Welcome Guest. Sign in or Signup

3 Answers

TAS – CAS Corrected for Nonstandard Temp?

Asked by: 6243 views
Aerodynamics

I understand how altitude and density affects TAS. I do not understand why CAS needs to be corrected for nonstandard temperature.

Ace Any FAA Written Test!
Actual FAA Questions / Free Lifetime Updates
The best explanations in the business
Fast, efficient study.
Pass Your Checkride With Confidence!
FAA Practical Test prep that reflects actual checkrides.
Any checkride: Airplane, Helicopter, Glider, etc.
Written and maintained by actual pilot examiners and master CFIs.
The World's Most Trusted eLogbook
Be Organized, Current, Professional, and Safe.
Highly customizable - for student pilots through pros.
Free Transition Service for users of other eLogs.
Our sincere thanks to pilots such as yourself who support AskACFI while helping themselves by using the awesome PC, Mac, iPhone/iPad, and Android aviation apps of our sponsors.

3 Answers



  1. John D Collins on Jul 21, 2014

    CAS is calibrated airspeed. IOW, it provides the corrections for installation errors of the pitot static system as installed on a particular aircraft type to adjust the indicated airspeed to obtain the effective airspeed. As an example, a C172P shows the following corrections from flaps up indicated airspeed to calibrated airspeed at 110 IAS the CAS is 107. If you were to indicate 110, you would correct 107 for the temperature effect and not the 110 value

    0 Votes Thumb up 0 Votes Thumb down 0 Votes



  2. Drew on Jul 23, 2014

    Up at altitude, the air is less dense, so the aircraft must have a lower CAS for a given TAS. So, if CAS is corrected for density, I get that it would yield TAS. I dont understand why non-standard temperatures are taken into account when calculating TAS. Is TAS calculated based on the standard temperature lapse rate? Why does temperature even matter?

    0 Votes Thumb up 0 Votes Thumb down 0 Votes

  3. Best Answer


    John D Collins on Jul 24, 2014

    For low speeds and altitudes that most light piston aircraft fly at, the calculation to determine the true airspeed can be simplified to be equal to the CAS times the square root of the ratio of the standard air density at sea level (p0) divided by the air density at the altitude (p), TAS = CAS SQRT (p0/p).

    You can derive the density of dry air from the ideal gas law PV=NRT where P is the pressure, V is the volume, N is the number of molecules, R is the gas constant for dry air, and T is the absolute temperature. The density is the number of molecules (N) per unit volume (V). Restating the ideal gas equation, N/V (density) = P/RT = p. As you can see, the air density is an inverse function of temperature, the higher the temperature, the lower the density.

    This means that the higher the temperature at altitude, the higher the TAS since p is a smaller value and appears in the divisor of the TAS calculation. Dividing by a smaller value yields a higher result.

    +1 Votes Thumb up 1 Votes Thumb down 0 Votes


Answer Question

Our sincere thanks to all who contribute constructively to this forum in answering flight training questions. If you are a flight instructor or represent a flight school / FBO offering flight instruction, you are welcome to include links to your site and related contact information as it pertains to offering local flight instruction in a specific geographic area. Additionally, direct links to FAA and related official government sources of information are welcome. However we thank you for your understanding that links to other sites or text that may be construed as explicit or implicit advertising of other business, sites, or goods/services are not permitted even if such links nominally are relevant to the question asked.