Airbus optimum cruise levels -

[ispra]

When the FMGC offers an optimum cruise level on the PERF page, is it advisable to to accept it? On short sectors, especially light, there can be a big difference between OPT levels, Max levels and computer flight plan levels (based of fuel efficiency). I know traditional thinking is for 'go as high as you can then descend' but I understand OPT CRZ is to give sufficient time for the engine to operate at a stable power setting for engine life and maintenance considerations, at least this is what I have picked up over the years. Notwithstanding the above, when there is a big difference between OPT and higher cruise levels, is the extra fuel used by staying low factored into the computation? I.E. would a wise and company minded crew tend to follow the OPT CRZ value?

[The African Dude]

Ok, I know you will have read this, but just to expand my reply, FCOM says "The optimum flight level (OPT FL) indicates the most economic flight level for a given cost index, weight, weather data. It is continuously updated in flight. It requires a 15 min minimum cruise time. The OPT FL is a compromise between fuel and time saving."

So although you could argue that the part of the Cost Index, which balances airborne time vs. fuel, would take in to account the airframe and engine life maintenance/management, it doesn't take in to account other more complex factors such a cyclic fatigue on airframe, engine stabilisation etc. I also don't think that the CFP accounts for these factors. The CI is the all-in bundled compromise.

My perspective would be that, actually, climb thrust is fairly stable and for our average, mostly continuous, climb there is limited thermal shock/stabilisation to consider. The benefits of a longer, slightly more stable engine cruise segment may not be significant when compared to factors such as ATC route planning/restrictions.

[autoflight]

If your company regularly has a relatively short sector in a busy area, it might be worthwhile to set a max normal level to avoid stress on ATC trying to give you climb and descent clearance through overflying traffic that should normally be higher.

On a longer sector, go for it.

[BlackandBrown]

African dude - the 15 mins cruise bit is easyJet specific - airlines can choose how long minimum cruise time is and I believe our software has been update to calculate for as little as a 5 minute cruise now.

[ispra]

Puzzling differences

Yesterday the OPT CRZ outbound on a 80min leg was FL 360 then inbound with a similar payload and a bit of a headwind the OPT CRZ was FL280. I cannot understand these differences.

[nitpicker330]

It's an Airbus, don't try to understand it.

I've seen the Opt Alt drop to FL280 as well for no good reason when it was FL360 5 mins before that!!

[The African Dude]

B&B I'd forgotten about that.

If today's newsletter is anything to go by, the CFP profile should be the most accurate as the OPT FL is only predicted for the current position, if conditions are as forecast.

Ispra, seen the same. Did the box have all levels wind or just the trip average?

[Superpilot]

Wind direction and speed mixed up over next way point? (easy mistake when reading from a SITA CFP)

[nitpicker330]

You didn't ask me but.......

In my case the flight plan had all the winds uplinked pre departure and from memory they were pretty accurate. The trip distance was considerable so there was no sense in it saying OPT FL 280....

I've seen it a few times.

[autoflight]

Depends on CI and what winds are entered. Give me 150K headwind and CI 35, I would be expecting to see a lower level.

[The African Dude]

Sorry nitpicker - moment of absentmindedness
Interesting though, and I will treat the OPT FL with a little more skepticism in future.

[ok45]

Eratic

I've experienced what nitpicker330 said, Im flying A320 though, so in my case it is usually several hundred feet of OPT ALT "jump" in flight. I've been where it said OPT ALT was FL380, and once we started the climb to FL380, it says OPT ALT was FL360 (our previously held altitude) very annoying to be honest.

[ispra]

Trip average

In the habit of entering trip average winds, bit lazy I suppose, except when very strong then I put in the descent wind gradient through a few levels. Do you think entering average + or - winds results in these OPT CRZ discrepancies?

[Fly3]

The OPT ALT calculation is quite complex. It requires the entry of forecast winds along the entire route and at levels above and below your planned cruising level. For instance if you are planned to fly at FL350 then enter the winds from say, FL330 upwards to at least the maximum level you intend to achieve en-route. It is also important to enter the forecast temperature at the level you plan to cross each waypoint. Not doing this can lead to the situation where the OPT ALT was FL380 and you climb only to find that because of the new temperature at that level the OPT has reduced to FL360.

[Phoebus]

It depends a lot on the FMGC model and software version. The legacy Honeywell (FM1) had little wind data to work on, took no account of leg length and was completely upset by the cruise temp/tropo inserted on INIT A if the departure airfield was in a different airmass to en-route - especially going from hot Mediterranean back to cold Europe.
I have no recent Honeywell FM2 experience, but Smith/Thales FM2 has had issues with INIT A data until the latest Release 1A software. You can really screw the optimum by; not putting winds above and below current cruise, not inserting temperature and best of all; only putting lower wind data up to the expected step climb. This is the best way to screw it up - as the last entered wind is copied right to the last cruise waypoint. If you have a tailwind at the beginning of a flight and headwind later on, this will give false optimum cruise too low. As they say - garbage in, garbage out...