Why do I get penalized if I STEP down to OPT?

[andreadebiase 5]

I am trying to understand why i would be penalized if I descend to an OPT altitude from an altitude slightly above the MAX. Example, i am on FL370 and the FMC is telling on the CRZ page that my OPT is 344 and MAX is 369. If I STEP from 370 to 344 I am shown a penalty of 6,%....shouldn't i get a GAIN instead? am I misinterpreting the concept of OPT? isn't it referred to optimal fuel use? 

 

ps

on another note if i want to add a pic to a post here i am asked to have on a URL....is there a specific website where to upload pics so i can put them here?

thank you

 

 

[BrianW 124]

The optimum altitude is partially derived from the cost index, so it may sometimes favor time over fuel depending on the cost index you use.

[downscc 1,327]

 

 

i am on FL370 and the FMC is telling on the CRZ page that my OPT is 344 and MAX is 369.

 

The problem with being higher than MAX is that at ECON speed you do not have a Vmo buffer, I bet you were in the amber zone on the speed tape.  There are more things to consider than cost.

[andreadebiase 5]

The problem with being higher than MAX is that at ECON speed you do not have a Vmo buffer, I bet you were in the amber zone on the speed tape.  There are more things to consider than cost.

 

mhhhh....next time I will check but i was actually on along haul going from Hawaii to KSAN and i was on a LRC CRZ not the ECON....

[downscc 1,327]

I've flown the Hawaii trips numerous times and never found a need for LRC.  In fact i just went PHLI-KCRP (that is So Texas) non-stop in a B736 with CI of 40.

[BrianW 124]

Why LRC?  ECON below a CI of 35 will give you better range than LRC if that’s what you’re looking for. A CI of 0 will give you the best range.

[andreadebiase 5]

Why LRC?  

mostly because I don't know yet how to use it. I thought LRC is used every time you do a very long flight but clearly that is as simple as that. I just need to do some more homework. My point though is that I was not on ECON.

[BrianW 124]

No worries, but that was an important detail to leave out since ECON is the more common mode.  The route length is also important because there are differences in the FMC OPT calculations between short flights (<400 NM) and long ones, but again only in ECON mode..

 

LRC differs from ECON in that the OPT altitude is only based on aircraft weight. Do you remember what the winds were?  Maybe you just lost a good portion of tailwind at the lower altitude.

 

[andreadebiase 5]

Food for thought here http://www.boeing.com/commercial/aeromagazine/articles/qtr_4_07/article_05_3.html

I think i am starting to understand why I was getting penalized although I was aiming for OPT but i need to check on this by flying a few more times.

See I had chosen a CI in the FMC AND I also did choose LRC at the same time. Seems like you do one OR the other...still unclear on this..... Anyway, I did both and I do wonder if this why I was getting conflicting instructions meaning being penalized while aiming for OPT. 

[downscc 1,327]

 

 

Seems like you do one OR the other...still unclear on this.....

 

Spot on... one or the other, or the third often used option when on oceanic routes is a fixed mach number.