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.