Welcome Guest. Sign in or Signup

1 Answers

“Standard atmosphere at sea level”

Asked by: 2706 views General Aviation

The CAS is equal to TAS in a standard atmosphere at sea level. Isn't the "at sea level" redundant since standard atmosphere implies a constant temperature and pressure decrease as the altitude changes (eg: -2 deg C and 1''HG)?

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.

1 Answers



  1. Skyfox on Sep 22, 2016

    Standard atmosphere refers to 15°C (59°F) and 29.92″Hg (1013.25 mb) at sea level. What you\’re referring to is the standard lapse rate of 2°C (3.5°F) per thousand feet. Calibrated airspeed (CAS) is just indicated airspeed (IAS) corrected for installation and instrument errors. Here\’s something pasted from the Pilot’s Handbook of Aeronautical Knowledge:

    “True airspeed (TAS)—CAS corrected for altitude and nonstandard temperature. Because air density decreases with an increase in altitude, an aircraft has to be flown faster at higher altitudes to cause the same pressure difference between pitot impact pressure and static pressure. Therefore, for a given CAS, TAS increases as altitude increases; or for a given TAS, CAS decreases as altitude increases. A pilot can find TAS by two methods. The most accurate method is to use a flight computer. With this method, the CAS is corrected for temperature and pressure variation by using the airspeed correction scale on the computer.”

    So to answer your question, no, the CAS and TAS won’t remain the same as altitude increases above sea level, and the FAR definition of CAS isn’t being redundant by including “at sea level” in the definition.

    0 Votes Thumb up 0 Votes Thumb down 0 Votes


The following terms have been auto-detected the question above and any answers or discussion provided. Click on a term to see its definition from the Dauntless Aviation JargonBuster Glossary.

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.