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Mark in CA
12th Sep 2019, 19:39
A320neo ans A321neo experiencing CoG issues?

https://simpleflying.com/lufthansa-a320-cog-economy/

Following concerns regarding the center of gravity issues (https://simpleflying.com/airbus-a321neo-pitch-problem/) with the A321neo, the European Union Aviation Safety Agency (EASA) has uncovered a similar problem (https://simpleflying.com/excessive-pitch-problem-on-second-a320-family-aircraft/) with the A320neo

fdr
12th Sep 2019, 23:56
A320neo ans A321neo experiencing CoG issues?


Probably not the case that the world has forgotten how to build. Information flows faster today than previously, and is less fettered or filtered. Issues that arise are now aired in various arenas including online news, chat rooms, and social media.

svhar
13th Sep 2019, 01:10
I don't know, but I flew the 757 since it first came out of the factory. I don't recall any issues with the design.

AerocatS2A
13th Sep 2019, 01:36
It's worth noting that all that happens with the neo is that it behaves like a normal aeroplane under a specific set of circumstances, and all the pilot has to do is fly it like a normal aeroplane. It is vastly different from the MAX issue.

gums
13th Sep 2019, 01:50
Salute!
Prolly a flight control law that needs some work.

As the bus model(s) in question go, they do not have to meet the same control column pressure WRT AoA requirement that the 737Max does (no direct or indirect control surface posiiton/force feedback required for the FBW bus) They also do not have a stick vibrator like the Boeing stick shaker for stall warning, just "bells and whistles".

Besides using direct AoA values for the control laws, we also have gains due to CAS ( Q, or dynamic pressure) and some rate variables. So unless there are serious aero pitch moment problems that the existing control surfaces cannot overcome, this should be a quick fix,and an effective one.

EDIT: The AoA limiter ( they call it "protection") should have some degree of "rate" or pitch onset to prevent overshoots, but we would have to see the curves/plots and time constants for all the variables to gain a good understanding..

This problem appears to be an overshoot one, does it not? So maybe the rate variable could use some attention, as well as the gains in effect got TOGA scenarios.. END EDIT

Gums opines...

Atlas Shrugged
13th Sep 2019, 03:07
it behaves like a normal aeroplane under a specific set of circumstances, and all the pilot has to do is fly it like a normal aeroplane

And therein lies the problem.

As the systems have improved, the point at which they fail keeps getting deeper and deeper into the areas that make the aircraft unnecessarily hard to fly. At some point, as they keep progressing, the 'system' will hand over a larger and larger bag of **** each time.

Airbus has had the effect of weakening pilot skills because they've automated a lot of the day to day stuff...but at the same time, when it all goes tits-up, you'll need those very same weakened skills.

Just a thought.....

robdean
13th Sep 2019, 05:57
The problem is essentially mitigated if the rear row of seats is unoccupied. That’s not a fundamental aeronautical design issue, it is a 'Well, head office insist on another row up back' issue.

Joe le Taxi
13th Sep 2019, 06:29
As the bus model(s) in question go, they do not have to meet the same control column pressure WRT AoA requirement that the 737Max does (no direct or indirect control surface posiiton/force feedback required for the FBW bus) .. It's an interesting point - how come Airbus gets away with this exemption just because the stick is on the side, while Boeing are required to create perfectly progressive stick feedback force, all the way to the extremes of the envelope, and in trying to meet the requirement, create MCAS? Progressive pitch force is a major design headache, from single engine homebuilts, right through to airliners. Meanwhile there have been several Airbus incidents, arguably attributable to the lack of stick force feedback.

Paul Lupp
13th Sep 2019, 07:23
Why are we allowing electronics to make planes safe to fly? Should they not be designed to be "stable" in normal flight conditions without computer intervention, with override only happening when something very unusual happens?

DaveReidUK
13th Sep 2019, 08:04
The problem is essentially mitigated if the rear row of seats is unoccupied. Thatís not a fundamental aeronautical design issue

Sounds like a pretty good description of one to me.

Less Hair
13th Sep 2019, 08:10
That's what happens with the modular approach airplane families are designed today. Stretch them, make them heavier, install more powerful engines and change the cabin interior and onboard installations (tanks) to upset weight and balance - But keep the wings and rudders the size they were before.
No surprise you end up with some surprises at the corners of the flight envelope. On computer controlled aircraft those things can be modified more elegant than on old style (MAX) mechanical controlled airplanes.

