A CALIBRATION GUIDE TO PITOT STATIC SYSTEMS
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A CALIBRATION GUIDE TO PITOT STATIC SYSTEMS

INTRODUCTION

A pitot static system is the driving force of an aircraft’s speed. There are many components that make up this crucial system of an aircraft. This guide will walk you through the components of a pitot static system, the importance of calibration, how-to calibrate and more.


WHAT DOES A PITOT-STATIC SYSTEM CONSIST OF?

A pitot-static system is a system consisting of static air pressure and dynamic pressure due to motion of an aircraft through air. The combination of these two pressures is key in aircraft operation because it is the driving force of an aircraft’s airspeed, Mach number, altitude and altitude trend. Failure of this system can be extremely dangerous and can result in life-threatening situations. In order to ensure a system is functioning properly and precisely, it is vital to have all components within the pitot-static system calibrated on a frequent basis.

COMPONENTS OF A PITOT-STATIC SYSTEM

A pitot-static system is made up of several important components. Each component plays an important role within the system; therefore it is crucial that each individual component is working properly and accurately.

PITOT TUBE

The L-shaped pitot tube is a device located on the exterior of an aircraft with the purpose of measuring airspeed. It contains a small opening at the front that allows total pressure to enter. This total pressure is then transmitted to the airspeed indicator (ASI) through a tiny tube. Once the dynamic and static pressure is delivered to the ASI, airspeed is indicated.

The pitot tube also has smaller hole located in the posterior of the tube. This small opening functions as a drain for moisture intakes during precipitation.

STATIC PORT

The static port is located on the side of an aircraft and functions as a small air inlet. Its purpose is to measure static air pressure, also known as barometric pressure. The pressure within the static port is collected and then utilized by the altimeter and the vertical speed indicator (VSI).

Many aircrafts will contain multiple or alternative static ports in case of possible blockage.

INSTRUMENTS

The pitot-static system consists of three instruments, including:

The airspeed indicator (ASI) is a sensitive pressure gauge that measures the difference between the pitot and the static pressure. This instrument must be properly calibrated to ensure measurements are precise and accurate. If the airspeed indicator is reading improperly, airspeed can fluctuate which could result in high safety risks.

The altimeter is one of the most important instruments installed in an aircraft. This is because it measures the height of an aircraft above a given pressure level. The proper settings for an altimeter can’t be overemphasized; therefore frequent maintenance and calibration is a necessity. If the altimeter is not functioning properly or is not on the correct setting, a pilot will not be able to determine the altitude.

The vertical speed indicator (VSI) is responsible for indicating whether an aircraft is climbing, descending or in level flight. If this instrument is properly calibrated, the VSI should read zero. Proper functionality of the VSI is pivotal during aircraft flight.

WHY PITOT-STATIC SYSTEM CALIBRATION IS IMPORTANT

Due to the many running parts within a pitot-static system, it is of great importance to ensure that each part is functioning properly and accurately. Many devastating tragedies have occurred due to error of the pitot-static system. Airspeed and altitude are two key factors in safely operating an aircraft. In order to ensure overall safety of any individuals associated with an aircraft, calibration of the pitot-static system should be performed frequently. Calibration will ensure the pitot-static system is measuring total pressure precisely and accurately, which in turn will lead to correct readings of airspeed, altitude and altitude trends.

THE IMPORTANCE OF AIRSPEED INDICATOR CALIBRATION

The airspeed indicator (ASI) is arguably one of the most important instruments installed into an aircraft and is a key piece to the correct operation of a pitot-static system. For these reasons alone, the accuracy and proper functionality of this instrument is extremely necessary. In order to establish certainty and reliability, an ASI must be calibrated and maintained on a regular basis.

Calibration of an ASI is of great importance because it not only calculates pressure accuracy, but also measurement uncertainty. Due to the many different types of airspeeds, such as indicated airspeed (IAS), calibrated airspeed (CAS), true airspeed (TAS) and ground airspeed (GAS), pilots and technicians must be familiar with what margin of error is okay and what is not. When an ASI is frequently calibrated the margin of uncertainty is calculated and should be repeatable. These calculations are extremely beneficial to pilots during flight and help ensure overall safety of correct airspeeds.

