When TAS=IAS?
08-16-2023, 07:34 PM
#1
New Hire
Thread Starter
Joined APC: Aug 2023
Posts: 2
When TAS=IAS?
Since the airspeed indicator is calibrated to sea level ISA, the only realistic scenario of where the TAS=IAS would be when we are at sea level or very close to it and the pressure is 29.92 and the temp is 15 degrees?
Thank you guys!
Thank you guys!
08-18-2023, 02:59 PM
#2
Gets Weekends Off
Joined APC: Nov 2018
Posts: 121
Originally Posted by Breaach
Since the airspeed indicator is calibrated to sea level ISA, the only realistic scenario of where the TAS=IAS would be when we are at sea level or very close to it and the pressure is 29.92 and the temp is 15 degrees?
Thank you guys!
Thank you guys!
08-18-2023, 05:09 PM
#3
Disinterested Third Party
Joined APC: Jun 2012
Posts: 6,025
Calibrated airspeed looks at position and installation error, which also looks at errors with increases or decreases in speed: the error at 40 knots is not the same error at 400 knots. At the high end of the spectrum, we dont' look so much at Calibrated Airspeed as much as Equivalent Airspeed, or EAS, which is indicated airspeed adjusted for installation error, adjusted for compressibility (what happens when we begin to experience mach effects at higher speeds).
True airspeed looks at indicated airspeed adjusted for temperature and pressure altitude.
As we climb higher and higher, indicated airspeed becomes lower and lower, and to an extent, true airspeed increases...but that depends on temperature. Because true airspeed is indicated airspeed adjusted for temperature and for pressure altitude, and because pressure altitude assumes a standard atmosphere (we set our altimeter to 29.92, for pressure altitude), then we're adjusting that for temperature deviation from standard, or deviation from ISA (standard atmosphere).
If you're flying in a light airplane, much of the time, indicated airspeed will be close enough, and will be indicative of airplane performance. Most light airplanes don't fly fast enough or high enough for it to become too big of an issue. (Performance based on density altitude is a much bigger concern, or should be). In a light airplane, True airspeed will begin to diverge from indicated with both temperature and pressure changes. It's true airspeed, generally speaking, that you'll use for your navigation calculations, adjusted for wind. Indicated speed for performance and limitations (or calibrated speed, if you have that available). Once you climb high enough, about 27,000 to 29,000,' the use of indicated airspeed is traded for a more useful reference at higher speeds, which is M1 or Mach, and most aircraft that climb that high use air data computers to adjust and correct the information received in the cockpit.
True airspeed looks at indicated airspeed adjusted for temperature and pressure altitude.
As we climb higher and higher, indicated airspeed becomes lower and lower, and to an extent, true airspeed increases...but that depends on temperature. Because true airspeed is indicated airspeed adjusted for temperature and for pressure altitude, and because pressure altitude assumes a standard atmosphere (we set our altimeter to 29.92, for pressure altitude), then we're adjusting that for temperature deviation from standard, or deviation from ISA (standard atmosphere).
If you're flying in a light airplane, much of the time, indicated airspeed will be close enough, and will be indicative of airplane performance. Most light airplanes don't fly fast enough or high enough for it to become too big of an issue. (Performance based on density altitude is a much bigger concern, or should be). In a light airplane, True airspeed will begin to diverge from indicated with both temperature and pressure changes. It's true airspeed, generally speaking, that you'll use for your navigation calculations, adjusted for wind. Indicated speed for performance and limitations (or calibrated speed, if you have that available). Once you climb high enough, about 27,000 to 29,000,' the use of indicated airspeed is traded for a more useful reference at higher speeds, which is M1 or Mach, and most aircraft that climb that high use air data computers to adjust and correct the information received in the cockpit.
