Descent planning

Hi lenthamen ;-)

The same like for the climb…there is the best climb speed Vx and Vy

Same is true for the decend…if you wanna decend in a holding you choose the minimum decend per minute …. If you descend on cruise you choose the best rate of gliding / same like cruise climb. All depends of course on mass as well.

From gliding there are various apps which help you to optimize you altitude. It calculates wind and terrain in it as well.

For a da40, you have the comfortable situation that the power idle doesn’t affect the engine like with lycomings. The water cooler stabilizes the temperature, so,theoretically, you should fly as long as possible on cruising FL, and once you reached TOD you go idle and use Vy to reach the approach fix (this would be the most fuel efficient way)

When it comes to time there are different parameters…I would assume that choosing an decend rate with brings you to the max speed possible in your environment until you reach the approach fix (I guess 100-250ft/min an min 85% power at the FL and increasing to 100% when you reach the feets) I guess ATC won’t accept this minimum descend speed but I never tried?

And since you love formulas. Why not creating your own one..

Take your POH and an excel sheet with the three phases climb, cruise and decend…enter plane and Wx parameters and the output is time or fuel efficient …it will give you the best altitude and speed setting for each trip…:-))))))))

And if you want to do it even more professional, you can name a new indicator for this and call it cost factor…like 1-10. You enter your cost factor and the excel sheet gives you speed setting and FL to take…just like an airliner ;-))))))

Just kidding sorry;-)

lenthamen wrote:

Is there a way to determinate which settings (power / descent rate / IAS) give the best time (or fuel) -efficient descent profile?

It’s negligable for SEP (or MEP).

In the Comanche I will just take 700ft / minute then keep speed just out of the yellow arc in case of Turbulence (so reduce power in order to keep it just outside of the arc), I wouldn’t accept anywhere near the 1500ft/min personally either. From what I was taught in my IR training, for commercial operations in non pressurised aircraft the maximum allowed descent rate would be 850ft/min – and ATC cannot ask you for quicker descent.

LFHNflightstudent wrote:

and ATC cannot ask you for quicker descent

They can ask you anything I once was asked 1000 or greater and did 1001fpm

Normally I’m planning for 500fpm at Vno

1500 fpm is unaccepable, i think. I run the Vertical Nav calculator on my #2 navigator when approaching the destination and i ask ATC for the descent the first time when it shows “500 fom required” (“standing by for descent”). Normally i get it soon after (sometimes i didn’t in Greece), and 1000 fpm is no problem.

europaxs wrote:

They can ask you anything

True :-)

For me it depends on the situation.

Let’s say I am going UK to Brac LDSB. Over the Alps I will be at maybe FL170 and then I have well over 100nm in which to descend. There is no point in going down at 1000fpm or even 500fpm, only to stop the descent at the next cleared level, say FL150, especially if it puts me low over water. Also the speed goes way up during such a descent and the MPG is thus sub-optimal. So it is better to achieve a slower descent, say 200-300fpm, which produces only a modest speed increase, and with a bit of luck one gets (or asks for and gets) the next stepdown before one actually reaches it, so a continuous descent is achieved. And I try to do the same coming back to the UK, from FL100 or whatever, down to 1600ft. Sometimes I use the Garmin 496’s VS calculator (set up a DCT and it just works) but usually it works well enough to do the 200fpm for every 1000ft to lose with 10nm to run (120kt GS assumed; usually one is a lot faster so pro-rate the VS accordingly). There are several other rules of thumb… Once, on a difficult flight from Zagreb to Shoreham which due to crazy headwinds (much stronger than forecast) was about 8hrs, I did a -100fpm continuous descent over about 300nm, to squeeze out every last drop of fuel. I played this with the fuel totaliser all the way and landed with 14.6 USG.

However sometimes ATC want a faster descent, in which case they will tell you. I have never been asked to do 1000fpm or higher.

ATC is entitled to expect 500fpm for a climb or a descent and youa re supposed to advise them if unable, but in practice they know how piston GA works and they are not bothered.

In theory wouldn’t the “most efficient” way be whatever rate of decent allows you to maintain best glide speed with the engine at idle? I guess there is an argument that even at idle the engine is consuming some fuel (I doubt they do ‘fuel off on overrun’ like most modern cars) and there may be some speed faster than best glide which produces a lessor fuel consumption due to taking less time.

Peter wrote:

Sometimes I use the Garmin 496’s VS calculator …

The problem with these things is that (most of them at least – dont’t know about this Garmin unit) do not have an atmospheric model and know nothing about winds. All they can do is divide your distance to go (again it does not know about any shortcuts or detours which are about to come) by your momentary ground speed and compute a rate of descent from that. Which will be way to high in most cases when you come from high altitude. In a piston single or twin I would wait until the calculator shows 1000ft/min and then commence my descent at 500ft/min and find that I don’t have to change that much until touchdown. And in the event of an unforeseen shortcut nobody’s eardrums will pop if the rate needs to increased briefly to 1000 or 1200 ft/min.