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BWIAirport
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CRZ OPT - How is it determined?

Wed Jan 08, 2020 8:01 pm

The optimal cruise altitude is displayed on the MCDU/CDU under either VNAV or PROG, as most of you are aware, and always increases as the flight progresses. Is this calculated based off the weight and fuel capabilities of the aircraft, and in that case why is the optimal flight level not the same as the CRZ MAX, which also increases as the aircraft sheds fuel weight? Or is it calculated based off winds aloft?

Traffic aside, is this always the best altitude to cruise? In other words, should I always try to stay at the CRZ OPT level (so long as I remain consistent with ICAO step climb)?
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mmo
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Re: CRZ OPT - How is it determined?

Wed Jan 08, 2020 8:29 pm

This is for the 777

OPT:

• with ECON speed selected, displays altitude which minimizes trip cost based on weight and cost index

• with LRC, EO, CO, or SEL speed selected, displays altitude which minimizes trip fuel based on weight

• does not reflect the effect of speed if speed intervention (MCP IAS/MACH window) is selected.

• Penalty for being OFF Optimum Altitude (at LRC Speed) is:

2000 feet below - 1%
2000 feet above - 2%
4000 feet above - 3%
6000 feet above - 5%
8000 feet above - 7%
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GalaxyFlyer
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Re: CRZ OPT - How is it determined?

Wed Jan 08, 2020 9:15 pm

Simply put, OPT is the altitude where the plane, under the current conditions gets the most nautical air (or ground) miles per pound (easier math, per 1,000#) of fuel. It’s not MAX.

GF
 
e38
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Re: CRZ OPT - How is it determined?

Wed Jan 08, 2020 9:54 pm

Quoting BWIAirport (thread starter), "always increases as the flight progresses."

BWIAirport, no, it doesn't always increase as the flight progresses!

On the aircraft I fly, the FMS is taking the actual winds, along with the forecast winds our meteorology department provides, as well as the aircraft gross weight to give you, as GalaxyFlyer stated above, the greatest air nautical miles per pound of fuel.

Very often, as the flight progresses, the optimum altitude will decrease if the FMS determines that a lower altitude will result in a more efficient fuel burn based on current and forecast winds.

Are you talking about some type of flight simulation program? That may not be realistic relative to the real world.

e38
 
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AirKevin
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Re: CRZ OPT - How is it determined?

Wed Jan 08, 2020 11:04 pm

BWIAirport wrote:
why is the optimal flight level not the same as the CRZ MAX

Max would put you at the limits of the aircraft for a given weight, and you're hitting coffin corner at that point. There's really no margin for error between your stall speed and overspeed. I suppose you could fly up to max cruise altitude if you really wanted to, but just because it can be done doesn't mean it's a good idea.
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Starlionblue
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Re: CRZ OPT - How is it determined?

Wed Jan 08, 2020 11:38 pm

Yes, OPT is the best altitude to cruise. REC MAX is the altitude with 0.3G buffet margin to overspeed and stall, and it is not recommended to exceed it. Exception: If the calculated REC MAX is at or higher than the service ceiling, the service ceiling is displayed.

That being said, there are sometimes other considerations than fuel burn. On a busy airway, you may want to "claim the high ground", climbing earlier than optimal to avoid potentially being kept low later. Also ATC considerations, quadrantal rule, semi-circular rule....

In the MCDU, if you got into STEP ALTS (either from the PERF CRUISE page or the vertical revision page), and enter a higher cruise level, the optimal time/distance to climb to that altitude will be displayed. If the difference is less than 1 minute trip time or 100kg fuel, "NO OPTIMAL" is displayed.
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gloom
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Re: CRZ OPT - How is it determined?

Thu Jan 09, 2020 7:44 am

As already ststed, OPT is the max range altitude (more or less). Or speaking in terms of aero, minimum drag altitude.

If you go lower, the air is thicker. Aero drag will be thus higher, even with lower angle of attack.
If you go higher, the air is thinner, but angle of attack causes increased drag on wings and horizontal stabilizers.

Also, there's a CoG effect - depending on CoG selected for a flight during dispatch, cruise level for a given flight and weight could differ a bit. This is due to trim position, resulting in aoa and trim drag. Of course, CoG is optimised during preflight, but sometimes few more cargo pieces are worth getting CoG a bit from optimal position.

