# How to find TAS for given CAS, FL and OAT?

Here is another question for my PPL Exam. If anyone can illuminate me with explaining every step to achieve TAS with given parameters, it would be great.

Given: Calibrated airspeed (CAS): 155 kt. Flight level (FL) 80. Outside air temperature (OAT): -15° C. The true airspeed (TAS) equals...

When seeing this question there is only one thing is coming up to my mind and which is: Temperature drops 2° C every 1000 ft but I cannot connect the dots to find TAS.

• This answer can help you. All you need to do is convert the FL into a pressure and plug the numbers into the formula given in that answer... – Bianfable May 26 '19 at 10:24

My trusty E6-B flight computer (I can't believe we still teach people to use what is essentially a slide rule) has the following printed on it:

AIRSPEED CORRECTION
Set PRESS. ALT opposite °C in window. Opposite CAS on inner scale read TAS on outer scale. Read Density ALT Centre ▲

Following the instructions and setting -15°C against 8,000 ft reads a TAS of 170 kts for a CAS of 155 kts

• There is a huge advantage to a slide-rule over a calculator: slide rule won't have flat battery! – Jan Hudec Jul 10 '20 at 19:49
• Nor even a dead battery, @JanHudec (for the Americans in the crowd. :D) – FreeMan Jul 11 '20 at 15:00
• @JanHudec that may be an advantage while flying. But in pre-planning not so much. – Jamiec Jul 13 '20 at 7:45
• @Jamiec, having it as a backup in your flight bag in case you need to update your plans in flight is good enough reason to learn basics of its use even if you'll almost never actually use it. – Jan Hudec Jul 26 '20 at 11:38

Maybe, you can use this Airspeed Calculator. Using this, you can easily convert between TAS, EAS, CAS. They have also explained the theory behind it.

• Your link is broken. – bogl Jul 10 '20 at 8:41
• @bogl It's working for me. You can also google search "AeroToolbox Airspeed Conversions", you might get to the webpage. – Pavan Jul 10 '20 at 17:46
• Phew, looks like my whizz-wheel skills still work. This is why I cant understand why people need to learn how to use it any more. It's not like anyone busts it out while actually flying, and in prep there are a hundred apps/websites which do a better job of explaining it. – Jamiec Jul 10 '20 at 19:17