True Airspeed
- Thread starter A Pilot
- Start date
ananoman
New Member
True airspeed is the actual speed the aircraft is traveling through the air. If there is no wind, this will be equal to ground speed.
True airspeed (TAS) will be within a few knots of indicated airspeed (IAS, what you read off the airspeed indicator in the cockpit) on a standard day at sea level. There is usually a few knots difference due to errors in the instrument itself and other minor errors caused by the location of the pitot/static ports on the aircraft.
As you increase in altitude, air density decreases and there are less molecules of air that get rammed into the pitot tube. This causes a problem since the airspeed indicator measures the difference between ram and static pressure to determine your airspeed. Thus, the higher you are, the less accurate your airspeed indicator.
In a standard atmosphere, this difference is about 2% per thousand feet. For example if you are at 5,000' MSL, and your indicated airspeed is 100 KIAS, then your TAS is about 110 kts. As an extreme example, the space shuttle travels at something like 17,000 mph when orbiting the earth and would have an airspeed of zero. Since there is no air in outer space, the airspeed indicator cannot function.
From your airplanes standpoint, IAS is all important. It determines how many molecules of air are traveling over the wings and making lift. No matter what altutude you are at, you will use the same indicated airspeeds to rotate for takeoff, or to fly final on landing. Your flap extension speeds, stall speeds, etc. do not change with altitude.
Takeoff and landing performance are another matter. We just said that you will rotate at the same indicated airspeed for takeoff, or flare for landing at the same speeds, irregardless of altitude. But, we also said that if there is no wind, TAS and groundspeed are equal. So, if you need to accelerate to 100 KIAS to rotate for takeoff at sea level, you now need to accelerate the aircraft to a groundspeed of 110 knots for takeoff at 5,000' MSL inorder to get 100 kts of indicated airspeed. This will obviously eat up more runway. In addition as altitude increases, engine power decreases, requiring even more runway. Landing distances will increase for the same reason, your brakes will need to do more work to stop the airplane.
I hope that was not too confusing!
True airspeed (TAS) will be within a few knots of indicated airspeed (IAS, what you read off the airspeed indicator in the cockpit) on a standard day at sea level. There is usually a few knots difference due to errors in the instrument itself and other minor errors caused by the location of the pitot/static ports on the aircraft.
As you increase in altitude, air density decreases and there are less molecules of air that get rammed into the pitot tube. This causes a problem since the airspeed indicator measures the difference between ram and static pressure to determine your airspeed. Thus, the higher you are, the less accurate your airspeed indicator.
In a standard atmosphere, this difference is about 2% per thousand feet. For example if you are at 5,000' MSL, and your indicated airspeed is 100 KIAS, then your TAS is about 110 kts. As an extreme example, the space shuttle travels at something like 17,000 mph when orbiting the earth and would have an airspeed of zero. Since there is no air in outer space, the airspeed indicator cannot function.
From your airplanes standpoint, IAS is all important. It determines how many molecules of air are traveling over the wings and making lift. No matter what altutude you are at, you will use the same indicated airspeeds to rotate for takeoff, or to fly final on landing. Your flap extension speeds, stall speeds, etc. do not change with altitude.
Takeoff and landing performance are another matter. We just said that you will rotate at the same indicated airspeed for takeoff, or flare for landing at the same speeds, irregardless of altitude. But, we also said that if there is no wind, TAS and groundspeed are equal. So, if you need to accelerate to 100 KIAS to rotate for takeoff at sea level, you now need to accelerate the aircraft to a groundspeed of 110 knots for takeoff at 5,000' MSL inorder to get 100 kts of indicated airspeed. This will obviously eat up more runway. In addition as altitude increases, engine power decreases, requiring even more runway. Landing distances will increase for the same reason, your brakes will need to do more work to stop the airplane.
I hope that was not too confusing!
Bandit_Driver
Gold Member
Good Explanation but a bit of generalization for big airplanes...
Our landing and rotation speeds change based on Temp, aircraft weight, and airport elevation.
Another way to think of airspeeds are this way "ICE T"
I = indicated airspeed
C= Calibrated air speed (indicated corrected for installation error)
E= Equivalent Airspeed (Calibrated A/S corrected for compressability)
T= True Airspeed (Equiv A/S corrected for environmental Factors(non std temp and pressure)
I however cannot remember the formulas to convert one into another...
Our landing and rotation speeds change based on Temp, aircraft weight, and airport elevation.
Another way to think of airspeeds are this way "ICE T"
I = indicated airspeed
C= Calibrated air speed (indicated corrected for installation error)
E= Equivalent Airspeed (Calibrated A/S corrected for compressability)
T= True Airspeed (Equiv A/S corrected for environmental Factors(non std temp and pressure)
I however cannot remember the formulas to convert one into another...