Max Alt A320

[Neupielot]

Hi. Can someone shed some light as to how to estimate what the Rec Max Alt on the FMGC at XXX time?
I know some aircrafts used 6-8 mins ( if i remember correctly) for every 100ft increase on the max alt on fmc. Is that the same for A320 case?
I know u can find out where the Optimum alt would be from the step alt vertical revision page but I'm not sure how to find what the max alt would be at a certain time.
Comes in handy when asked by atc as to when u can climb up to a certain FL.

Thanks.

[rudderrudderrat]

Hi Neupielot,

FCOM 1.22.20.28 Perf says:

"A maximum altitude using a 0.2 g buffet margin is also computed. It is not displayed, but the system uses it to limit CRZ ALT entry."

Normal displayed buffet margin is .3 g, but if you type your maximum requested FL into the Cruise Field when it shows say Max FL377, then it may accept FL 380 if its buffet margin is =>0.2 g.

Best make sure there is no forecast turbulence / wind shear though, else you will want to return to a lower FL. Otherwise refer to the performance graphs FCOM 2.5.20 Optimum & Max FL and open the little hidden picture at the bottom.

[Neupielot]

which brings me to the next question(s).

1) rec max is 0.3g buffet margin. Hmm...isn't the " normal " margin 1.3g or 1.4g? Or is it up to Optimum the margin is 1.3g. Above Optimum it gradually reduces from 1.3g to 0.3g at rec max? I'm a little confused here.:O


2)the chart/graph @ 2.5.20 (optimum/max FL), The "n" sign= 1.4g and 1.3g buffet margin. The max alt line is below those 2 lines, which means max alt margin is greater than 1.4g? If it is, then it doesn't agree with the 1st question above. I do realise that rec max alt and max alt has different definitions but .....surely it can't be that big a difference in margins?:confused:

Many thanks. :)

[rudderrudderrat]

Hi,

1) 0.2 delta g buffet margin = 1.2 g margin. Slightly different ways of expressing the same margin.

2) There are 2 graphs shown - @ 0.78M and @ Long Range Cruise.
at 0.78 M you'll be limited to rate of climb 300 feet per min to get to Max FL.
At LRC you'll be limited by your buffet margin.

[Neupielot]

thanks again rudderrat :).