack of aneroid wafers which expand and contract with changes the static pressure. Like the ASI, the altimeter works by indicating a pressure differential between the static pressure and a constant “standard” pressure of 29.92 inches of mercury.
As the airplane climbs in altitude, the static pressure decreases. The opposite happens as the plane descends. This causes the aneroid wafers to move, which are linked to the needle of the altimeter. One thing a pilot must keep in mind is that changes in weather conditions along their route causes changes in the atmospheric pressure, regardless of maintaining the same indicated altitude. There’s a knob on the altimeter that can be used to set the pressure reading to the local atmospheric pressure, which is read through a setting window called the Kollsman window. This constant calibration by the pilot in flight is vital to safety of flight, especially in lower visibility. Not making these adjustments during flight could result in a midair collision or flight into terrain/obstacles.
How does the pitot-static system operate the vertical speed indicator?
The vertical speed indicator (VSI) indicates if the aircraft is climbing, descending or in level flight. The indication given is a rate of climb or descent in feet per minute (fpm). Much like the altimeter, the VSI works off a differential pressure measurement. There is a diaphragm located inside the instrument and is connected to the static line. Around the diaphragm, the empty case space is also connected to the static line, but with a calibrated leak.
The diaphragm receives unrestricted static pressure while inside the case around it receives metered static pressure, creating the differential in pressure. When the plane climbs or descends, the static pressure changes instantly, but the case pressure is gradual. The difference acts on the diaphragm, which is connected to the instrument’s needle, and gives the pilot an indication of the rate of climb or rate of descent.
Troubleshooting Piot-Static Errors
Like any aircraft system, the pitot-static system can have errors. Usually these are created by blockages in the pitot tube and/or static port, caused by ice, bugs, or dirt. Pilots are trained to cross-check their instruments and identify possible blockages and errors. The openings that can get blocked are the pitot tube’s main front hole, the pilot tube’s drain hole, and the static port opening. One or more blockages in these holes, alone or in combination, create different errors in instrument indication. It’s critical for pilots, especially those flying in instrument meteorological conditions (IMC), to know how to identify these errors and what secondary instruments can be used as a back-up.
Some aircraft are equipped with an alternate static source. If the static port becomes blocked, the alternate static source can be opened, which will then read the static pressure inside the flight deck. The readings on the three instruments won’t be as accurate, but it’s better to use than dealing with a full failure. Always refer to the aircraft’s Airplane Flight Manual (AFM) or Pilot’s Operating Handbook (POH) for corrections to make when using the alternate static source.
When ice is believed to be the culprit of a frozen pitot tube, aircraft are equipped with pitot heat. A switch is turned on in the panel, which provides an electrical current to the metal pitot tube, warming it up. It will depend on the severity of the ice buildup how long it will take to melt the ice. Checking the pitot heat operation is a part of the preflight checklist.
Special technicians trained in avionics can perform a thorough system inspection. This inspection is required every 24 calendar months if the aircraft will be operating under IFR. The inspection consists of testing and calibration to make sure the lines don’t have leaks and that the instruments are operating within standards. A few things pilots and mechanics can do regularly to prevent blockages are regular cleaning of the pilot tube and static port. Keeping a cover on the pitot tube while on the ground is also a best practice.
Conclusion
The pitot-static system and its operation is critical for safe flight. Pilots should be well educated on the operation of the system and know how to recognize possible errors. There are many more details about the pitot-static system and the three instruments in the Pilot’s Handbook of Aeronautical Knowledge (PHAK) in Chapter 8. Take some time to brush up on the knowledge of the system. Not only could you be asked about it on your next checkride, but knowing how the system works will help you be more prepared for a safe flight.
Leslie Caubble, CFI/IGI
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