A321 Neo altitude capability

[tubby linton]

For the pilots who have flown the NEO and CEO A321 both with sharklets the altitude capability for the NEO seems to be a bit on the ambitious side. The margin seems to be much smaller and Turb speed seems to be very close to green dot and Vlsin the Neo.
Have Airbus got their data correct?It is not a pleasant aircraft to be in turbulence and the autothrust seems to be very sloppy.
The aircraft being compared are an A321 Neo with Leap amd an A321 Ceo with IAE V2533, both with sharklets.

[vilas]

Strange! Because A320 CEO CFM is definitely better than IAE. Why should Leap not be even better?

[tubby linton]

The A321 Neo optimum certainly seems to be higher than a Ceo at the same weight.The margin is much smaller in the Neo and turbulence speed can be almost coincidental with green dot with Vls very close as well.

[hikoushi]

Our NEOs tend to like living a couple thousand feet below REC MAX, and even then the margins can be very tight at altitude when heavy. Some examples below from early on. The trop heights were correctly programmed into the FMS, and both were using low cost index and bracketing the optimum altitude within 1000 feet (per FCTM). First one is from a fairly heavy flight. Cruising at 350 with a REC MAX at 367. The second is from a lighter flight. Cruising at 370 with a REC MAX at 389. The first one was resolved by increasing Mach by 0.01; the second one by descending back 1000 feet.

In both cases smooth conditions with occasional light turbulence, but didn't take much air movement for the speed to decay. In these airplanes it seems like when the managed cruise speed gets within about 5 knots of Green Dot, the thrust margin to recover from a speed decay gets a lot more critical; this is when the A/THR gets "sloppy" as was stated previously.

Moral of the story in the NEO if your flight planning people offer you a high altitude right off the bat, if there is any degree of turbulence or higher than average temps at cruise, stay at least a couple thousand feet below the recommended maximum, optimum or computer-planned altitude be damned.

[speedrestriction]

Thrust margin is better on the NEO LEAP vs the CEO CFM. Whilst I wouldn’t fly a CEO with a managed speed as close to Green Dot at cruising altitude as in the second picture above, in smooth conditions I am quite happy to operate the LEAP since the thrust margin is usually between 8 and 11%, ie you have sufficient reserve thrust if you fall a few knots behind to overcome the additional induced drag.

[Check Airman]

Originally Posted by hikoushi

Our NEOs tend to like living a couple thousand feet below REC MAX, and even then the margins can be very tight at altitude when heavy. Some examples below from early on. The trop heights were correctly programmed into the FMS, and both were using low cost index and bracketing the optimum altitude within 1000 feet (per FCTM). First one is from a fairly heavy flight. Cruising at 350 with a REC MAX at 367. The second is from a lighter flight. Cruising at 370 with a REC MAX at 389. The first one was resolved by increasing Mach by 0.01; the second one by descending back 1000 feet.

In both cases smooth conditions with occasional light turbulence, but didn't take much air movement for the speed to decay. In these airplanes it seems like when the managed cruise speed gets within about 5 knots of Green Dot, the thrust margin to recover from a speed decay gets a lot more critical; this is when the A/THR gets "sloppy" as was stated previously.

Moral of the story in the NEO if your flight planning people offer you a high altitude right off the bat, if there is any degree of turbulence or higher than average temps at cruise, stay at least a couple thousand feet below the recommended maximum, optimum or computer-planned altitude be damned.

What CI were you using? Those speeds seem VERY slow. Especially for a 321

[speedrestriction]

Originally Posted by Check Airman

What CI were you using? Those speeds seem VERY slow. Especially for a 321

CI8 with a light-moderate tail wind will produce a managed target like this at the mid-thirties and above.

Descent Economy speeds on the NEO are also 15kts or so slower than the CEO.

[FlightDetent]

The CEOs with Pegasus or Thales FMS had the DES speed constrained at not less than 275 kt. Is that also changed?

[CanadianAirbusPilot]

Technique not SOP. I find selecting speed vs mach on the CEO makes the a/thr much more active vs the soft capture

In both those photos, why can't you just bug up to .78 or higher? We run our 321s frequently in smaller margins than that but I find having selected .79 or .80 to be the best way to not worry about it. I would probably not let the plane ride at that low of an airspeed.

[speedrestriction]

Airbus and CFM spend billions optimising the airframe and systems. Airlines buy in software to help them optimise schedules matching runways slots to flight times and trip costs, one output of which is cost index. The company file a flight plan telling ATC that we will be flying a certain Mach No.