Of course, for a given altitude pilot can always override. Back in the days when I received my share of knowledge, there was always alternate plan (below/above) for a given loadsheet that announced the difference to fuel (so, for example optimum F340, alternate plans for 320, 360; trip fuel stated for all three), so that pilots could recalculate how much fuel penalty it would be in case of suboptimal FL for whatever reason. I guess that's part of EFB now.

Cheers,
Adam
 
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Starlionblue
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Re: CRZ OPT - How is it determined?

Thu Jan 09, 2020 8:20 am

gloom wrote:
As already ststed, OPT is the max range altitude (more or less). Or speaking in terms of aero, minimum drag altitude.

If you go lower, the air is thicker. Aero drag will be thus higher, even with lower angle of attack.
If you go higher, the air is thinner, but angle of attack causes increased drag on wings and horizontal stabilizers.

Also, there's a CoG effect - depending on CoG selected for a flight during dispatch, cruise level for a given flight and weight could differ a bit. This is due to trim position, resulting in aoa and trim drag. Of course, CoG is optimised during preflight, but sometimes few more cargo pieces are worth getting CoG a bit from optimal position.

Of course, for a given altitude pilot can always override. Back in the days when I received my share of knowledge, there was always alternate plan (below/above) for a given loadsheet that announced the difference to fuel (so, for example optimum F340, alternate plans for 320, 360; trip fuel stated for all three), so that pilots could recalculate how much fuel penalty it would be in case of suboptimal FL for whatever reason. I guess that's part of EFB now.

Cheers,
Adam


Indeed we still have that on the flight plan in the EFB, so we can see what would happen to the burn if we are kept low.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
gloom
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Re: CRZ OPT - How is it determined?

Thu Jan 09, 2020 10:25 am

Starlionblue wrote:
Indeed we still have that on the flight plan in the EFB, so we can see what would happen to the burn if we are kept low.


Slightly off topic, but are there still weight below/above fuel calculations as well, for last minute load changes? Or is it a part of preflight to recalculate should the change come?

Cheers,
Adam
 
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Starlionblue
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Re: CRZ OPT - How is it determined?

Thu Jan 09, 2020 11:57 am

gloom wrote:
Starlionblue wrote:
Indeed we still have that on the flight plan in the EFB, so we can see what would happen to the burn if we are kept low.


Slightly off topic, but are there still weight below/above fuel calculations as well, for last minute load changes? Or is it a part of preflight to recalculate should the change come?

Cheers,
Adam


When you punch in the final ZFW in the app, it will automatically change the total and trip fuels. I still feel lazy doing it that way. :D

But yes, the correction factors are still there, which is good for a gross error check. And so that we have the necessary supporting information in case we want to "manually set" the fuel figure.
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dennypayne
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Re: CRZ OPT - How is it determined?

Thu Jan 09, 2020 4:55 pm

gloom wrote:
Of course, for a given altitude pilot can always override. Back in the days when I received my share of knowledge, there was always alternate plan (below/above) for a given loadsheet that announced the difference to fuel (so, for example optimum F340, alternate plans for 320, 360; trip fuel stated for all three), so that pilots could recalculate how much fuel penalty it would be in case of suboptimal FL for whatever reason.


On most of the flights where I was able to listen in to ATC on Channel 9, it seemed that a substantial portion of enroute communication was along the lines of where the ride was better and could we please have that altitude instead. So do these calculations pretty much get overriden then, or is potential turbulence also factored in to the initial determination of optimum?

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gloom
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Re: CRZ OPT - How is it determined?

Thu Jan 09, 2020 7:50 pm

dennypayne wrote:
is potential turbulence also factored in to the initial determination of optimum?


Well, from my experience it is hardly ever a problem for whole flight. On our flightplans, we had something called "turbulence factor", graded 0 to 10 (0 being no turbulence, 10 being really severe). If I remember correctly, I've never seen a flightplan without low values. At least part of the flight was at 1s or 2s. However, once or twice we were asked after briefing to alter the requested flight level. I don't quite remember if it was for turbulence, but possibly could happen for that. A friend of mine, flying as navigator on Tu134s in the old days, said once they had AMS on a day when factors were 9+ all the way. He claimed the toilet was the cleanest part of the plane after flight ended.

Still, it will not be optimum, for obvious reasons. It will become selected/planned, and obviously plan will include all standard elements, but flight level will not be optimum anymore.

Cheers,
Adam

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