NEDude
13th Sep 2019, 08:18
Engineers have not forgotten how to build airplanes. It is just that today, it is the CEOs, accountants, and marketing people who are driving the design process from the top down. In the past it was the engineers saying what could be done and the upper management marketed and sold what the engineers could build.

AerocatS2A
13th Sep 2019, 08:25
And therein lies the problem.

As the systems have improved, the point at which they fail keeps getting deeper and deeper into the areas that make the aircraft unnecessarily hard to fly. At some point, as they keep progressing, the 'system' will hand over a larger and larger bag of **** each time.

Airbus has had the effect of weakening pilot skills because they've automated a lot of the day to day stuff...but at the same time, when it all goes tits-up, you'll need those very same weakened skills.

Just a thought.....

As I understand it, this happens when flying manually so you arenít being ďhandedĒ anything, you already have it, you just need to push the stick forward a bit.

cattletruck
13th Sep 2019, 11:26
Has everyone suddenly forgotten how to build airplanes?

Also applies to many other facets of engineering while they pay corporate flunkies a mega salary to do top level designs that an eight year old could do in five minutes.

Lake1952
13th Sep 2019, 11:37
Although they may look like airplanes, they are really giant computers. That said, it doesn't hurt to all step back and remember that the recent era of the aviation industry is safer now statistically that it ever was. That is not an argument against continuing to improve.

a5in_the_sim
13th Sep 2019, 12:03
As is ever the case we pilots love to debate niff naff and trivia. Airliners may be giant computers but airlines buy them to MAKE MONEY! Airlines are run by accountants whose sole purpose is to MAKE MONEY! Safety is not in the ledger. When have you ever heard of an accountant being held responsible for anything safety related in ANY industry? Industry are under intense pressure to produce these aircraft quickly and cheaply. This isn’t a “rocket science” problem. A few high profile corporate manslaugter convictions when accidents happen will soon focus the mind, before these slippery corporate types slither away to take charge of a nuclear power station project or the next space shuttle.

As a pilot I don’t give a rip how many computers or widgets the manufacturer has incorporated into the design. I don’t care whether it’s a Boeing or an Airbus. I just want the hunk of metal to be a quality product and safe. And I want to be properly instructed to fly it. I’d personally be willing to take a lower salary to be sure of those things. Sadly the thing we senior pilots seem far to preoccupied with is MAKING MONEY!........... and round and round we go!

jthg
13th Sep 2019, 13:01
Engineers have not forgotten how to build airplanes. It is just that today, it is the CEOs, accountants, and marketing people who are driving the design process from the top down. In the past it was the engineers saying what could be done and the upper management marketed and sold what the engineers could build.

See I cant agree with that. Commercial pressure has been there since day one, and sales/marketing have exerted pressure on the designers to build what they perceive the market desires from the get go.

Take the 777, in which the market had a seat at the table. It makes no sense for it to work any other way, it just requires engineers to be clear about what can or cant be done

aterpster
13th Sep 2019, 13:01
I don't know, but I flew the 757 since it first came out of the factory. I don't recall any issues with the design.
Wasn't that circa 1983?

lomapaseo
13th Sep 2019, 13:15
"The problem is essentially mitigated if the rear row of seats is unoccupied. Thatís not a fundamental aeronautical design issue

Sounds like a pretty good description of one to me.

Rarely does the designer specify where each row of seats are placed or baggage stowed. They tend to specify the pressure bulkheads fore and aft and then provide info for an operator to use the plane.

I believe we have talked about not sitting in the last row on our own flights on some planes

DaveReidUK
13th Sep 2019, 14:09
Rarely does the designer specify where each row of seats are placed or baggage stowed. They tend to specify the pressure bulkheads fore and aft and then provide info for an operator to use the plane.

Yes, that's true.

But the designer will have determined the C of G envelope within which the airlines can configure the cabin and operate the aircraft. For a regulator to step in and retrospectively impose a more limited C of G range indicates that something hasn't gone according to plan.

Maninthebar
13th Sep 2019, 14:21
It's an interesting point - how come Airbus gets away with this exemption just because the stick is on the side, while Boeing are required to create perfectly progressive stick feedback force, all the way to the extremes of the envelope, and in trying to meet the requirement, create MCAS? Progressive pitch force is a major design headache, from single engine homebuilts, right through to airliners. Meanwhile there have been several Airbus incidents, arguably attributable to the lack of stick force feedback.

....because Boeing philosophy is to give haptic feedback to the PF, whereas Airbus does not. So if that feedback is expected then it must be given.

EEngr
13th Sep 2019, 14:36
But the designer will have determined the C of G envelope within which the airlines can configure the cabin...

