Why is econ climb faster than econ descent

[calypso]

Quick question for those on the know. Why econ climb speed is faster than econ descent speed for the same cost index?. Some of the variation can be explained by the heavier weight on climb and even on a given day by differences in wind or temperature. Still in short sectors the weight difference is small and the wind quite similar.

For a low cost index econ climb and econ descent should roughtly equate to best rate of climb/ best glide speeds. In my A/c climb is usually 285kts but descent 255.

Any ideas?

[OPEN DES]

Let us take CI 0 as an example for clarity: CI 0=minimum fuel
(CI 19 will be the same idea, but ofcourse less pronounced)

Then:
Climb will give you max rate of climb: around 250 kt.
Descent will give us green dot.

For minimum fuel you want to be at your cruise-level in the minimum time, hence the max ROC climb speed.

Green dot will give you the best glide angle, so at the same time also the earliest possible TOD in idle.

In other words:
descent=most fuel efficient flight phase (idle)
cruise=2nd most fuel efficient flight phase
climb=least fuel efficient flight phase (high thrust)

So we want to minimise our time in CLB, maximise our time in Descent and just accept the cruise bit in between. :)


Cheers

[hawk37]

I like Open Des's argument. However something in me is a bit skeptical that CI = 0 would be green dot speed. Can anyone verify this, for airbus or boeings? There's been suggestions in other topics that descent at green dot saves sector fuel, but unless mistaken, I haven't seen anyone actually put CI = 0 descent speeds in a pprune post.

Anyone with access to CI = 0 descent speeds versus altitude?

[Keith.Williams.]

CALYPSO,

I think that the answer is in the bottom line of your post

".......econ climb and econ descent should roughtly equate to best rate of climb/ best glide speeds. In my A/c climb is usually 285kts but descent 255."

Best glide range speed is Vmd and for jets best rate of climb speed is faster than Vmd.

Using your figures, if 255 Kts is Vmd then 285 Kts is about 1.12 Vmd. These figures fit the general theory.

[F4F]

Another thing taken into account is the weight loss due to fuel burn.
Yeah well though there are some guys picking up enough ice to make their craft heavier upon landing than at take-off :8

live 2 fly 2 live

[LYKA]

Climb speeds vs. Descent speeds:
Normal climb speeds are generally higher than descent speeds because during climb, the engines are producing thrust and are not part of the overall aircraft drag. During descent, the engines are being dragged along. To minimize increased drag, the descent speeds are normally lower than the climb speeds

[calypso]

Interesting...

I would imagine that green dot equates to minimum sink speed, that is also quite close to best angle of climb and only good if you are looking for maximum endurance (ie holding, descending with a dual engine failure, etc) For econ descent we are looking for best glide speed and this should be quite close to best rate of climb speed. Hence the question.

Wether the engines are producing thrust or not should not negate their own drag. Surely the drag is still there even if they are producing lots of thrust. I do wonder about the increased efficiency from the ram air effect on the engines at higher speeds though?

[FlyingApe]

We aften climb at less than max rate in our regional jet- ie at high speed climb.

When light and climbing at max rate our cabin crew would be unable to move their trollies due to the high deck angle - and providing a full service on short sectors this is necessary.

[jonny dangerous]

One answer that explains the difference between the two:

The wind.

ECON Climb IAS is adjusted for the Top of Climb (TOC) winds entered (PERF INIT page Boeing). A strong headwind component entered gives a very high IAS for the climb segment, with the opposite for a tailwind.

ECON Descent IAS is NOT variable with winds entered. Headwind, tailwind, no wind, the planned IAS does not change.

I can't find the document right now, but Boeing or Airbus has a PDF document available on the internet that explicitly states the above.

Jonny

[411A]

Why is econ climb faster than econ descent?


In actual fact, sometimes it is not.
It very much depends on the individual airline, and how they want the 'econ' selection programed.

Case in point.
SV with L1011's and dual Hamilton-Sundstrand FMS's...the first widebody to have a true FMS, by the way.

Econ climb had several EPR settings and in addition, the speeds were 350 knots/M.85.
Cruise M.865, normally.

Descent (normally) M.85/350 knots.

Why? SV has very cheap fuel, so the airline wanted the fastest forward speeds.
OTOH, other airlines used slower descent speeds as their 'econ' selection.

As it is in most things in flying...sometimes it just 'depends'.

[calypso]

I had not thought about the head/tail wind effect changing the climb but not the econ descent speed. It makes sense but in that case econ climb would be sometimes faster (headwind during the climb) and some times slower (tailwind during the climb) than the descent speed, in my experience however climb speed is consistently faster.

The price of fuel is included in the cost index and my question was about cost index generated speeds rather than generic (airline generated) descent speeds. Can and do airlines especify a particular cost index logic? I thought the FMC had just one cost index program?. Airlines just choose the number that suits their operation at any particular time. Operators who don't care about the cost of fuel just choose a very high cost index and the speeds will be very close to VMO/MMO. In that particular case I guess my question is not relevant.