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#### true airspeed, calibrated airspeed, ground speed, indicated airspeed...what do I use???

true airspeed, calibrated airspeed, ground speed, indicated airspeed...what do I use??? So many kinds of speeds gets me confused, what do I use for what?? what speed do I use when using my computer to compute for time from A to B? am I the only one who gets frustrated for this? please only people with experience answer tis question

Submitted: 2400 days ago
victor q

Submitted: 2400 days ago

true airspeed is indicated airspeed corrected for temperature and altitude.

calibrated airspeed is indicated airspeed corrected for airflow dynamics of the aircraft the airspeed indicator is installed in.

ground speed is your speed over the ground (gps shows this)
used most often for time from a to b.

indicated airspeed is what the airspeed indicator "indicates" or shows as you are flying, and is used in above calculations.

good flying!

1/5

born west coast

Submitted: 2400 days ago

True airspeed will be used to calculate groundspeed

Groundspeed is used to calculate ETA and fuel burn

Indicated airspeed (or Calibrated airspeed in older aircraft) is read off the Airspeed indicator and is used to calculate true airspeed. It is also useful because most performance numbers (Vs, Vso, Va, Vy, Vx, etc) are published in KIAS (knots indicated airspeed) and can therfore be read directly off the airspeed indicator.

In order to calculate True Airspeed, you can go through a fnacy calculation on your E6-B, or simply use a common rule-of-thumb calculation that says: add 2% for each 1000' above sea level. For instance at an IAS of 105 knots and 5000', add 10% (5 x 2 = 10) and add to 105 to get to 116 KTAS.

Calibrated airspeed is defined as "indicated airspeed corrected for position and instrument error." This is basically a meaningless term...what that REALLY means is in some airpseed indicators, there is a built in correction for variable things like angle of attack and disturbed airflow at various airspeeds. However, this correction only applies at sea level and on an ISA standard day. For this reason, the term was abandoned. Airspeed indicators now read in Indicated Airspeed, and you probably won't need to worry about Calibrated Airspeed.

2/5

Jason

Submitted: 2400 days ago

Don't worry abotu calibrated airspeed, it isn't especially important. Indicated airspeed is the airspeed the plane "thinks" it is moving through the air. True airspeed is indicated airspeed corrected for nonstandard pressure. for example, the air is less dense up at high altitudes. So even though you are moving the same speed, there are less air mollecules flowing into the pitot tube, and the indicated airspeed goes down, even though you are auctually still at your initial speed. Groundspeed is your speed over the ground, IE if you were flying directly into a 10 knot headwind, and you were flying at 120 knots airspeed, your ground speed would be 110 knots. your airspeed will be relatively consistent, but groudn speed will be dependant on what the winds are doing.

3/5

Walter H

Submitted: 2399 days ago

I think I can give you a simple summary of these speeds that will help you out.

Indicated airspeed is the most important speed for actually operating the airplane. This is read straight off the airspeed indicator. It tells you when to rotate for takeoff, when you should retract or extend flaps and landing gear, how fast to fly for short field landings, etc. All of the operational speeds for the airplane, such as stall speed, manuvering speed, and never-exceed speeds are indicated airspeeds. It also is the speed you use to fly when Air Traffic Control directs you to fly a certain speed. This is the speed that is most used by pilots to fly the airplane.

Calibrated airspeed...not so important to pilots. Think of this speed as being a mechanical error in your airspeed indicator. But, the manufacturer knew about the error when they painted the numbers on your airspeed indicator to make up for the error. That is why, it is not really important that you know or use calibrated airspeed.

Ground speed...is the aircraft speed over the ground. This is the speed used for flight planning, and how long it takes to get from point A to point B. This is important to pilots in knowing how much fuel will be needed, how far away from an airport to start descending, and maybe which direction to land (if no other wind info is available). Groundspeed is either shown on a GPS (or DME, etc.) instrument, or it must be calculated. Most aircraft today have some sort of groundspeed indicator, so calculation is not often necessary.

True airspeed is a little bit complicated. The good news is, you don't really need to use if very often. It is mostly needed to do calculations in order to determine groundspeed, which I already discussed. If you don't care about groundspeed, then there is little use in using true airspeed for anything. True airspeed is used when manufacturers talk about how fast their planes will go, because that is the actual speed you travel in a zero wind condition. They could just as well call it "no-wind groundspeed." (In case you're curious, true aispeed can be determined by a calculation using indicated airspeed corrected for altitude and non-standard outside air temperature. Also, as you may have figured out, the only difference between true airspeed and groundspeed is the wind.)

So really, all you NEED to use regularly is indicated airspeed to fly the plane, and groundspeed to know how long it will take to get where you're going.

Good luck to you.

4/5

sfsfan1

Submitted: 2398 days ago

for calculating A to B navigation use ground speed when inflight, true airspeed /if u are able to calculate it/ is used during preflight preparation. idicated airspeed shows you your aircraft "aerodynamic behavior" this speed is used when your instructor or handling book says "perform maneuver at 250 knots". you remember that your stall speed is X and the number is constant regardless of altitude. your aircraft is handling as if flown at X speed at the sea level, but in fact it is flying much faster at FL200. the stall speed is increasing with the altitude, -true airspeed- but the indicated speed shows you imaginary number that tells you well clear the aircraft doesnt have enough lift, stall !!
for preflight preparation, and planning use true airspeed together with wind drift.
in flight use ground speed from GPS when calculating time-over-next waypoint.
when maneuvering, use indicated airspeed. the thick aroow at the speed indicator.

5/5

Honza_Urba n

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