CALIBRATION OF AN AIRSPEED INDICATOR

An ASI is an extremely sensitive pressure gauge that must undergo the correct form of calibration to avoid damage to the instrument and inaccurate results. Due to the sensitivity of the gauge, it is in your best interest to have a professional calibrate your ASI. There have been many instances reported of individuals simply blowing into the ASI to apply pressure. This should NEVER be done and could cause serious damage and expensive consequences. If you choose to calibrate your ASI on your own, consider the following steps:

STEP 1: OBTAIN A MANOMETER – THIS WILL BE USED TO MEASURE PRESSURE

STEP 2: DISCONNECT THE ASI FROM THE PITOT-STATIC SYSTEM

STEP 3: CONNECT THE MANOMETER TO THE PITOT LINE OF THE ASI

STEP 4: APPLY KNOWN PRESSURE SLOWLY AND STEADILY TO THE MANOMETER

STEP 5: CALIBRATE THE AIRSPEED INDICATOR BY CHECKING ACCURACY OF THE MEASUREMENTS. IN OTHER WORDS, COMPARE RESULTS WITH A TABLE OF PRESSURES.

STEP 6: RE-INSTALL THE ASI, ENSURING THAT ALL HOSES ARE HOOKED UP PROPERLY AND THE INSTRUMENT IS IN THE SAME POSITION IT WAS IN PRIOR TO REMOVING. ALSO ENSURE THAT ALL CONNECTIONS ARE TIGHT

As previously mentioned, it is highly recommended that calibration of an ASI is done professionally by an accredited calibration lab. Although step-by-step instructions seem simple, it is important to keep in mind that an ASI is a critical instrument in the pitot-static system. If a pitot-static system is not calibrated properly or if the instruments within the system are not calibrated or hooked up properly, chances of aircraft failure are high. This could leave to tragic consequences which, in the long run, would be more expensive than sending your instrument to a professional lab.

THE IMPORTANCE OF ALTIMETER CALIBRATION

An altimeter, also referred to as the aneroid barometer, is an important instrument for proper operation of the pitot-static system and navigation of an aircraft. Without proper altimeter functionality, an aircraft would not be able to measure altitude, which is necessary for safe flight. Because altitude varies in different settings, it is extremely important to ensure that the altimeter is properly measuring the correct amount of atmospheric pressure. In order to ensure this, altimeters not only need to be set before every fight, but also calibrated frequently.

ALTIMETER CALIBRATION PROCESS

Essentially, an altimeter is a type of pressure gauge. The inside of the altimeter consists of a sealed casing which contains a stack of sealed aneroid diaphragms, also referred to as wafers. These wafers are calibrated to a standard atmospheric pressure and are responsible for reading the intake of pressure. Calibration of an altimeter ensures that pressure measurements from the wafers are accurate and reliable. During calibration, an accredited calibration laboratory will apply known pressure to the altimeter to check that the instrument is indicating pressure levels that correspond with the International Standard Atmosphere (ISA). Similarly to other pressure gauges, if calibration of an altimeter is ignored, high chances of inaccurate results, unreliability and extreme safety risks are likely.

It’s important to note that the calibration of an altimeter before each flight is much different than actual instrument calibration. The instrument itself should be calibrated regularly by an accredited calibration lab to ensure that the absolute altitude measurements are calculating accurately. Calibration of an altimeter is essential to assure proper functionality of the pitot-static system as a whole.

THE IMPORTANCE OF VERTICAL SPEED INDICATOR CALIBRATION

A vertical speed indicator (VSI), also known as a vertical velocity indicator, is a vital component within the pitot-static system. Proper functionality is crucial for determining whether an aircraft is level, climbing or descending during flight. To ensure accuracy and safety of both the VSI and the pitot-static system, instrument calibration should be maintained and scheduled regularly.

WHY CALIBRATION OF THE VERTICAL SPEED INDICATOR IS IMPORTANT

Similarly to the ASI and the altimeter, the VSI is a type of pressure gauge. The VSI is sealed within a case and is connected to the static line through a calibrated leak. Within the case is a diaphragm, which is key to the measurement of pressure drops to indicate climbing/descending. When pressures are equalized during level flight, the needle of the VSI is calibrated to read zero. During pre-flight, it is extremely important to verify the VSI needle is reading zero. If the needle indicates anything other than zero feet per minute on the ground, then the calibration of the VSI is off. The instrument can still be used, but it is highly discouraged. If a VSI is not reading properly on ground, chances of serious hazard in flight are likely, especially in weather.

Many will argue that malfunction of the VSI is no big deal and that flight can still take place. Although it is true there are other ways to calculate the climbing of an aircraft, proper functionality of the pitot-static systems relies on the correct functionality of the VSI. This is why calibration of the VSI should be performed on a frequent basis – to ensure overall accuracy of the VSI itself and also of the pitot-static system.

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