A320 NEO optimum altitude

[Flyman35]

Noticed that optimum altitude in A320 neo aircraft when flown it left us with a small margin between the vls and vmax not like classic A320 which gives
us a wider margin.another thing is Mach control on cruising level it seems like the thrust is a little bit lazy and don't respond quickly like the classic A320, was wondering if any similar notes have been observed by pilots?
for example below I am flying 2000 feet below optimum , imagine if I climb to optimum which is 370 where the vls and vmax margin will be significantly low.



https://cimg1.ibsrv.net/gimg/pprune.org-vbulletin/1125x1361/pfd1_2136aa84351aff61fad1d4aaccad126d289042dd.jpg
Flying at FL350 while FMS optimum is FL370, wind inserted at both levels.
https://cimg1.ibsrv.net/gimg/pprune.org-vbulletin/1125x1490/progress_22eea155e54a88b279ca70b3b08b21c636a40032.jpg
optimum is FL370

[Fly3]

Am I between missing something? There appears to be a 45 knot gap between the two. I remember my Airbus instructor telling me that anytime you have at least a 40 knot gap the aircraft should be able to handle any upset.

[FlightDetent]

Depends on the cost index. A low one will go for a higher level, getting you a smaller margin.

Inserting the winds actually complicates the aerodynamical / performance understanding as you're basically offering the airplane two different operating theatres. I.e. with less headwind in higher levels, the OPT figure will push further up into tighter margins.

Fly3: He's 4000 below the MAX, hence questioning if perhaps there should be a bit more than those 45 kt.

[Check Airman]

That margin seems generous enough to climb to 370. That being said, most of my time's on the 321, so I'm accustomed to seeing smaller margins than that at altitude.

[Jimmy5]

Both CEO and NEO A320 variants will be thrust limited (as opposed to buffet limited) at the rec max altitude. However the NEO has slightly more thrust and generally isn’t derated for airway charge benefits (in my company anyway) and therefore at it’s rec max limit it will be closer to the 1.3g buffet. I Imagine this situation or something similar with regards to optimum fuel flows for the different engines, plays out in some form at the optimum level as well. If you’re flying higher or closer to the buffet limit then the margins will be less.

I haven’t noticed the sluggishness that you describe on the NEO.

[WhatShortage]

Don't fly the LR at optimum then... That 45kts gap was like 15 TOPS, had to go lower. Honestly we were at 380 but nevertheless, it lacked authority on the thrust.

[A320251N]

Reparding the thrust control in CRZ... I do find it a bit lazy as well !

Looks like switching from managed MACH to selected SPEED improves this behavior.

I don't know if it is something related to the SOFT ALT control...

[speedrestriction]

The Leap NEOs definitely have more thrust margin available at cruise than the CFM56 CEOs. In my company there seems to be an informal SOP not to cruise at anything higher than Rec Max -1000ft. This is to prevent getting below GD where the CEOs might not have the puff to power out of the increased induced drag. This behaviour has lead to pilots not being accustomed to flying at altitudes where the aerodynamic margins are slightly narrowed. (Realistically, we are always flying waaaaay above stall speed and a lot closer to MMO).

Unfortunately this informal SOP has carried over to the LEAP equipped aircraft which certainly do have the power to cope with deviations below GD. I have seen a 321 NEO at OPT/Rec Max 380 accelerate from Vls -6 (approx GD-9) without any difficulty after encountering a gradual shear. The thrust was slow to come on but when it did it accelerated reasonably smartly (N1 had been sitting at 87.5ish).

Flying the NEO at Opt/Rec Max gives aerodynamic margins similar to other types I have flown whilst retaining superior thrust margins. When Opt/Rec Max coincide and at low cost indices, managed speed is within a knot of GD. This spooks people but having flown the NEO for hundreds of hours in this exact configuration I can reliably say that this is unwarranted - it is a perfectly safe and stable configuration. I am also flabbergasted that people feel more
comfortable in these situations to select Mach and put it half way between GD and MMO. It makes the picture look a little more “normal” but significantly degrades your margin to MMO while increasing your margin from V alpha prot which you are already well away from. It betrays a real lack of understanding of the aircraft’s actual performance.

To my way of thinking, flying the NEO as a CEO is very conservative and a little wasteful not to mention the additional airspace congestion caused by aircraft flying 2000ft below their ATC filed levels.

[ahramin]

Speedrestriction while I completely agree with your philosophy, the specific example you cite (vls -6) would have me thinking that I don't have enough thrust margin and need to descend as soon as possible. Was this an extreme situation due to an abnormal outside factor or are you saying there's no problem getting into the vls in cruise with a NEO?

[speedrestriction]

It was more a case of sluggish auto thrust response an increasing tailwind over a period of about 40 seconds in ALT CRZ.

The aircraft had been stable in the ALT CRZ with the thrust set at 87.5%. The wind shifted and the IAS washed off by about 9kts or so over the period of about 30-40 seconds but the thrust remained at the previous setting. Eventually the auto thrust decided that the deviation warranted the addition of power, the engines wound up to full power and the aircraft smartly returned to the managed target speed.

I have found that there is a bigger delta between thrust required to sustain cruise speed at/near green dot and rated thrust at Rec Max on NEOs vs CEOs. Typically 8-9% on a NEO vs maybe 3-4% if at Rec Max on a CEO.

In the scenario described above, if the aircraft has been a CEO, I am certain I would have had to initiate a descent to get back on the right side of the drag curve. I certainly consider the 1000’ margin prudent on the CEO and I practically never push above it unless operationally necessary, being well above the tropopause and in very smooth air, not crossing high terrain.

[Manual Pitch Trim]

Stay at FL350 its safer. If you can .....

yes burns a little more fuel, but if theres turbulence and if you have to do a emergency descent, its better😉

[speedrestriction]

Poling around 4000’ below optimum for a 100 fleet airline is probably somewhere in the region of 15m - 20m extra kilos of fuel burn per year. That ain’t a little.

[Alex Whittingham]

There is a paper by Artur Bensel from Hamburg University at this site (https://www.fzt.haw-hamburg.de/pers/Scholz/arbeiten/TextBensel.pdf) which disputes the old academic derivation of best range speed still air of 1.316 VMD (which relies on the assumption that TSFC is constant) and suggests 1.09 VMD is more likely (actually 1.05 to 1.11), hence possibly giving managed speeds closer to GD than you would expect from distant ATPL studies...and slower still in a tailwind