How does airplane find its own speed?

Question

How does airplane find its own speed? Like does it use relative motion or does it just set one point on the ground as a basis with respect to time or do they use satellites?

Answer 1

You've gotten lots of partial answers that together make a fairly good one. There are three speeds that are interesting to know, and they're all determined a little differently.

Indicated airspeed (IAS) is the easiest to find. The airspeed indicator on the panel tells you what it is. It is determined by measuring the pressure of the air striking what's called the pitot tube, which is located in the airstream outside the airplane -- usually below the leading edge of a wing, but it can be in other places.

Because indicated airspeed measures the pressure of molecules of air against some surface, it can be tricked as you climb into less dense air (or as you move from an area of high pressure to an area of low pressure or vice-versa). The air has less density at higher altitudes, so it will appear as if you're going slower even if you're actually going the same speed with respect to the airmass.

To offset this effect, we also calculate "True Airspeed" (TAS). TAS is the indicated airspeed adjusted for density. Density is related to barometric pressure, temperature, and altitude. You can ignore all that and just think of TAS as your real airspeed through the air because most of the problems that beset indicated airspeed have been removed.

Indicated airspeed is important because your stall speed and red-line (never exceed) speeds are based on indicated airspeeds. But if you're calculating how long it takes to get somewhere, or if you're monitoring engine performance, you need to know true airspeed.

As I said, indicated airspeed is your speed within the mass of air in which you're flying. Remember that air moves across the surface of earth, and it moves at different speeds and directions at different altitudes at any given time. So the other important speed to know is ground speed.

Your ground speed is your true airspeed plus or minus the effect of the movement of the airmass you're flying in. So if you're flying into a strong wind, your TAS might be 200 kts but your ground speed might only be 150 kts. Turn around and fly the other way and your TAS is still 200 kts but now the wind is working for you and you're going 250 kts over the ground.

Ground speed can't be determined by measuring the outside conditions. You need a reference point of some kind. The most commonly used system these days is GPS. GPS uses stationary satellites in orbit around the earth to triangulate and determine your position. Changes in position over time equals your speed over the surface of the earth.

There are radio-based ways of determining ground speed, such as DME. You can also measure it visually by calculating the time it takes to travel between two obvious landmarks, then measuring the distance between those landmarks on your map. But GPS is a lot easier.

Ground speed is important for flight planning, but it's important not to use it for flying the airplane. An airplane can stall (quit flying) while it is moving 200 kts over the ground. That's why the portable GPS devices that display ground speed on a display that looks like an airspeed indicator are so dangerous.

TAS is important for flight planning and performance monitoring, but again not for flying the airplane.

IAS is important for flying the airplane but not really relevant to flight planning at all.

There are other airspeeds that pilots worry about (CAS comes to mind) but these three are the easiest to understand.

Answer 2

And if you are lacking any better/easier way, call the nearest ATC radar facility and ask them to start a track on you.  After a minute or two of averaging, your ground speed will be calculated and the controller can provide it to you.

Answer 3

it uses forward airspeed, because even if there is a wind being part of this airspeed, that air is flowing over the wings and creating lift. this is measured by a tube exposed to the forward airflow and measures the pressure of the ram air and displays the airspeed on a gauge. However, there are measurements such as ground speed which is usually given from a gps.

Answer 4

im guessing the speed of the turbines and velocity.

Answer 5

There are several different airspeeds used for flying that pertain to all aircraft. Most are in knots.There is indicated airspeed that is displayed on the panel(derived from atmospheric pressure). Then there is calibrated airspeed and then groundspeed, now calculated from GPS.

Answer 6

Airspeed indicator measures the airflow against the aircraft through a pitot tube.

GPS gives true track ground speed.

airspeed is relative to conditions (i.e. wind direction and speed, as well as altitude and airpressure)

so gps satelite navigation gives the most logical speed.

also you have to remember an airplane flies 1-6 miles above the earth surface. So if its GPS ground speed is 100 Knotts, a nautical mile 6 miles above the earth is a differnt length than on the ground.

Another way is with radio beacons and distance calculations.

Most pilots refer to Indicated AirSpeed or IAS

EDIT: Also GPS has lag which makes it not up to date, you can suddenly stop and the GPS for a few moments (depending on quality of signal and device) will display an incorrect v alue.

Answer 7

The first answer is pretty much on the dot!