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A320: Opt altitude exceeds rec max

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A320: Opt altitude exceeds rec max

Old 20th Feb 2016, 00:53
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A320: Opt altitude exceeds rec max

Many years flying 320/330's but never seen this until my current airline that I just joined.

A320 opt altitude very close to rec max and on some sectors actually exceeding rec max. We saw it recently on both east and west bound sectors on the same day where the opt was several hundred feet above the rec max.

Granted I have been 330 flying for a long time and only just getting re acquanted with the 320 but I am used to looking down and seeing the opt generally several thousand feet below rec max.

Is this normal?
oicur12.again is offline  
Old 20th Feb 2016, 02:44
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Probably a rather low cost index (single digit), and winds inserted into the fms for several levels.
If the wind aloft at max altitude is much more favorable than below the optimum altitude can go up till maximum. If then the temperature is changing inflight the maximum altitude may drop below optimum.
Maybe that's what you saw.
On most flights you will find that optimum levels are some 2000 ft below maximum. Lower on high cost index, higher on low ones.
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Old 20th Feb 2016, 06:02
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Never saw it or heard of it. Just for personal interest, what is the FMS setup?
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Old 20th Feb 2016, 08:25
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Saw this on an A320 quite recently. It was only ~200ft, but it was indeed odd that the optimum altitude was the higher number. CI was 75 if I recall.
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Old 20th Feb 2016, 10:03
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Have a look in the QRH Perf section and see if Rec Max is what it should be given the ISA temps and weights.
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Old 20th Feb 2016, 10:12
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I've been flying the bus for about 6 years and it's kind of normal to find the optimum and maximum altitude on the same level. But the optimum above the maximum happened to me last month for the first time.
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Old 20th Feb 2016, 12:54
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I've never seen it in 10 years on the A320. We operate SE Asia and enter 53000 for tropo and ISA +10. CI 20 or 30.

A cruise level above the recommended max will be accepted with a 0.2g bufett margin, normal is 0.3g. Company policy is not above REC MAX.
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Old 21st Feb 2016, 15:43
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Safelife,

Yes i have been keeping an eye on the winds and it doesnt appear to be the cause. Yesterday east bound strong tailwind opt and max were the same and on the west bound the same daywith strong headwinds up high it still had opt and max together.

We tried ci changes and both opt and max remained together from ci 0 to 60. In old airlines with same engines we would fly ci 40 and there was always a split of several thousand feet.

Interestingly, the Qrh rec max agrees with the fmc each time I have checked but the qrh opt is often 2-3000 feet less than the fmc opt.

My take thus far is that there is something odd about the fmc opt alt predictions in these aircraft.

Flight detent, fms setup. Honeywell fms 2
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Old 22nd Feb 2016, 04:57
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Anything high in the APD?
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Old 22nd Feb 2016, 05:50
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Actually, I see it all the time, and it's quite normal, especially in the tropics.

Even though the two values are displayed on the same page, there are 2 different algorithms at work (or 2 different apps running; analogy for the iGeneration)

For the REC MAX, see FCOM DSC-22-20-40-30, Pg 10/18. I think the keyword here is "IS". i.e. the calculation is done in real time, using actual atmospheric data, and the info on the INIT A page, to predict a maximum altitude that satisfies the conditions mentioned.

For the OPT, FCOM DSC-22-20-40-10, Pg 5/8: The key here is that "It requires a 5 min minimum cruise time"... i.e. during CLB phase, it will give you a standard value, based on input weight and standard weather data. For this value to have any meaning, it needs actual weather data (gathered in real time, and from the CRZ wind data).

So, to take an example:
1. You are in climb phase
2. It's hot and humid (i.e. ISA +15 or more, lots of haze, clouds around at high altitude, etc)

For the OPT calculation:
1) the FMGS is assuming applying an ISA deviation based model (with standard lapse temp rates) on the CRZ FL/TMP data, hence OPT is not "optimized" yet, since you're not yet in cruise.

For the REC MAX:
2) REC MAX is calculated using a temp model of e.g ISA +15 (real time data) interpolated to the CRZ LVL, using the temp entered on the INIT A page. Since it doesn't know that the ISA deviation will significantly reduce as you get higher, the best it can do is give you a really low number that will meet the requirements. (0.3g buffet, etc etc)

I see it a lot when you have a lot of moisture in the air with high temps, and you are still climbing.

Looks weird, but actually quite logical.
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Old 22nd Feb 2016, 09:17
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Seagull, please let it be, or they will insert another "FMS specificity"...
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Old 22nd Feb 2016, 10:01
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from Elite

"Have a look in the QRH Perf section and see if Rec Max is what it should be given the ISA temps and weights" unquote


Elite`s post made me think that maybe its to do with the diff between QNH and QNE on days where there is a large difference . . . ? or press higher than 1013.2 . . ?
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Old 22nd Feb 2016, 11:42
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Set the tropopause level and first cruize temperature on init A.


Cheers
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Old 23rd May 2021, 07:49
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Optimum altitude higher than max

Pl refer QRH graph. Here also at higher weights the optimum goes above max altitude.
why so?
any fresh perspective ?
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Old 25th May 2021, 09:07
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Were both FMGCs showing the same? I've noticed in the past that when one TAT measurement differs from the other, you'll get conflicting suggestions for optimum and recommended max.
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