A320 : fuel efficient descent

[HazelNuts39]

Setting cost index to zero should not get you flying at green dot.

So I understand. See OPEN DES post #5.

However, if Airbus hadn't selected a minimum descent speed of 250/270 kt, it would lead you to flying at green dot.

[mikedreamer787]

Why stop there, start the APU and shut both down.

Well why stop even there?

Keep the APU off and just dead stick the using EMER GEN.
The cost of towing plane off the runway and restowing said genny
afterwards might still be below the monetary gains made by shuttin'
down everything on board at TOPD that burns kero.

[Ultra Glide]

My understanding is Green Dot is minimum fuel flow per hour and Cost Index 0 is maximum miles per pound / kilogram of fuel.

Therefore, if there will be no delays, CI 0. If you can predict any delays, the sooner you slow to Green Dot the better... unless you do it so soon that you end up flying unnecessary additional time.

[HazelNuts39]

Green Dot is minimum fuel flow per hour

That is approximately correct in level flight. In descent at idle thrust it is maximum miles per foot of altitude.

[Ultra Glide]

Found this online:

"You can find the speed for the maximum lift to drag ratio (L/Dmax) right in your Pilot's Operating Handbook (if you have one). Look under emergency procedures for engine failure. Find the recommended glide speed for maximum range. This glide speed is the speed for L/Dmax (I'll save you the derivation). This speed also corresponds to the bottom of the drag polar. After all, if lift is a constant for level flight, L/Dmax will occur at the minimum drag. According to Carson, these three speeds are related by factors of about 1.316. So this means:

Vmax endurance = Best Glide Speed ÷ 1.316

Vmax range = Best Glide Speed

Voptimum cruise = (Best Glide Speed) * 1.316

So now you can find these speeds for your airplane."

Here's the link to the complete article. Came from EAA website.

Maximum Endurance, Maximum Range, and Optimum Cruise Speeds

[Ultra Glide]

So Nuts, you're correct. Green dot is best L/D. (Just looked it up to make sure.) Therefore it's best glide, therefore it's max range.

But looking at the article, if best endurance is Green Dot ÷ 1.316, that means if Green Dot is 210, best endurance is 160???

Something's wrong here.

[HazelNuts39]

Something's wrong here.

No, nothing wrong, but you should be aware that the article you're referring to is for prop-driven airplanes. Jets are different.