So, was the envelope specified incorrectly? Or did the 'airline' mis-configure the cabin. 'Airline' because (on this side of the pond) it's actually Boeing that builds the cabin per the customer's requests. The original envelope is checked as a part of the type certification using analysis and perhaps a few flight tests with test ballast. If it flies OK, the design is signed off. Then the customer comes along and requests a certain seating and cargo configuration. The design for that particular aircraft will again have to be analyzed and signed off for it's airworthiness certificate to be issued.

So, where did the process break down? A poor envelope specification up front? Or marketing caving in to a customer to cram in just one more row of seats? The article says Lufthansa is affected rather than all customers. So I suspect it's the latter.

DaveReidUK
13th Sep 2019, 15:37
So, where did the process break down? A poor envelope specification up front? Or marketing caving in to a customer to cram in just one more row of seats? The article says Lufthansa is affected rather than all customers. So I suspect it's the latter.

Good questions. The reduced C of G envelope that EASA has imposed obviously applies to all A320neo and A321neo aircraft, but it could well be that only Lufthansa's particular LOPA causes them a problem as a consequence.

nevillestyke
13th Sep 2019, 16:04
All you need is fatter folk in row 0.

FlightDetent
13th Sep 2019, 18:00
When I trim my Airbus in DCT law for clean speed and then bring towards stall using a pitch up command on the stick, there is progressive force that needs to be overcome. No issue.

Next.

Oh, wait. If I have a stroke mid-way and let go off the stick, the nose drops down and does not hang up, or - heaven forbid - keep raising.

For those interested in the particular CG restriction on A320, please see my comment on the A321 thread.

Speaking of which: if the manufacturer discovers on a test rig something about the pitch response is not aligned with the book exactly, WITHOUT any prior in-service event or SIM observation, just by doing their QA homework properly, that is actually how you DO build an aeroplane. Not faultless, but responsible is the key word.

If I wanted to be mean, a little note would slip about how you can fully trim the Bus manually in and out of all settings at all corners of the envelope. But I am a much nicer person than that.

Rick777
13th Sep 2019, 22:21
Expecting a pilot who had 250 hours years before he started flying Airbus airplanes to actually know how to fly a normal airplane is a little much. I flew the A320 and loved it, but I spent years flying most of the Boeings first. There is a huge difference.

AerocatS2A
14th Sep 2019, 02:01
It’s “stick forward to lower the nose” type skills, not something esoteric like landing a tail dragger.

Oriana
14th Sep 2019, 03:40
I cannot understand how being a pilot of an FBW airliner is any lesser than a cables and pulley airliner pilot.

Does anyone criticise F16 or current FBW fighter pilot as being a lesser pilot than an F4 or A4 pilot?

Strange logics (no pun intended).

You are there to fly/operate a flying machine that 'makes money' for 'shareholders'. If you're lucky you have an aeroplane that makes doing that day in, day out easier and 'backs you up'.

If you wanna go 'flying', dislodge some quids out of your wallet and go fly a Maul!

Australopithecus
14th Sep 2019, 03:54
Having flown both, I'd say “not at all”. Each has their challenges. But please do note that somewhere in the first few pages of the Airbus FCTM is the helpful advice that, when in direct law, one simply flies the aircraft like a conventional jet transport.
What do you expect that pro tip might mean to somone who has never done that?

i never really loved the manual flying on FBW, but then again I never HAD to, it was always done for skill retention only.

rog747
14th Sep 2019, 06:45
Is the C of G issue because of too much of a forward or an aft trim, or both scenarios?

If so, this maybe the case of a full Business class load in the front (and maybe cargo), and too few Y pax down the back, or vice versa...
Hardly anyone has checked bags anymore in Business so one cannot offset out of trim situations with the hold baggage.

We had a particular problem with Transavia Holland on a 737-200 used for the LGW-AMS scheduled services, which had a movable business cabin so you never knew much in advance of the variable loads - It was a pig to trim which for a 737-200 was unusual, but caused by a lot of extra metal installed in the front area due to some re-skinning (post Aloha HNL)

DaveReidUK
14th Sep 2019, 07:56
Is the C of G issue because of too much of a forward or an aft trim, or both scenarios?

If the reports are correct, Lufthansa is unable to seat pax in the rearmost row, which would indicate an aft C of G limitation.

Uplinker
14th Sep 2019, 18:59
...............how come Airbus gets away with this exemption just because the stick is on the side, while Boeing are required to create perfectly progressive stick feedback force, all the way to the extremes of the envelope, and in trying to meet the requirement, create MCAS? Progressive pitch force is a major design headache, from single engine homebuilts, right through to airliners..........

