Hey guys,I got few questions that I am hoping you guys could help me on 1) what is the "Packs" in the cockpit, what does it do? is it something you turn off during flight,etc? 2) CRZ altitude recommendation in FMC, does it take in account also the distance of the route , or only plane weight,etcwhen it suggests the optimum Hight for cruse? i.e. if my plane/FMC is recommending FL380 for optimum fuel burn, however my flight is only 130-160nmtotal, or in many cases I do routes of 70-80nm, which in away makes no scene climbing to such high altitude. would it suggest the optimum fuel burn CRZ altitude based also on flight distance or it would still stick to the FL380, etc? 3) most of my flights that I do are no longer then 1 hour route path (e.g. WSSS - WMKK, VTSP - VTBS, KJFT - KBTV, etc), for those kind of short distance flightwhat is the cost Index usually used in real airlines in the FMC (give or take)? (for NG of course) 4) when doing a short distance flight (like 30 - 45 min' flight route/time, e.g. WMKK - WSSS), how does one know what Altitude to CRZ at (as for best burn fuel saving, etc)?it obviously can't be based on weight only, etc as by the time I climb to let's say FL380 (e.g.), or even half way through climbing, I need to descend back and by climbing I actually burn more fuel by doing so and is costlyif I am not going to take advantage of that CRZ altitude for a whileso it's kinda brings me back to my second question in a way.. if FMC doesn't take distance into consideration, how does one know the best hight to pick (best compromise for cost / performance)?

Packs are the air conditioning devices that cool or warm cabin air, they work using bleed air (from engine,apu,or ground cart*) and provide air for aircraft pressurization. For the other questions, most is related to the cost index, altitude, climb and descend calculation are made by taking the cost index value in count. It represent how economically the flight must be.

3) most of my flights that I do are no longer then 1 hour route path (e.g. WSSS - WMKK, VTSP - VTBS, KJFT - KBTV, etc), for those kind of short distance flightwhat is the cost Index usually used in real airlines in the FMC (give or take)? (for NG of course)

20-28 (BA, short routes, source BAV website)

20-28 (BA, short routes, source BAV website)

That would be for the Boeing 737-436..... we're talking about an 737 NG. And there is a difference, a big one. In the BAV forum it is recommended to use a CI of 0 for the 738 to get the same time conformance as a 734.

what about question 2 and 4?it wasn't really answeredquestion 3.. CI of "0"? is this reality?

That would be for the Boeing 737-436..... we're talking about an 737 NG. And there is a difference, a big one. In the BAV forum it is recommended to use a CI of 0 for the 738 to get the same time conformance as a 734.

Oh yes, you are correct. But I do get 5min+/- time conformance with the recommended CI setting. The other one says that 0 is gonna give you 2minute difference in the cruise... that BA is in RW using 0 for takeoffs and 28 for cruises.

question 3.. CI of "0"? is this reality?

No.

ok , guys I really appreciate your help, but I never really got any real answers here apart from "davierosoft" that answeredthe Packs question.the rest were either not mentioned or contradicting information was given.and I am seeing where this thread is heading now , on arguing on the CI of B734 (sorry guys, I know it's a free forum and freedom of speech), which mean I will never been answered and I'll be forced to open a new threadwhich I reluctant doing.

3. The cost index is changing very often and different from company to company and route to route but since the companies try to save money the cost index is usually 8-50 give or take. Manfred

Virgin Australia use a CI of 40 I'm told......0 or 100 is probably unrealistic. Most airlines are using lower CI's due to the increased cost of fuel compared to cheaper airfares. Hope this helps.

any one knows about Question 2 and 4?

2) The optimum cruise alt is the one that acheives the best economy for the CI that you have entered. You can enter different CI's for different parts of the flight. It is not dependent on the length of the flight. 4) Your assumptions are not correct. If your Opt Alt is FL380 and you only spend 5 minutes there before starting your descent, you will still be more efficient then if you spent longer at a lower altitude. If you can't reach Opt Alt, you fly keep climbing until you reach TOD.

For questions 2 and 4 CI is the answer as the FMC will calculate the right altitude, ramps for climbing and descending for the CI inserted. So, the answer is always CI.For TOD calculation you can also fill the des forecast page, this will help you to descend correctly to the destination (if forecasts are updated)

you totally miss

For questions 2 and 4 CI is the answer as the FMC will calculate the right altitude, ramps for climbing and descending for the CI inserted. So, the answer is always CI.For TOD calculation you can also fill the des forecast page, this will help you to descend correctly to the destination (if forecasts are updated)

you totally missed my question. I know it is based on CI.I'll rephrase in a simpler way.you filled your plane with passengers and Cargo. You loaded the fuel, you set route, etc and CI.Great FL385 (e.g.) is recommended by FMC,But my flight is only 110nm!!what , would you really climb to that high? and sure won't be able to decent on time without missing your landing spot. CI by itself won't be good enough.... on short flight, you will end up burning more fuel climbing rather just staying at lower altitude.to make it more extreme for the question to sink in better. take a short flight like LLBG - OJAI , only 70nm (25min flight), would you still climb to FL385 as FMC suggests? that was my question.how does one know what altitude to pick on shorter flights that would best match fuel burn saving, etc?

No you would never get to that FL anyway for that distance. Assuming terrain was not an issue, then for 70nm I would probably aim for a max of 15000 ft. Not sure if that is realistic in RW ops or not..G