FlightAware Discussions

FlightAware KNOTS and MPH VS Aircrafts GS and TAS.

flightaware.com/live/flight/SWA1 … S/tracklog

Using the above flights track graph as an example, what would the aircrafts cockpit displays be the equivalent of the KNOTS/MPH values of FlightAware?



It is very confusing to me.


In other words;
MPH are not the same as KTS, MPH are statute miles (the same as your car if you are in the US, Kts are Nautical miles…roughly a 15% difference. Flight Aware shows MPH as a courtesy for people unfamiliar with Knots. Aircraft have nothing in the cockpit showing MPH except for the airshow or a calculated difference in case the boss asks. The Russians use Kilometers per hour and if the Roman Empire was still around we might be using cubits per second. It doesn’t matter, just stick to the same standard…no switching from MPH to KTS.

To simplify a bit, pilots have an indicator that shows Indicated Air Speed (IAS) in Knots. At sea level on a “standard day” (15 degrees C/ 59F, no wind) IAS and TAS (true airspeed) are the same. However as the airplane climbs the air gets thinner, there are fewer molecules of air to impact the mechanism. So as you climb the IAS reads low compared to the TAS, the higher you go the worse the error. Today aircraft have computers that calculate the change and display TAS to the crew.

Next: Wind. It’s all fine and dandy to know how fast you are going through the air, after all that is what we need for takeoff and landing. The wing only cares about the airflow going around it, Mr. wing doesn’t care about ground speed. That is why pilots use IAS for takeoff and landing. However TAS won’t tell you when you will arrive at your destination with any accuracy. You need to compensate for the wind. GS is easy to calculate, pilots have been doing it since 1903, or even before if you believe the history channel. Take TAS add or subtract the headwind or tailwind and you get GS, That is normally done on the ground before flight when you are dong all of the computations to figure out things like how much fuel do I need for this flight? In the cockpit you can read GS directly from the GPS without doing that calculation. Using GS you can calculate when you will arrive.
Also computers compare TAS, GS, Heading and Course and come up with what the wind must be.

IAS compensated for altitude/temperature = TAS
TAS compensated for wind = GS

Hope that helps,

It does help, thank you for the in depth explanation and reply. I would say it would be a safe bet to assume that the KNOT value of the track graph would be GS on the PFD.


Yes, sort of.
Until Flight Aware goes exclusively to ADS-B data what you see here is based on ATC radar which typically updates every 6-10 seconds so the GS they have is an average over the last few seconds. Over the ocean GS is an average based on position reports which may only happen every 45 minutes or so. Again an average. When ADS-B/C is used the updates are much more frequent. As you have seen FA only gives you position graphs every minute, or oceanic reports as they happen.
By contrast the cockpit instruments are reading real time GS based on whatever sensor is in use, GPS, DME, IRU etc. In the cockpit GS variations of one or two knots every couple of seconds is common even in cruise flight. What you get here is esentially a smoothing out of data over a longer period of time.