In Europe this data is fed into a computer which in turn tries to optimise ATC slot allocation to minimise overall delay.......only for Captain Bloggs thinks, mneh - I don’t like the look of it, I’m gonna add on a few extra knots because I like the way it looks on the PFD.

Quick question: in the pictures above, at the managed speed are you closer to the over speed protection or under speed (alpha) protection?

Having spent maybe 40 hours in the last 6 months cruising around in a profile similar to the second picture, it would appear that speed stability is not an issue. Don’t get me wrong - I wouldn’t try it in a jet stream crossing a big mountain range. It hasn’t caused any problems in light-moderate chop though.

[Check Airman]

Originally Posted by speedrestriction

CI8 with a light-moderate tail wind will produce a managed target like this at the mid-thirties and above.

Descent Economy speeds on the NEO are also 15kts or so slower than the CEO.

I've also observed the NEO speeds are slower. Still trying to figure out why. The first picture is uncomfortably slow, but I'd do it if requested by ATC, and if the environment permitted.

I would not be as slow as your captain opted to fly in that second picture. The computer can spit out whatever CI it likes, it's still my butt strapped to the plane up there. If it all goes wrong, not much the computer can do to help.

[hikoushi]

Originally Posted by CanadianAirbusPilot

Technique not SOP. I find selecting speed vs mach on the CEO makes the a/thr much more active vs the soft capture

In both those photos, why can't you just bug up to .78 or higher? We run our 321s frequently in smaller margins than that but I find having selected .79 or .80 to be the best way to not worry about it. I would probably not let the plane ride at that low of an airspeed.

These were from early on, as the aircraft was just beginning to reveal it's behavioral idiosyncrasies! As mentioned lower altitudes and hard-selecting a slightly higher speed were the temporary fixes, followed by the permanent fix of bumping up the CI a bit.

[pineteam]

Originally Posted by CanadianAirbusPilot

In both those photos, why can't you just bug up to .78 or higher? We run our 321s frequently in smaller margins than that but I find having selected .79 or .80 to be the best way to not worry about it. I would probably not let the plane ride at that low of an airspeed.

I agree. The penetration mach number on the checklist is .76 but it’s definitely not the best course of action at high altitude on an heavy A321. Bette select a higher Mach number sitting nicely in the middle or even closer to MMO that taking the chance to enter the back side of the drag curve in case of sudden moderate/severe turbulence.

[tubby linton]

I would suggest a read of ths article by Airbus before thinking about cruising at speeds around green dot.
https://safetyfirst.airbus.com/contr...eed-in-cruise/

[vilas]

Are we reaching a full circle? The very purpose of Neo's existence is fuel saving. If it requires increasing the COI or flying higher select speed then some saving will be compromised. Rather than randomly each one doing his bit Airlines should get the new data about actual fuel saving with these practical constraints and the recommended cost index.

[pineteam]

I was talking purely for the CEO. We don’t have NEO yet. What drives me nuts if that some skippers will slow down to .76 in Moderate turbulence at very high altitude when the managed speed of .78 was nicely in the middle of the coffin corner unaware of the danger of flying close to green dot at high altitude. Or the guys flying exactly at the penetration speed of 275kt around 20 000 feet because it’s on the checklist when obviously slowing down would be much smoother... SMH.

@t Tubby Linton: That’s a great article. One of the best I have read from Safety First.

[tubby linton]

I do feel that the data package for the Leap engined A321 is incorrect and Airbus needs to revisit it. I have been in continuous light chop in one at Optimum altitude and the autothrust was extremely sluggish and was doing nothing to try and hold the speed target. I now consistenlty fly lower if there is any cat alomg the route on the sigweather chart.
Pineteam, I am glad that you found it useful, lets stay safe in flight.

[tdracer]

Something that may be coming into play here. I know that on the LEAP for the 737 MAX, they slowed down the engine accel/decel rates at altitude - it allowed for a more aggressive active turbine clearance control (e.g. turbine case cooling - ACC or TCC) without risking turbine rub - but aircraft performance in turbulence naturally suffers. I assume they did pretty much the same thing for the NEO.

[Gear Operator]

Contributing factor in my opinion is that the actual weight of the A/C is higher than assumed (and entered in the FMS). The speeds (Greendot, Vls) calculated by the FAC is in most cases several kts higher than those according the FMS.