Airspeed is an important element that no pilot can do without. It is through this reading that pilots know the speeds of their planes. But how do pilots determine its value? An airspeed indicator is the instrument dedicated to carry out this noble task. Understanding how the ASI works is essential so you can be prepared to use one or know the alternative should it ever fail.
How the ASI Works
The airspeed indicator displays the dynamic pressure before it is corrected for any errors such as pressure or non-standard temperature. It is for this reason that this reading is also known as indicated airspeed. To come up with this reading, a series of mechanical activities must first take place. The ASI has to compare the dynamic airspeed with the static pressure to give its final reading.
Pressure Differences
Dynamic or ram airspeed is the speed of air being forced onto the aircraft. Its value is obtained through a pitot tube; a port facing to the forward of the aircraft. Another port on the side of the aircraft measures static pressure which is then compared with the ram air input. The difference in these pressures is converted through mechanical linkages for display on the airspeed indicator.
Indicated Airspeed Errors
Airspeed indicators may display the wrong reading due to a number of errors. Outside air conditions are always affected adversely by variations in temperature and air pressure. However, these density errors can be compensated through an OAT knob.
As with any other instruments, ASIs age with time and become prone to errors. Errors also occur if any of the input ports is blocked by insects, ice, or water. When the pitot tube is blocked, the ASI only shows change of airspeed relative to the change in altitude. On the other hand, the indicated airspeed will be higher or lower than normal if the static port is blocked.
Operating in Emergency
Aircraft manufacturers provide mechanisms to enable the safe operation of aircraft in the event of airspeed indicator failures. For instance, pitot tube heaters help to melt any ice frozen on the tubes. Pilots should activate these heaters whenever flying in cold weather. Many aircraft are also fitted with an alternate source for static speed. In the event that static tube port is blocked, pilots can activate these ports from the cockpit. Readings from the alternate sources may not be as much accurate as the original ports but are reliable in the case of emergency.
Although modern aircraft are installed with other options to indicate the dynamic speed, pilots should always be prepared for emergency operation. With proper training and simulation, flying or landing without an ASI should not be an issue.