It’s not because Airbus has a “stick on the side”, and Airbus are not “getting away with anything” - they went through extensive certification to have the system approved. It is because the Airbus FBW has a well sorted, properly integrated fly by wire flight control system - as do many military jets. The side-stick instructs the five computers which in turn operate the flight controls, (even when flying manually). The Boeing 737 however, only has rods and cables and mechanical linkages from the 1950’s to operate its control surfaces.

An analogy might be modern cars having ABS braking systems. Cars without ABS require the driver to prevent locking the wheels in slippery conditions, ABS performs the anti-locking function ‘in the background’. Cars with ABS are safer in slippery conditions than those without.

(ABS also works on individual locked wheels. The driver of a non ABS car can only release or apply brakes to all the wheels simultaneously).

oliver2002
14th Sep 2019, 19:05
The Lufthansa A320neo is tail heavy because they moved the lavs into the tail using the spaceflex option and did some modifications in the front to squeeze in two extra rows of seats. The tail heavy situation was known and because of it they didn't fit the inflight WiFi equipment either.

krismiler
15th Sep 2019, 22:54
i never really loved the manual flying on FBW, but then again I never HAD to, it was always done for skill retention only.

I never liked direct law in the simulator on the A320, in this case cables, pulleys and a yoke felt better as they gave the sort of feedback you needed to manually control an aircraft without the computers.

Any new type is going to have teething problems, the airlines always finish off the testing program for the manufactures by which stage anything major should have been picked up. It would be uneconomic and probably impossible to have a new aircraft absolutely perfect before the first delivery, some faults will only manifest themselves under very specific conditions or after a certain length of time in service.

b1lanc
16th Sep 2019, 12:30
This according to Aviation Week. Begs the question. If you removed the last row and re-configured the seat map so the ac would have one less row total but the 'new' last row would be in current position of the blocked row, would the CoG issue still remain?

Aihkio
16th Sep 2019, 13:19
[QUOTE]would the CoG issue still remain?/QUOTE]

Maybe, the CG of pax stays at the same point but with a little less weight. So it depends.

DaveReidUK
16th Sep 2019, 14:15
This according to Aviation Week. Begs the question. If you removed the last row and re-configured the seat map so the ac would have one less row total but the 'new' last row would be in current position of the blocked row, would the CoG issue still remain?

Even if it cured the C of G issue, there would be little point.

Either way, you still have 6 sellable seats less than you started with, plus you'd have the cost of re-aligning all the PSUs, etc with the revised configuration.

FlightDetent
16th Sep 2019, 20:00
Even if it cured the C of G issue:ugh:

There is a pitching moment attenuation issue, not a CoG one. To avoid manifestation of the problem a CoG restriction was put in place, until the flight controls are sorted out.

DaveReidUK
16th Sep 2019, 21:10
There is a pitching moment attenuation issue, not a CoG one. To avoid manifestation of the problem a CoG restriction was put in place, until the flight controls are sorted out.

Glad we got that sorted out. :O

Rated De
16th Sep 2019, 21:22
The problem is essentially mitigated if the rear row of seats is unoccupied. Thatís not a fundamental aeronautical design issue, it is a 'Well, head office insist on another row up back' issue.

That sounds exactly like the Commercial Tail wagging the aeronautical dog.

Therefore, if there were no extra seats, the aircraft would have requisite stability?

gums
16th Sep 2019, 23:15
Salute!

Sound like another plane's problem of late?

Gums sends......

FlightDetent
17th Sep 2019, 03:57
Therefore, if there were no extra seats, the aircraft would have requisite stability? DaveReidUK Right, did we?

Aihkio
17th Sep 2019, 05:26
There is a pitching moment attenuation issue, not a CoG one. To avoid manifestation of the problem a CoG restriction was put in place, until the flight controls are sorted out.
These two kind of go together, the more aft CG the more sensitive the aircraft is to pitch input. Finally it becomes unstable, which in itself should not be a real problem to a FBW plane but presently not allowed.

Rated De
17th Sep 2019, 08:55
Right, did we?


Presented as a question not an outcome...

TURIN
17th Sep 2019, 09:36
I don't know, but I flew the 757 since it first came out of the factory. I don't recall any issues with the design.
As an eager apprentice in the early 80s I was involved with a Boeing mod team that made extensive Slat/flap modifications on in-service 757s. These mods were so extensive that a full CAA flight test was required afterwards. I was fortunate enough to be on one of them. Full stalls on one engine out over the North Sea were quite interesting.

