View Full Version : A320...Fmgc Opt FL

13th Sep 2007, 22:07

some times i do short sector flight (~ 250 n.m ) i.e Kuwait-Bahrain-Kuwait.

when i load the flight plan info in the fmgc...with the winds...at various level (FL 220.240.260 and 280) and also i plan the arrival with the expected routing ...i got the optimum flight level as FL 200 or 205 somthing like that. but the planned level the computer flight plan is FL270 .

so...is flying at or close to the Opt FL is recommended in this case or to stick to the flight plan FL ?

FL 270 will give a cruis time almost 10 mins.

what i do usually is to fly at FL250 just to increase the Cruise time so i got enough time to make the P.A ...prepare the Approach...Breif the Approach if it is not breifed while on ground at the departure airport...etc.

any ideas.

thanks in advance.

14th Sep 2007, 02:08
All FMCs (even silly ones like A320s) are mere computer tools of trade. Optimum level is merely advice to be taken into account with real world realitys (like your FL 250 for extra time to get stuff done). From what you say FL 270 sounds like a short-sector canned level for averaged wieghts and seasonal winds.

I just done an A320 conversion in Tooloose. When the bloke wasnt looking I played around with the box including what your asking here. After you stick in all the winds for varius levels, cost index etc go to the Progress page and stick in varius cruising levels (incl the optimum) and read the flight-time and arrival fuel of each level on the flight plan page. On some sectors I done in the sim, even a level slightley under rec max level saved 100kg over optimum.

14th Sep 2007, 03:38
The A320 FMGC will provide an optimum level based on wind, gross weight, cost index and time. If the cruise segment will last less than 15 minutes, the FMGC will recalculate an optimum level that will provide a 15 minute cruise (ie, 270 gives 10 minute cruise, FMGC will call for 205 as it will provide a 15 minute cruise).

I've not seen it written explicitly in the FCOM, but my understanding of the aerodynamics of the A320 (at least V2500 powered ones) suggests that it is more fuel efficient to climb to (for example) 270 at CLB thrust, cruise and then descend in DES mode than it is to adhere to the suggested optimum level in this situation - the argument being that the extra fuel burnt in the climb is offset by the less fuel burnt in the shorter cruise and longer descent.

On shorter sectors I tend to select the company generated flight plan level, in conjunction with my own analysis of the shear-rate and wind at different levels, up to the point that climbing too high will make my job harder. In other words, too little time in the cruise to set up the arrival!

Tricky explanations over; do whatever it takes to make your job as simple as it can be!

14th Sep 2007, 15:18
Thanks guys for this great reply's

17th Sep 2007, 09:12
The optimum level of an Airbus FMS take into account the Cost Index. So setting it higher, it will first increase your speed, that is for climb, cruise and descent. After reaching max speeds, and further increasing your Cost Index, it will start lowering the Optimum Level, to be even faster. It has nothing to do with fuel efficency. It is just the optimum level for your CI. To find the real Optimum Level, check the table in the AOM (It was provided in the Quick reference Handbook at my company).

And the most efficent flight has no cruise at all, it is a ballistic curve. i always try to achieve that, but usually fail to do so because of ATC and the busy skies of Europe. You can set up for the approach and make your announcement during descent also - just hand the control to the other guy in the Cockpit, he has a rating too.


17th Sep 2007, 09:25
Ah, optimum FLs in the A320 series. Mounts up on hobby horse.........
The FMGC Optimum FL in the 320 series depends on, as others have said, Wt (mass), CI, wind, Temp and 15 mins in the cruise. But what do we tell it? Wt - yes, CI - yes, wind - maybe, Temp - maybe one. But the CFP knows the whole dynamics of the atmosphere that day ie all the winds and temps at all the levels. Therefore, provided the CFP projected wt (mass) is close to the aircraft's actual mass, the CFP optimum will be the level that will be best for fuel in the light of the CI used.
Time and time again we see guys load up the FMGC with a crz level and temp, maybe a trip wind and then wonder why the FMGC optimum differs from the CFP optimum and elect to fly the FMGC optimum which is invariably lower thereby wasting fuel.
There is, of course, more to it than that.....dismounts.

17th Sep 2007, 12:06
You aren't altogether incorrect, 346. If you do change the CI your optimum level will also change. CI=0 is equivalent to minimum fuel consumption (maximum range). CI=999 is equivalent to minimum time ("defined in FCOM 4.02.20 p16). However, even with an amendment to CI, the effects of wind, temperature and gross weight will still alter the OPT FL.

Further, if you refer to FCOM 4.02.20 p15, the "OPT FL is a compromise between fuel and time saving". What part of that sentence does not scream out "fuel efficiency"? That is why pilots are allowed to manually edit the CI, be it at the behest of the company or as part of a pilot-initiated strategy.

As for the "ballistic" flight path, I agree - that's exactly what I inferred earlier. But it never happens because of ATC. You said it yourself.

And yes, you can set up the FMGC for arrival on the descent, but I'd much rather be monitoring the descent!

17th Sep 2007, 13:21
In the PERF section there is a small paragraph about flight planning higher levels on short routes ( ie, less than 15 min in the cruise) which ignores the OPT level. An example is on YMML-YMLT route( 246nm).

Perhaps this may suit a company's particular flight plan requirement as in the above and Kuwait340's scenario.