Which Optimum level to use? CFP vs FMS?

[Stuck_in_an_ATR]

In the mob I fly for, there are two schools of thought as to which cruise Flight Level should be chosen for cruise - the level provided by the CFP, or the optimum level calculated by the FMGS.

The former school reasons that the Computerized Flight Plan has been optimized for winds at all altitudes, while the "box" only knows the current wind.

The latter school says that the CFP has been created for forecast conditions and weights, which usually differ from the actual ones, and therefore the FMS Opt level is more valid.

I tend to lean towards the FMS supporters, but would like to learn others view on that.

Also, can anyone tell my why the A-320 FMGS won't calculate step-climbs? While we mostly operate quite short sectors (up to 3 hours), the FMS-computed optimum level in the beginning usually differs from the one at the end. Still, when I try to insert a step-climb, I get the message "no optimal". Why?

[mutt]

Optimum Levels are based on unit of fuel per distance, Lb/Nam, it doesnt take into account the present winds temps etc, they are only accounted for when you use ECON, Cost Index.

You are seeing discrepancies due to differences in residual rates of climb and buffet margins. If your CFP is always lower, then the company issued parameters are more conservative than the FMS.

Both the CFP and FMS are right, but just different, its up to you to decide if you want the additional conservatism. You could start by asking your CFP provider about the input parameters.

Mutt

[GlueBall]

Stuck_in_an_ATR ...you speak as if in your piece of the sky you are able to get the exact level at the exact time your FMS or CFPL calls for it. Must be nice. :)

In my environment we seldom match the exact CFPL altitude profiles with congested air traffic realities. :{

[PT6A]

Funnily enough I had just logged on to post the same question.

In my company we are told to use the OFP as that is computed to a higher degree than the FMS.

However with our new EFB software upgrade we are getting a program called Altitools? I think that's what they call it... But anyway this will do real time processing and suggest re-routes or levels to get in / keep out of the wind.

The theory behind this is the EFB has MUCH more processing power than the FMS.

At the moment I think CityLine have been "playing" with it for a couple of years.

[CONF iture]

and therefore the FMS Opt level is more valid.
... only if all the latest winds along the route have been inserted for a variety of Flight Levels.
The FMS OPT FL is immensely related to the winds as and if inserted in the box.

So I would say it all depends on how old in your CFP versus how fresh are your inserted winds.

[PT6A]

In most cases the FMS is what 1980's technology? With a tiny amount of processesing power... Verses a flight planning system running on something much faster with a full worldwide set of GRIB winds.

As CONF mentioned its important to A, check how old the OFP is and B, to make sure they used the correct set of wind(s) for the flight.

In the current aircraft an OFP will be provide the best results I think, moving to the future we will see more EFB based applications to assist with tactical reroute and level optimization.

[629bus]

Use the fmc! here is why;

CFP
It might have been generated for a different weight. there might have been a last minute zfw of up to 10 tonnes (that's for my 777). Or the ZFW may have increased. In the odd scenario winds may be different then the FP (not very often)

FMC
Optimum = current weight and cost index
Maximum = current weight, coat index and temp
Recommend = current weight, cost index, temp and winds. (uses 100% of fms wind for actual position and a 50/50 mix 200nm ahead of the aero plane with forecast winds).

So in a nutshell CFP for planning, FMC for operational decision making.

[629bus]

Forgot to mention, the FMC has been loaded with winds at all levels and it's amazingly accurate!

[Slasher]

Don't forget some CFPs take into account CAT or the lowest
expected statistical level for that sector (eg lowest level that
can be expected FL280 and the CFP reflects that level's fuel
requirement). The box's optimum for same sector might be
FL340 so you take 280 fuel but you're ATC-planned at 340.

Talk to the dispatcher whether he took into account any SEV
CAT levels if the CFP level seems unusually low. Sometimes
the CFP can be out depending on the time of compilation and
the CAT may be simply moderate by then but sometimes the
reverse can happen.

[CONF iture]

FMC
Optimum = current weight and cost index
Maximum = current weight, coat index and temp
Recommend = current weight, cost index, temp and winds. (uses 100% of fms wind for actual position and a 50/50 mix 200nm ahead of the aero plane with forecast winds).
On the Airbus, what you call the Recommended is the Optimum OPT, and what you call the Maximum is the Recommended Maximum REC MAX.
With favorable winds the OPT can even match the REC MAX.

Also, the Optimum, as you defined it, is a QRH value, or the FMS OPT if winds were not inserted.

How do you load the winds, is it a facetious process by WPT and FL, or is it more straightforward ?

[FoxHunter]

I've found the FMS will always want to climb earlier than the CFP, MD11/MD10.
Since the fuel penalty is greater for being too high vs too low I've always used the CFP.

[Canuckbirdstrike]

The comparison of technology between an FMS and a flight planning system is like a basic calculator (FMS) and a top of the line PC (CFP).

The comments about the recency of winds in a CFP is misleading. A CFP uses a large data set of winds for various times and sophisticated atmospheric progression models to calculate winds and temperatures at the correct times as the flight progresses.

The wind/temp model in an FMS is very basic and the data is limited by what is entered and at no time will have the gird granularity of a CFP.

The FMS is a guidance tool to be used in concert with the CFP to fly the aircraft strategically.

Overall you are better to stick with the CFP, but we all recognize there may be constraints due to ATC and turbulence. Then it important to understand what is better for your aircraft, be a little high above optimum or a little low.

[nitpicker330]

The CFP in CX is very accurate BUT it's only as good as the info programmed into it pre departure.
The FMGEC in the Bus or the FMC in the Boeing is also very good but relies in the Pilot entering all the data correctly.

Once airborne start with the CFP and then after all the winds/temps are in the FM, use it. It will be more accurate as it blends in real time data.

[Last Ditch]

can anyone tell my why the A-320 FMGS won't calculate step-climbs? While we
mostly operate quite short sectors (up to 3 hours), the FMS-computed optimum
level in the beginning usually differs from the one at the end. Still, when I
try to insert a step-climb, I get the message "no optimal". Why?

"no optimal" happens if the FMGS calculates that you cannot save any fuel by climbing to the new opt FL. i.e. too close to TOD.

[PA38-Pilot]

I have wondered the same thing as Stuck_in_an_ATR (http://www.pprune.org/members/127367-stuck_in_an_atr) for the past 4 years.

"no optimal" happens if the FMGS calculates that you cannot save any fuel by climbing to the new opt FL. i.e. too close to TOD. But it still shows "no optimal" with 4-6 hours to go. And if you force a step climb further down the road, it will show an increase of EFOB at destination.

[babotika]

I'm still trying to understand why the Honywell fmc standard on the 320 is completely oblivious to step climbs. Used to operate with the Thales standard and if a climb was worth it (including the fuel burnt to climb) it would recommend the step.

Just one of many areas where the Honywell box is a piece of junk compared to Thales even in it's latest (buggy) version.

[Natstrackalpha]

I have to say, that nearly every time I have compared the aloft winds in the OFP to the actual winds experienced, they've been pretty much spot on! I`ve found that too - especially in the last year, or so . . maybe its something to do with global roasting or maybe their computors are better than before.( I am still beating them at predicting the weather though., am scoring 8 out of ten bang on to their 7 out of ten).