I assume this was due to a problem with the initial design.

atakacs
17th Sep 2019, 09:39
I am still a bit baffled by this one. Few questions

the 320 and 321, obviously related, are fairly different airframes. Can they both be impacted by the same issue ?
is this LH specific (due to their cabin seating) ? If not all operators ? oops...
is this just a matter of "tuning" the FBW software or something more fundamental ?
am I correct im my understanding that this was "discovered" in simulation ?

AerocatS2A
17th Sep 2019, 10:34
CoG limit applies to all operators, however not all operators find the limit to be limiting, if you know what I mean. I know of one operator that checked their historical load data and found they’d never loaded aft of the new limit, not that they’ve had the neo for long.

The 320 issue is not the same as the 321. The 321 will be fixed with ELAC L103. The 320 problem was discovered by computer simulation and requires a very specific set of circumstances. The 321 problem was discovered in flight testing.

FlightDetent
17th Sep 2019, 11:34
atakacs

For the A320 specifically

Reference Airbus Operational Telex 999.0059/19

- In case of unfavourable aft CG position behind the newly outlined limits
- when not in clean configuration
- while the aircraft is decelerating i.e. from a certain max flaps speed down to minimum reasonable speed ("Vls") -10 kts
- then, if in close sequence TOGA thrust is applied and full back stick is pulled

A computer simulation executed as a part of further FCS development efforts revealed that an excessive pitch attitude might result. It was never encountered in service or during a flight test.

-----

The pitch can still be controlled with normal sidestick input, just by releasing the full aft grip.

Yet that excessive pitch is not supposed to happen under normal control of the FCS system. There should be a "hard stop" where the elevators are manipulated (behind pilot's back) in such a manner so that the nose would stop raising - even with full back stick being pulled.

A speculation on my part is that the "over-ruling" of the pilots full up input and the pitch-up moment of the underslung thrust is too soft in the "wobbly CoG area" (see Aihkio above). Thus in the future versions the FCS will be allowed to flex more muscle.

It is not a stability nor controllability issue of the airframe aerodynamical configuration. E.g. with abnormal hydraulic the A/C can still be flown properly through the G/A manoeuvre with only one elevator or frozen horizontal stabiliser - in the full original CoG range.
https://cimg7.ibsrv.net/gimg/pprune.org-vbulletin/1050x1123/content_0018_720b0d1fa888f3c9cfc07a61ca1d9672b4aa0554.jpg

That's the cart and horse.

----

If the airline's specific configuration is very tail heavy when empty, loading upfront might be a challenge. That being said, in the common tkof range 60t+ I really do not remember seeing TO CG above 35 all that often. Allegedly LH needs to take action.

What can you do in case you had to?
- Load bags to front, or even carry ballast there
- Play with the pottable water tank load (tiny little)
- Put your half-load pantry into the front galley
- If fuel is less than 8t (around-ish), transfer the outer wing tanks into inners
- Carry ballast fuel in the centre tank (the issue looks more critical at low weights and this would work nicely)

But none of those are, ehm, "operation personnel proof". The solution no to sell the last few seats is rather charming if it does the trick,
- the seats are easily blocked in the DCS systems
- you can put pressure on the OEM due to lost revenue
- with a good pricing system, you never really sell full 100% seats (though I believe LH to be super-efficient)

Aihkio
17th Sep 2019, 11:43
Thx, makes a lot of sense. In any control system when the first time derivative is not properly taken into account one easily has overshoots.

FlightDetent
17th Sep 2019, 12:34
The red/green lines are the new limits, cca 4 points of % more forward than the original. A common operational rule is that additional 2 points % padding is applied due to uncertainties between reality and the paperwork.

AerocatS2A
17th Sep 2019, 18:56
Flight Detent, those details are for the A320. The A321 issue is different.

Mark in CA
18th Sep 2019, 13:11
And therein lies the problem.

As the systems have improved, the point at which they fail keeps getting deeper and deeper into the areas that make the aircraft unnecessarily hard to fly. At some point, as they keep progressing, the 'system' will hand over a larger and larger bag of **** each time.

Airbus has had the effect of weakening pilot skills because they've automated a lot of the day to day stuff...but at the same time, when it all goes tits-up, you'll need those very same weakened skills.

Just a thought.....

Yes, and this thought is now the central theme of the article I just posted from the NY Times Magazine focusing on the Lion Air and Ethiopia 737 Max crashes.