# How to get from IAS in MPH to KCAS using POH graph of best TAS/GPH?

My understanding is that in order to find the V(L/D)max I can do the following steps:

1. Look in the Pilot's Operating Handbook (POH) and find the performance reflecting the best TAS/fuel flow.
2. Find the KTAS for the determination from step 1.
3. Use a flight computer to get the KCAS from step 2.
4. The number from step 3 is V(L/D)max.
5. V(L/D)max / 1.316 = Vbe
6. V(L/D)max * 1.316 = V-Carson

While this sounds logical, I do not see any clear path from step 1 to step 2.

I looked up the definition of TAS, and it is related to EAS. But when I look up EAS, it is dependent on TAS. So when you do the algebra, it simplifies down to TAS = TAS and EAS = EAS, which is nonsensical, circular, and unhelpful. Maybe I am not looking at the right equations, I’m not sure.

Do most pilots have flight computers with which they can determine KCAS starting with the value described in step 1? If not, how should this best be done? I could estimate air pressure using altitude, though it would not be very accurate, but I feel like there must be a better way given the large amount of instruments and tools available to most pilots in the cockpit.

• If you're looking for best glide speed V(L/D max), isn't that an aerodynamic speed, and as such is based on CAS, not TAS? Most aerodynamic speeds happen at the same CAS at different altitudes and temperatures, and so should use IAS instead of TAS. Second, I'm curious about why you want to use MPH. Usually since cockpit instruments are in knots, mph is only used for ground speed. (Ground speed is TAS with wind speed and direction taken into account). Aug 12, 2016 at 16:23
• Someone told me that a lot of recreational pilots use mph, I don't know if it's true. I use kts. Glide speed is not the same as V(L/D)max because glide speed does not account for having a working engine (it's calculated for when your engine busts, thus why you find it in the emergency procedures section). Aug 12, 2016 at 16:58

Several questions here that need answers;
V(L/D max) is best glide speed. Look it up under emergency procedures in the POH.
Airspeeds:
TAS is EAS corrected for temperature.
EAS is CAS corrected for compressibility.
CAS is IAS corrected for installation errors.

IAS/CAS correction is available from the POH.
Pilots can use an E6-B flight computer to convert between CAS, EAS, and TAS.

For more on this topic; here's a good link

• I am in communication over email with Mr Erb currently, and have read his article and Carson's. I think et glide speed is only an approximation of V(L/D)max because t does not take into account the difference in wetted area and power you get from a working motor and prop. Glide speed in the emergency section of the POH is for when your engine fails. Aug 11, 2016 at 17:19