FMC Performance data vs. O.M. Performance Tables

[jorgvaz]

Hi everybody...

Some days ago during a night flight, we were heavy trying to achieve our final level. As you know sometimes we have "to fight" with other colleagues to obtain "the level" before.
That was the case, the same route same destination same levels..who first reached it.....won!
We were leveled off at FL330 waiting for the step climb to FL370, the FMC stated 54 minutes to S/C, and the optimum at this moment was FL338 and maximum FL360, the weather conditions were fine, and very good tail winds, so we requested to climb "Special Level"350, and we did it. After several minutes the Control asked us if we were able to climb 370, because has another traffic opposite direction same level. Checked out FMC optimum FL354 max FL364, we answered the Control that unable to climb, so we had to descent until FL330. Although for the FMC the max. value for CRZ was FL364 at LRC, checking the Operations Manual Tables LRC. for the same weight, FL 370 was possible.

Anybody knows why the FMC data base is more "conservative" than the papers?? I mean the Operations Manual??

And...the end of the tale?..the other guys as soon as we leveled off at FL330, requested to climb FL370, so we were down and on their tail for the rest of the round.

Thanks all very much for the time and for sharing knowledge!!!
:)

[mutt]

I would guess that you are using the staight ops manual data which doesnt have the same residual rate of climb as the Airline Policy Files for the FMC.


Mutt

[mustafagander]

I wonder if you had an ISA deviation which the paper manuals didn't allow for?

[MasterGreen]

What I will never understand is the mentality that demands that you must always climb to maximum altitude.

Have you no understanding of jet performance at all?

Whatever - your companies' loss I guess for not teaching you...

MG

A tip - learn about the wind trade tables and the concept of SFC. Lots of good stuff out there on the web. Try :

Basic Performance (http://www.iasa-intl.com/wtt.html)

[BOAC]

As MG says, OPTIMUM is just that. When OPTIMUM is closer to 370 than 330 - then climb, not before, unless there is a very good reason like weather/terrain or trying to 'bag' a flight level - assuming you have done the step climb fuel extra v. cruise savings x time-to-go check, plus taken the wind into account. FMC 'step climb' point does NOT take fuel economy into account, it is purely weight based (737).

Again assuming you have taken into account ALL the factors regarding max altitude from your paper tables (eg ISA dev etc as the FMC does) AND your company operating licence ALLOWS you so to do, then climb outwith VNAV to the level you have calculated as safe. Some FMCs have a cruise C of G option which can give you a higher 'max' level but may NOT be approved in your ops manual.

[jorgvaz]

MasterGreen

quote


"Have you no understanding of jet performance at all?

Whatever - your companies' loss I guess for not teaching you... "

MG

Well... that's our company operational philosophy:

There is an optimum altitude to fly a plane at based on its weight.
As the weight of a plane changes, so does its optimum altitude.Therefore as fuel burns during cruise, the optimum altitude changes. In addition, as fuel burns off, the airplanes's tendency is to climb.
Typically airlines are not allowed to do a climbing cruise i.e. allow the airplane to climb as fuel burns off. Rather they must fly at specified altitudes. For this reason a step climb is used.

A step climb starts at an altitude that is above optimum altitude (typically 200 feet above optimum altitude) and stays at that altitude until it is a given amount under the optimum altitude (again, typically 2000 feet)

At this point, the airplane will climb and be above optimum altitude again.The typical climb is 4000 feet to again be 2000 feet above optimum.

This method keeps planes at specified altitudes yet lets planes climb in increments to save fuel.

By the way, thanks for your information about the site.