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What Cost Index to enter in FMC?

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That's just the thing, though. Ask your average simmer (who knows what a CI is) what you should use and they'll likely say "well, Operator X uses 42" (note the specific value, without stating a range). Ask them if it changes and they'll likely say no, even if it does. The very fact that the linked thread simply lists set values without a range lends to this idea. There's enough misinformation in this hobby. I really don't like contributing to it, so I speak up when I see something that can be potentially misleading. In this case, a thread with specific values is potentially misleading, particularly because the other thread does not note that the values may change, and the reasons for why. The amount of confusion on the topic is a clear indicator of this.

 

Moreover, CI doesn't only affect cruise, so saying that it doesn't matter for a short flight is somewhat misleading. Granted, shorter segments mean that the CI has less of an effect on a single flight, but remember that even winglets aren't very effective from a single flight perspective. It's the savings over time that are the issue. For companies like Emirates, CI should be even more of an issue, since their business model is limited to longer haul flights for the most part, which would see even more of a benefit to tailored values. Then again, they may not be as constrained by fuel costs, given their advantage in that market.

I know CI doesn't only affect cruise but very rarely are you flying vnav descent unless you are the extreme cost cutting like Ryanair. ATC do not like jets descending at 250-260 knots at flight levels. Most of the time you are going down at a set speed like 280 or 300. So in that regard the CI is really only affecting cruise numbers and climb. But not climb much because even at low CI your climb speed is pretty high (above 280-290) - David Lee

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The world's largest 737-800 operator have been using just CI6 for the last three years.....

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I know CI doesn't only affect cruise but very rarely are you flying vnav descent unless you are the extreme cost cutting like Ryanair.

I wouldn't quite call this a qualified statement. Might you be able to provide a list of operators who don't use VNAV in the descent? I know some might not, but that seems like a very bold statement without any references provided.

ATC do not like jets descending at 250-260 knots at flight levels. Most of the time you are going down at a set speed like 280 or 300. So in that regard the CI is really only affecting cruise numbers and climb. But not climb much because even at low CI your climb speed is pretty high (above 280-290) - David Lee

This is definitely not a qualified statement. As someone who's spent quite a lot of time working with ATC, and traffic management from the TRACON to the the System Command Center (ATCSCC) here in the United States, the speed predominantly a non-issue. Granted, the speed drives the descent rate for an idle path descent, but unless you're unnecessarily tying up flight levels by descending very slowly (around 500 fpm), it really doesn't matter. For the lateral perspective, aircraft types are highly variable in their operation, so adjusting for closure between an operator descending at a higher speed and an operator using a lower speed is just business as usual. For the vertical perspective, it's a non issue unless you're descending so slowly that you're well outside of the "normal" assumption. A difference of 20 knots isn't going to cause that.

 

Ask most controllers what speed they'd prefer an aircraft to descend at and you'd likely get a blank stare. The only times they really see any issue is when you have a GA aircraft in the mix with large/heavy (GA aircraft climb like pigs, especially in the summer), or you have a military aircraft that climbs so quickly that the ATC automation blanks the value because it assumes the rate is unachievable (see F/A-18 climb rate). ATC isn't here to micromanage the flying of aircraft. They're here because aircraft are different, and operated differently, and need to have someone with a different perspective ensuring that they are separated. If you're not fitting into the mix, they're there to either prompt you to attempt to fit into the mix (usually not until the TRACON, at lower levels), or provide some sort of mitigation (like a "blow-by").

 

As I've mentioned in this thread already, please be very mindful of the information you're putting out there. People who are less knowledgeable will take information from those who appear knowledgeable at face value. There are already enough misconceptions about ATC out there. Let's not add more.

The world's largest 737-800 operator have been using just CI6 for the last three years.....

With no variance at all? One would imagine that such an operator would, well, harp on most efficient practices and cost balancing. Costs are rather variable in this industry. Using a fixed value is almost akin to using a fixed FL. Then again, developing some way of managing and balancing those costs itself is a cost, so maybe it's just resignation?


Kyle Rodgers

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I know CI doesn't only affect cruise but very rarely are you flying vnav descent unless you are the extreme cost cutting like Ryanair. ATC do not like jets descending at 250-260 knots at flight levels. Most of the time you are going down at a set speed like 280 or 300. So in that regard the CI is really only affecting cruise numbers and climb. But not climb much because even at low CI your climb speed is pretty high (above 280-290) - David Lee

We've been using just CI6 for the past 3 years, this gives you a descent of 245 kts on transition. ATC in Europe have absolutely no problem with what speed you descend at. As every operator flies at different speed, either with a fixed or variable CI, you might be asked by ATC to speed up or slow down to maintain separation. We plan to descend at our CI (245kts) and we accelerate if asked to.

 

The ECON CRZ and CLB speed is a function of not just CI but your planned cruise altitude, HW/TW component and Gross Weight and will vary every flight. Your ECON DES Speed is fixed.

 

There is also a common misconception that all we ever use is LNAV and VNAV. For our operator VNAV is the default CLB or DES mode. During DES we'll use it if on profile. As soon as we get a shortcut we'll be high on profile and you have two ways to recapture that profile, accelerate and/or use speed brake. We use what ever mode on the MCP to get the job done.

 

 

With no variance at all? One would imagine that such an operator would, well, harp on most efficient practices and cost balancing. Costs are rather variable in this industry. Using a fixed value is almost akin to using a fixed FL. Then again, developing some way of managing and balancing those costs itself is a cost, so maybe it's just resignation?

 

No variance at all, CI6 and that's it and from what I understand it will be staying that way. It's not all about saving money (although a big part of it!!) With the introduction of CI6 there was a notable decrease in the amount of High Energy Approaches being flown too.

 

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No variance at all, CI6 and that's it and from what I understand it will be staying that way. It's not all about saving money (although a big part of it!!) With the introduction of CI6 there was a notable decrease in the amount of High Energy Approaches being flown too.

 

That latter part makes sense, I guess. Still, I find it a bit shortsighted. Granted, I'm not running an airline, but fixed behavior is by nature inflexible, but flying by its nature requires flexibility for efficiencies.

 

In any case, interesting bit of info about that op and its fixed CI.


Kyle Rodgers

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This whole conversation is somewhat academic since the cost index is basiclly just a starting point for flight planning.  We usually end up tweaking the speeds in the box to our liking anyway during the flight.  Some pilots input speeds directly into the climb, cruise and descent pages, others just bump the cost index up or down until the speeds are what they want.  Throw in some ATC assigned speed changes along the way and the CI becomes even less relevant.

 

In short I wouldn't spend too much time worrying about the perfect CI to use.


I wouldn't quite call this a qualified statement. Might you be able to provide a list of operators who don't use VNAV in the descent? I know some might not, but that seems like a very bold statement without any references provided.
 

 

I think his statement may have been poorly worded.  A better way to say it that some operators don't use the computed ECON speed during a VNAV descent.

 

Our company standard descent speed is 290.  The FMC will never give us a speed lower than that in ECON, even if the true ECON speed would be lower.  Even then going into places like ORD you are going to end up changing it to at least 300, if not more.

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