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Lionofzion

Active member
Joined
Sep 14, 2005
Posts
28
Hello everyone, I'm a newly minted student pilot with a few questions I thought someone out there might know.
#1.-How do you figure out ISA and how important is it?
#2.-What exactly is TAS, how do you figure it out and is it the main airspeed number that a pilot uses for flight planning? (The reason I brought it up is I heard a pilot saying that TAS is better than groundspeed).

Thanks in advance.
 
Lionofzion said:
Hello everyone, I'm a newly minted student pilot with a few questions I thought someone out there might know.
#1.-How do you figure out ISA and how important is it?
#2.-What exactly is TAS, how do you figure it out and is it the main airspeed number that a pilot uses for flight planning? (The reason I brought it up is I heard a pilot saying that TAS is better than groundspeed).

Thanks in advance.

1. There isn't really anything to "figure out" about ISA (International Standard Atmosphere). All it is is a set of "standard" atmospheric conditions that anything else can be compared to (Namely, that it's 59 degrees F, and 29.92 inHg of pressure at sea level). Since most performance figures change based on pressure and temperature, it makes it easier giving them based on those uniform conditions, so you are always comparing apples to apples. Then you adjust for the real-world conditions at the time and place of your situation.

2. The concept of TAS (True air speed) is really simple; be careful you don' overthink it and make it more complicated than it has to be. All it is the speed the plane is moving, with respect to the air. That's it. So what about that little word "true?" Well, there are all kinds of errors that go in to the speed you read from your airspeed indicator, and the TAS has corrected for all of them and is your real speed through the air.

You'll read about all the other errors and types of airspeed later in your training, but for now the most basic one you need to focus is the one due to density (temperature and pressure). Indicated Air Speed (IAS) is what you read right off your instrument, and is NOT corrected for that error. Basically, your airspeed indicator is callibrated to read correctly at sea level, so down low your IAS is the same as your TAS. As you go higher, there is less air, and your IAS becomes lower for any value of TAS; meaning, you need to take the speed you are reading off the instrument and adjust it higher to figure out your TAS. There are several ways to do this, which you'll learn about later, but the important thing now is you get the concept.

So when is each type of speed useful? Well TAS lets you know how fast you're going through the air, so if you adjust it for the wind, you get your groundspeed, which is what you use for navigation and flight planning.

You might think, "well once I convert my IAS to TAS I can forget about it, since I just want to know how fast I'm going!" Hold it! Your airspeed indicator works based on how hard the air is "hitting" the pitot tube, just as your wings produce lift based on how hard the air is going over them. The same way that your airspeed indicator reads a lower-than-true airspeed up high where there is less air, your wings produce less lift. So, in essence, your airplane "feels the air" consistently according with IAS. Where does this lead up to?

When approaching to a landing at an airport up high in the mountains, you will have to make a faster approach because your wings need more airflow to make the same lift based off the thinner air. But, you don't need to screw around with converting your speed to TAS and figure out how much faster you need to go... Just use the same IAS as you use for an approach at a sea-level airport.

To summarize the difference... IAS is what the airplane "feels," while TAS is how fast it's really going. At sea level, they are the same, but at higher altitues, IAS becomes lower than TAS.
 
Lionofzion said:
#1.-How do you figure out ISA and how important is it?

ISA stands for International Standard Atmosphere. These figures are designed to establish a base from which to make calculations from. Most charts provide performance based on these conditions and make allowances for any deviation from these parameters. For example, lets say you're at seal level and under ISA, it should take 800' of runway to lift off. If the temperature were say 20 degrees warmer, you might need to add 200' to your figure. The chart will have a footnote indicating how much you need to compensate based on the temperature increments.

AT SEA LEVEL
Temperture:
59 degrees Fahrenheit or 15 degrees Celcius

Pressure:
29.92" inches of mercury or 1013.2 Millibars



Lionofzion said:
#2.-What exactly is TAS, how do you figure it out and is it the main airspeed number that a pilot uses for flight planning? (The reason I brought it up is I heard a pilot saying that TAS is better than groundspeed).

TAS stands for True Airspeed and is IAS (indicated airspeed) corrected for non-standard temperature and pressure. TAS is necessary in order to determine your ground speed.

TAS increases as altitude and temperature increase. Both of these factors decrease the air density. Under ISA conditions at sea level, IAS (indicated airspeed) and TAS are equal. As you climb to a higher altitude, the air becomes less dense or thinner which allows the airplane to travel faster.

TAS can be calculated by using a flight computer.

As a rule of thumb though, for every 1,000' you climb, TAS increases by 2% of your IAS.
 
Last edited:
Just so you're not confused, ISA does "not" stand for International Standard Temperature. It's international standard atmosphere. You definitely do "not" need to know your TAS (true airspeed) to determine your groundspeed. Just don't want to confuse newly minted pilot.
 
Thanks everyone for the help. Is there a rule of thumb or way to mentally figure out what ISA is at a given altitude? I think it has to do with the 2 degree lapse rate but not quite sure how it all fits together.
 
NYCPilot said:
AT SEA LEVEL
Temperture:
59 degrees Fahrenheit or 15 degrees Celcius

Pressure:
29.92" inches of mercury or 1013.2 Millibars.....

...or 14.7 pounds per square inch....
 

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