Airspeeds. Which ones do we care about?
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Airspeeds. Which ones do we care about?
Just a few general questions if you guys can just bare with me please.
Firstly, does indicated airspeed change with anything?
If we have wind shear for example, does the indicated a/s change?
Secondly, I believe when flying commercial, the aircraft speeds are already calculated AFTER wind` corrections, so that's why speeds like Vmcg, V1, etc are all indicated airspeed, is this true?
Once in cruise, it is the Ground Speed that matters to us?
What about calibrated a/s, true a/s?
Just some info from the more experienced please.
Thank you!
Firstly, does indicated airspeed change with anything?
If we have wind shear for example, does the indicated a/s change?
Secondly, I believe when flying commercial, the aircraft speeds are already calculated AFTER wind` corrections, so that's why speeds like Vmcg, V1, etc are all indicated airspeed, is this true?
Once in cruise, it is the Ground Speed that matters to us?
What about calibrated a/s, true a/s?
Just some info from the more experienced please.
Thank you!
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z.khalid - glad to bear with you!
Think about it; a body in motion tends to remain in motion (at the same inertial speed) until acted upon by a force.
So if you are flying in zero wind at 150kt, and are subjected to a sudden tailwind of 25 kt, your inertial speed (GS) cannot instantly change - Newtonian physics won't allow it. Thus your IAS will quickly drop by 25kt - and it will take some precious seconds to recover to 150kt IAS.
And the new GS after recovery will be 175kt! The extra energy must come from added power and/or trading altitude for airspeed.
If we have wind shear for example, does the indicated a/s change?
So if you are flying in zero wind at 150kt, and are subjected to a sudden tailwind of 25 kt, your inertial speed (GS) cannot instantly change - Newtonian physics won't allow it. Thus your IAS will quickly drop by 25kt - and it will take some precious seconds to recover to 150kt IAS.
And the new GS after recovery will be 175kt! The extra energy must come from added power and/or trading altitude for airspeed.
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The only speed a crew has is the one on the instruments - the indicated speed. (Even if you've got fancy air data computers, the speed is still the "indicated" speed, by definition).
So if I want a crew to fly using a speed reference, then I MUST provide that speed consistent with the displayed speed - so V1, V2, Vref etc must all be provided in IAS terms.
They are all airspeeds, so are measured relative to the air mass. The only time you worry about wind correction is either to get ground speed data for navigation purposes (because you navigate relative to the ground, but the aicraft flies relative to the air) or for certain specific cases where the procedures have you adjust a speed due to high winds etc. Those cases are not "correction for wind" - they are adjustments due to wind.
Here's a repeat of my answer to a previous post in a different forum:
This is my understanding of the various different speeds that can be used and their corrections:
ASIR - Air Speed Indicator Reading; the uncorrected reading of the instrument.
IE - Instrument Error; a correction made for errors in the construction and calibration of the instrument.
IAS - Indicated airspeed; ASIR corrected for IE.
PEC - Pressure Error Correction; a correction made for erroneous static port measurements caused by the position of the port and the local airflow disturbance cuased by airspeed, altitude and configuration.
RAS - Rectified Air Speed; IAS corrected for PEC.
CAS - Calibrated Air Speed; US term for RAS.
CE - Compressibility Error; the error induced by the increase in apparent air density at high speeds due to the compressibility of air
EAS - Equivalent Air Speed; the RAS or CAS corrected for CE. This is the speed that the aircraft 'thinks' it is flying at.
DE - Density Error; an error due to the fact that pitot pressure at a given speed varies with air density. The instrument is calibrated to read correctly with an asssumed air density of 1.225kg per cubic metre (i.e. the air density at sea level in the ISA)
TAS - True Air Speed; EAS corrected for DE.
So, the full sequence is:
ASIR
IE
IAS
PEC
RAS(CAS)
CE
EAS
DE
TAS
Hope this helps!
Eck
This is my understanding of the various different speeds that can be used and their corrections:
ASIR - Air Speed Indicator Reading; the uncorrected reading of the instrument.
IE - Instrument Error; a correction made for errors in the construction and calibration of the instrument.
IAS - Indicated airspeed; ASIR corrected for IE.
PEC - Pressure Error Correction; a correction made for erroneous static port measurements caused by the position of the port and the local airflow disturbance cuased by airspeed, altitude and configuration.
RAS - Rectified Air Speed; IAS corrected for PEC.
CAS - Calibrated Air Speed; US term for RAS.
CE - Compressibility Error; the error induced by the increase in apparent air density at high speeds due to the compressibility of air
EAS - Equivalent Air Speed; the RAS or CAS corrected for CE. This is the speed that the aircraft 'thinks' it is flying at.
DE - Density Error; an error due to the fact that pitot pressure at a given speed varies with air density. The instrument is calibrated to read correctly with an asssumed air density of 1.225kg per cubic metre (i.e. the air density at sea level in the ISA)
TAS - True Air Speed; EAS corrected for DE.
So, the full sequence is:
ASIR
IE
IAS
PEC
RAS(CAS)
CE
EAS
DE
TAS
Hope this helps!
Eck
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and yet...
... IAS is not a speed.
Sounds ilogical, but it is not.
Note that we "would like" to have EAS displayed on the instrument panel, even though GS or TAS can be available, EAS is the primary parameter for flight. Due to some errors (position, instrument, compressibility...) we can only have IAS. But let's consider IAS as a close enough EAS, ok?
TAS is the velocity of the airplane relative to the air mass
GS is the velocity of the airplane relative to the ground
IAS is the velocity of the airplane relative to.... oops, relative to what???
EAS or IAS are not velocities.
EAS is the TAS that an airplane would have in the ISA at sea level if it had the same dynamic pressure that the airplane has at that moment.
Basically, the airspeed indicator is a dynamic pressure indicator, only expressed in knots instead of PSI or inches of mercury. Dynamic pressure is what we need to fly, after all.
If you lack TAS or GS you will have problems from the navigation point of view, but if you lack IAS, then you have problems to fly the airplane.
Sounds ilogical, but it is not.
Note that we "would like" to have EAS displayed on the instrument panel, even though GS or TAS can be available, EAS is the primary parameter for flight. Due to some errors (position, instrument, compressibility...) we can only have IAS. But let's consider IAS as a close enough EAS, ok?
TAS is the velocity of the airplane relative to the air mass
GS is the velocity of the airplane relative to the ground
IAS is the velocity of the airplane relative to.... oops, relative to what???
EAS or IAS are not velocities.
EAS is the TAS that an airplane would have in the ISA at sea level if it had the same dynamic pressure that the airplane has at that moment.
Basically, the airspeed indicator is a dynamic pressure indicator, only expressed in knots instead of PSI or inches of mercury. Dynamic pressure is what we need to fly, after all.
If you lack TAS or GS you will have problems from the navigation point of view, but if you lack IAS, then you have problems to fly the airplane.
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Here's the math behind it if you all are curious...should be clear if you like math as much as I do.
http://www.calpoly.edu/~rcumming/Airspeed.pdf
http://www.calpoly.edu/~rcumming/Airspeed.pdf
