How Fast Am I Really Going?
Methods to measure speed
In my last post, The Need for Speed, I explained nautical miles per hour (knots), which is how aviation measures speed. Measuring speed through the air gets complicated and it’s a lot more than just knots. There are at least four ways to measure airspeed.
Indicated Airspeed (IAS)
Indicated airspeed (IAS) uses a pitot-static system to measure how fast an aircraft is traveling through the air. The pitot tube points forward and measures the pressure of air ramming into the tube. The ram air pressure compared to static pressure is used to calculate airspeed, as shown on an airspeed indicator.
The pitot tube was invented in the early 1700s by Henry Pitot, a French engineer. Pitot systems have been used to measure aircraft speed since 1912. A pitot-static system also powers a vertical speed indicator and the altimeter.
As altitude increases, indicated airspeed will show a significantly slower number than the plane’s true airspeed (TAS). The air becomes less dense as you go higher, resulting in fewer air molecules slamming into the pitot tube. A plane’s true airspeed will be two percent faster than the indicated airspeed for every 1,000 feet of altitude. So a jet showing 300 KIAS (knots indicated airspeed) at 30,000 feet will have a true airspeed of about 480 knots.
Indicated airspeed works fine for aircraft that don’t fly very fast and operate at lower altitudes. The OV-10 only had indicated airspeed. Showing true airspeed in the cockpit requires some sort of inertial or GPS navigation system. Modern military and commercial aircraft will all have a true airspeed indicator. In the F-16A, TAS was shown in the lower right corner of the HUD (heads-up display).
Ground Speed
Ground speed is true airspeed adjusted to the wind speed at the flight altitude. Knowing ground speed is essential for commercial airliners and other high-altitude jet traffic. Jet stream wind speeds can be over 200 mph, affecting flight times and fuel consumption.
While true airspeed and ground speed are more accurate measures of how fast a plane travels, indicated airspeed is often the most critical measure of speed. Indicated airspeed determines aircraft handling characteristics.
When I flew combat training missions, the indicated airspeed on the big dial right in front of me told me how hard I could maneuver the aircraft. For example, to get the tightest possible turn in an F-16A, I had to have at least 330 KIAS. That would be the same at low altitude or at 30,000 ft.
Mach
Now to the last measure of speed, the Mach number. The speed of sound sits at Mach One. Supersonic flight happens when flying faster than Mach One. The speed of sound is also altitude-dependent. At sea level, sound travels at 661 knots (TAS). At 40,000 feet, the number is 573 KTAS.
The fastest I ever went in an F-16 was Mach 1.82 at 40,000 feet. I was going 1,080 KIAS. As I noted in my post, FCF flights, F-16A jets would rarely be able to exceed Mach 1.5.
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Amazing, sounds like fun.