Types of Airspeed

"I am flying at either 130, 145, or 125 knots. Not sure which." - New student pilot

Indicated Airspeed (KIAS)

What you see on your airspeed indicator is called... indicated airspeed. If in knots, we abbreviate the term as KIAS (knots indicated air speed). Kinda easy, huh? On the image above, it's the 143 knots on the speed tape. This is the speed experienced by the pitot-static system.

That being said, indicated airspeed is only accurate at standard sea level conditions. Why is that? Well... remember how the atmosphere changes with altitude? Ha-ha! You probably guessed it (no you didn't, I know you too well). As altitude increases during a standard day (i.e., no inversions), pressure and temperature decrease. With decreasing temperature, there is a decreasing air density, too. In essence, the air becomes thinner. As the air becomes thinner, the pitot tube feels "less air" into it. That means it's feeling less dynamic pressure as compared to as sea level conditions. Get it?

At lower altitude, air is thicker (higher density), and the pitot-tube feels more air into it. At higher altitude, air is thinner (lower density), and the pitot-tube feels less air into it.

As you climb, the airspeed indicator will tell you a speed that is lower than the "real" speed. Yeah, in essence, it becomes more wrong the higher you go - or more appropriately- the colder the temperature gets.

Now, the more inquisitive of you probably wonder how to manage airspeed limits if the airspeed indicator is correct only at standard sea level conditions. Ha! Good question. The answer is: it doesn't matter. That is because if the pitot-static systems feels that "less air", then the wing will too! That means that all your V-speeds and structural limits do not change with temperature (i.e., altitude...). This is extremely important. It means that once you memorize your V-speeds, those will apply for any altitude.

True Airspeed (KTAS)

Now that we discussed indicated, let's chat more about true airspeed. When in knots, we abbreviate it as KTAS (knots true air speed). True airspeed is indicated airspeed corrected for non-standard temperature (i.e., NOT 25 degrees Celsius). That's right. We discussed in the previous paragraph how non-standard temperature affects airspeed, and how indicated airspeed gets increasingly incorrect as you climb through the atmosphere. Well, at what airspeed are you truly flying then? We call that True Airspeed!

There exists an equation to calculate true airspeed to the decimal, but as pilots, we are blessed with avionics that provide us with the actual value. On Garmin's G1000 image above, our true airspeed was 172 KTAS. Note the 'TAS 172 KT' below the speed tape. If you still wish to calculate it, there is an easy rule-of-thumb which works quite well (especially for public math)... Add 2% of your KIAS for each 1,000 feet above sea level.

From the above example, the aircraft is at 10,000 feet above sea level. So 2% x 10 = 20%. If I add 20% to 143 KIAS (or 1.20 x 143) = 171.6 KTAS. The avionics reported 172 KTAS. Pretty close!

Ground Speed

Ground speed is the speed you are traveling over the ground. As much as KTAS is a true measure of aircraft's performance, ground speed is used for calculations of actual performance (time enroute). Your speed over the ground is affected by the winds. If you have a tailwind, your ground speed will increase (winds are pushing you). If you have a headwind, your ground speed will decrease (the winds are blowing against you, slowing you down).

Look at the example above. The KTAS is 172 kts and the wind synoptic shows a 14 kts headwind. So, your ground speed is 172-14=158! You will then use ground speed to calcualte time enroute, given a distance. Also, remember that the unit of "knot" is "nautical mile per hour".

Equation for Ground Speed

Equation for Time Enroute

Notice how ground speed is really important in the generation of your navigation log. If you haven't started flght planning yet, ignore that. It's too painful and we want to postpone your pain as much as possible...