En-Route CI Mod

[Tonic Please]

Hello,

I was wondering if you would ever change the CI en-route from what was set at the gate? Why would you do this and is it legal or sometimes required?

I personally don't know why you would but perhaps you do?

[Ollie Onion]

Of course it is legal, there are many reasons for changing the cost index.

- need to conserve fuel so may set it to '0'
- need to save time at the cost of fuel so set it above flight plan level.

Only thing you need to keep in mind is you may need to update ATC of your new planned speed if you are not in a radar environment and it effects your estimated times.

[Natstrackalpha]

you need to keep in mind is you may need to update ATC of your new planned speed if you are not in a radar environment and it effects your estimated times.

Aha! - kewel! :ok:

[Tonic Please]

And one supposes that the CI figure is available on board so you know what to tap in? Why wouldn't you simply modify the cruise speed manually and see what revised figures for fuel-at-destination, eta, etc.? Wouldn't that be more accurate than a random, en-route CI change?

Thank you so far.

[wiggy]

I was wondering if you would ever change the CI en-route from what was set at the gate? Why would you do this and is it legal or sometimes required?

I'm not sure why "legality" fits onto it (other than as has been said ATC should be told of any major change from flight planned speed) ?

All C.I. is another tool in our tool box, i.e. a slightly clever method of speed control. In our airline we usually use a default of zero for departure/climbout and will change to the cruise CI at top of climb ( the CI the company has flight planned us at is displayed on our fuel plan). However there's nothing stopping us using that old fashioned skill, airmanship, and we often change the CI in the cruise (Long Haul), mainly to control the ETA to try and arrive on schedule, but sometimes we will reduce to 0 to conserve fuel.


Why wouldn't you simply modify the cruise speed manually and see what revised figures for fuel-at-destination, eta, etc.? Wouldn't that be more accurate than a random, en-route CI change?

That's another option available to us (and we're fixed Mach cruise anyway on oceanic sectors) but when flying Economy cruise/descent changing the C.I "carries" the speed change right through the sector (i.e. it modifies the descent all well as the cruise) so ETAs should be reasonably accurate right through to touchdown. We may well a combination of fixed Mach cruise for part of the cruise ( e.g. Oceanic), and then use C.I. controlled speed for overland sections - it all depends on the circumstances.

[Tonic Please]

That's very interesting. I didn't know that. Thanks a lot! :ok:

[FullWings]

Like Wiggy says, putting in a cost index instead of a fixed Mach/IAS allows the FMC to calculate the optimum speeds continually, taking into account mass, winds, temperatures, step climbs/descents, etc.

There are other reasons as well: our flightplans normally have the diversion planned at CI=0, whatever the CI for the leg to destination, so if you were on a long diversion it could be quite significant, especially if you were expecting to arrive with not much above reserves in the first place...

[Tonic Please]

Quite. It seems so obvious when you explain it. It's just something that never occured to me.

Thanks kindly once again.

TP

[Capn Bloggs]

QUOTE]when flying Economy cruise/descent changing the C.I "carries" the speed change right through the sector (i.e. it modifies the descent all well as the cruise) so ETAs should be reasonably accurate right through to touchdown.[/QUOTE]
We use CI all the time to comply with ATC-assigned Fix times. The most "econ"nomical way to minimse fuel burn whilst changing ETAs at terminal fixes?

We also play with the CI to get ourselves up higher (MAXFL changes).

Obviously the comments re updating your ETAs apply. :ok:

[Natstrackalpha]

That's very interesting. I didn't know that. Thanks a lot! Yep, the long range boys usually have a designated Machno. as opposed to their Econ. Oceanic its all in the CPDLC and Oceaninc Clearences briefs (which are long and deep, like the Oceans they fly over)

[Tonic Please]

Right. Now you have induced a follow-up question!

When I talked about 'legality', I assumed that the CI was fixed at take-off, provided on the various documents given at the gate and plugged into the FMC before pushback along with all the other palava, and that your company would be a little miffed if you changed it en-route since they fixed it to adhere to their own fuel policies/prices for that day of weather... the route planning chaps, etc.

So it seems that you don't have an official CI for a flight, so my question is: How do you know what CI to put in when you want to modify your ETA/Cruise FL? Is there a graph to look at? A data sheet of some kind? Or do you guess and say "Oh well CI80 is giving us FL360 and ETA of 1645, let's try, erm, 45 and see what that does"? I'd be surprised if that were the case? Hence my original reference to legality - having crews sodding around with CIs without the company knowing, costing more fuel perhaps, flying too fast, arriving to early and messing up slot/gate times...

