How do you know what cost index is right?

 

There is no "right" cost index.

 

 

 

I mean depends by company, or what? Or by the aircraft you use, or distance you fly?

 

It depends on the company, the day of the week, the number of high-mile/VIP passengers, the weather, ATC delays, the cost of fuel, how the dispatcher is feeling that morning, and many other factors.

 

Basics?

 

High CI - get there faster, cost of fuel isn't an issue.

Low CI - get there slower, likely because the cost of fuel is an issue.

Right! I mean what the intro.pdf say:"HGS AUTO AIII MODE: This option determines whether your HGS includes the Autoland AIII mode for use during instrument approaches. The AIII mode improves pilot situational awareness during low approaches by providing additional information such as the runway outline displayed on the HGS in front of the pilot’s eyes during the final phase of an instrument approach."

 

 

 

Now this confuse me! I notice that B747 and B777 comes by default with cost index 80, while B737 or A320 - A321 comes with cost index 40. So assuming that what you say is right, should be wrong the cost index. How do you know what cost index is right? I mean depends by company, or what? Or by the aircraft you use, or distance you fly?

AIII approaches aren't always autoland. AIII approaches can be handflown as well, allowing lower than published minimums to almost any ILS whether or not you autoland. Most US operators of the 737 do not have HGS installed, but almost all of them can autoland.

 

Cost index is not a given number... it depends per flight per day on a variety of factors. Inbound aircraft running late? The outbound flight will be planned at a higher CI. Nasty winter headwinds going west in December? Higher CI. Running early, good tailwind, want to save some gas? Lower CI. The index is a sliding scale between efficiency (cheaper operating cost) on the low end, and getting there faster on the high end. On a normal day on schedule with no other factors, my company plans most flights at a CI of 15, and flights to Hawaii at a CI of around 40. But it can and does vary. And yeah, a lower CI will give you a lower descent speed, but you can always bump it back up in the descent page of the FMC. We usually bump ours back up to .78/280.

How do you know what cost index is right? I mean depends by company, or what? Or by the aircraft you use, or distance you fly?

I don't know what cost index is right. It does vary by company, by type of plane but not distance. 737-800 SWA-10, Delta-17, American-20, and United 30 to 50. MD-11F-200 for all carriers. 77LR-20 777F-85 all carriers. It's all made up in my head.

 

 

...but not distance.

 

Not necessarily true.

 

Anything can be a reason for a change in CI.

 

 

High CI - get there faster, cost of fuel isn't an issue.

Low CI - get there slower, likely because the cost of fuel is an issue.

Got it! Good answer, but I have a curiosity: I made same flight from LIRF to LOWW first with B737 NGX and then with A320X ( I was testing dx10 with fixer shadows cockpit) with same GW and same cost index - 40 CI for both. The weather was "clear weather" the default from FSX with 1013 hp. The cruise was at FL350, but the cruise speed for B737 was 0.78 mach, while at A320 was at 0.76 mach. So the CI does have something to do with type of aircraft also?

Not necessarily true.

I am not writing about the real world. I am writing about what I do personally in my own home at my own computer. I don't use the distance of the flight as a determining factor in the CI.

So the CI does have something to do with type of aircraft also?

Absolutely: CI0 is usually defined as the minimum fuel cost, and by definition this speed will vary depending on the aircraft type. For instance, CI0 in the B747-400 is around M0.80-M0.81, which would be very fast indeed for a tiddler like the 737 or A320 ;-).

 

Worth also remembering that CI cruise speeds are dynamic and take in to account factors such as altitude, temperature and wind. If you have a big tailwind, your CI40 speed will be lower than if you have a big headwind. Also, in general, the cruise speed at a given CI will tend to decrease gradually over time.

 

Big Airways climb at CI0 (all fleets) and then normally cruise at CI90 in the B747. However, some eastbound transatlantics and flights to the Far East cruise at CI0 (i.e. if there's a stonking tailwind and the flight time is expected to be particularly short).

 

 

So the CI does have something to do with type of aircraft also?

 

Yes.

AIII approaches aren't always autoland. AIII approaches can be handflown as well, allowing lower than published minimums to almost any ILS whether or not you autoland. Most US operators of the 737 do not have HGS installed, but almost all of them can autoland.

 

Cost index is not a given number... it depends per flight per day on a variety of factors. Inbound aircraft running late? The outbound flight will be planned at a higher CI. Nasty winter headwinds going west in December? Higher CI. Running early, good tailwind, want to save some gas? Lower CI. The index is a sliding scale between efficiency (cheaper operating cost) on the low end, and getting there faster on the high end. On a normal day on schedule with no other factors, my company plans most flights at a CI of 15, and flights to Hawaii at a CI of around 40. But it can and does vary. And yeah, a lower CI will give you a lower descent speed, but you can always bump it back up in the descent page of the FMC. We usually bump ours back up to .78/280.

Interesting statement that most US carriers do'nt use HGS on their 737. I was wondering about your source of information on that particular point.

If you go to the Boeing.com web site and search on "cost index", you'll find a very good explanatory article that defines cost index and gives examples of its use, especially for fuel conservation strategies.

 

 

Now this confuse me! I notice that B747 and B777 comes by default with cost index 80, while B737 or A320 - A321 comes with cost index 40. So assuming that what you say is right, should be wrong the cost index. How do you know what cost index is right? I mean depends by company, or what? Or by the aircraft you use, or distance you fly?

 

The CI scale isn't the same for all aircraft - the Big Boeings are usually in that 80-100 range for a normal flight, while the smaller short/med range aircraft like the 737 or A320 tend to remain around 10-40 or so. The 737 and A320 have different FMS manufacturers than the Big Boeings too. (Smiths and Thales respectively vs. Honeywell I believe)

Interesting statement that most US carriers do'nt use HGS on their 737. I was wondering about your source of information on that particular point.

16 years flying for US airlines and commuting most of that time, meaning I got pretty familiar with most cockpits. Alaska has had them for a long time, SWA more recently, United does not, I do not know about delta, and I can't think of any others that do either. Just what I've seen...

16 years flying for US airlines and commuting most of that time, meaning I got pretty familiar with most cockpits. Alaska has had them for a long time, SWA more recently, United does not, I do not know about delta, and I can't think of any others that do either. Just what I've seen...

How about American?

Haha, forgot all about them. You know... I don't think I've ever been in the cockpit of AA 737... so I don't know. Anyone?

Both American and Delta do have the HGS on their 737NGs.