I have seen online some 'pilots' saying that "Southwest use CIXX for their 737's" or "Oh, we used to use CIXX but now we use XX for our triple 7's" - I mean, how can a company 'USE' a CI when it should be different for every routing, weather forecast, weight, etc... and now you're telling me you can even play around with it when cruising for various reasons?

Hope you understand my quest for clarification.

Thanks again, gentlemen :ok:

TP

[PT6A]

In my company we will not change the cost index with the exception of running short on fuel (less than the OFP says we should have) in which case we may tactically change it to a lower value to prevent and en-route landing.

Some airlines I know provide two OFP's to the crew one with a significantly higher cost index, the latter to be used in the event of a delay.

Lastly people like PACE labs are making EFB software that will advise en-route tactical changes of the cost index.

BA are the only airline I know of that changes their CI for t/o climb and cruise... I would be curious to the "briefing" that BA provide its pilots..l the theory of this idea

[wiggy]

PT6AI would be curious to the "briefing" that BA provide its pilots.

Scratches head and tries to remember something he was told a long time ago, so forgive me if I've got this wrong: as far as I recall it the logic behind C.I. zero for the climb was to give a quicker climb to cruise (:confused:) verses a non zero CI, and therefore you reduce engine costs (I believe paying manufacturers for "power by the hour" comes into it). Apologies again if that is BS, but that certainly was the story I've heard regarding climb C.I., I'd welcome other thoughts.

Tonic Please


So it seems that you don't have an official CI for a flight, so my question is: How do you know what CI to put in when you want to modify your ETA/Cruise FL? Is there a graph to look at? A data sheet of some kind? Or do you guess and say "Oh well CI80 is giving us FL360 and ETA of 1645, let's try, erm, 45 and see what that does"? I'd be surprised if that were the case?


Tonic -I'm surprised that you are surprised, because that is pretty much what we do....i.e. try changing the CI, see what that does to ETA/Fuel and modify the plan accordingly.

I suppose we do have what you could call "official" CI for each sector, it's detailed on the front page of the fuel plan we run off at briefing. If you've got very favourable winds you'll be presented with a CI zero plan (and also low fixed Mach numbers for Oceanic portions of the flight), if you've got unfavourable headwinds you'll have a high CI plan (and higher fixed Mach numbers over any ocean). If it's a high CI plan you're also given the fuel and time gain for CI zero as a note at the bottom of page one of the plan.........
however.... the company policy is that the CI/Fixed Mach portions of the flight can be varied by the crew during flight,e.g. to try to achieve a punctual arrival. The flight planners in the office can't factor everything into the plan and come up with a perfect CI. Just one example - an off schedule arrival can incur extra costs for the airline (e.g passengers missing connections), and stand plans (yes we do have one!!), ground handling personnel allocation and the like can be ruined if we arrive very late or very early at the likes of LHR, so being flexible with the CI once airborne is an official policy.

... your company would be a little miffed if you changed it en-route since they fixed it to adhere to their own fuel policies/prices for that day of weather... the route planning chaps, etc.


No my company wouldn't be miffed at all - see previous comments. Yes, on an ideal day you depart on schedule and fly the planners plan :D, but there would certainly be adverse comments on a Long Haul line check if you departed late, and then, despite fuel in hand, let the CI sit at the planned value for hours, and as a result arrived late at destination simply because the planners had said fly at CI X :(.

I'd be surprised if we were the only airline that had such a policy, fortunately we're not complete slaves of the machine, .........yet.

[mikha169]

Try to understand the cost index and how it works is like trying to understand your girlfriend or wife all the time and it's near impossible simply because of all the managerial factors involved in deriving that special number.

In my airline we operate at CI 40 in normal conditions and 120 in case we need to speed things up a little bit; it increases your mach number by approx .2 - .3

If you really want to get into CI here's a link to an explanation put out by Airbus if you're really interested in details...

SmartCockpit - Airline training guides, Aviation, Operations, Safety (http://www.smartcockpit.com/pdf/flightops/aerodynamics/23)

[PT6A]

Wiggy,

Thanks for taking the time to reply.. It makes sense.

I think the key thing to remember these policy's will and do differ between airlines as all sorts of things come into play.

It's not just about fuel but a whole host of other services and costs that maybe affected by a change in CI.

I would suggest that many airlines don't even know their true cost index, this is highlighted in one of the Airbus getting to grips documents.

[TopBunk]

In my airline we operate at CI 40 in normal conditions and 120 in case we need to speed things up a little bit; it increases your mach number by approx .2 - .3


Wow! Me thinks you may have got the decimal in the wrong place

[BOAC]

Maybe he/she flies Concorde? Yes sirree - we are now cruising at M1.1 folks:rolleyes: