PDA

View Full Version : A321neo Pitch Warning


Longtimer
17th Jul 2019, 14:43
Original design modified and then.....A321neo operators alerted over 'excessive pitch' anomaly

17 July, 2019
SOURCE: Flight Dashboard
BY: David Kaminski-Morrow
London

Airbus has introduced a temporary revision to A321neo flight manuals intended to prevent the possibility of the aircraft reaching excessive pitch attitudes.
The European Union Aviation Safety Agency has disclosed that the revision follows analysis of the re-engined type's elevator and aileron computer.


It has not elaborated on the situation beyond stating that "excessive" pitch could occur under certain conditions and "during specific manoeuvres".

EASA cautions that this could result in "reduced control" of the aircraft.

Airbus has issued temporary revisions to the aircraft's flight manual incorporating operational limitations.

EASA has ordered operators of the A321neo to amend the flight manuals accordingly, within 30 days.

The order covers both the CFM International Leap-1A and Pratt & Whitney PW1100G versions of the aircraft.

Airbus has issued seven relevant temporary revisions to the flight manual, on 21 June and 10 July. EASA says the safety measure is an interim action and could be followed by further requirements.

TechnicalPilot
18th Jul 2019, 13:16
Would someone be able to advise what are the specific limitations in the AFM regarding this AD please.

Onions
18th Jul 2019, 15:14
Does anyone have a definition of reduced control?

Parahelio
18th Jul 2019, 16:42
Does anyone have a definition of reduced control?
Its just something that happens
"during specific manoeuvres".

halfofrho
19th Jul 2019, 11:00
Would someone be able to advise what are the specific limitations in the AFM regarding this AD please.

ZFW / TO / LDG CG needs to be less than 34%.

Excessive pitch attitude can occur with:

CG aft of 34% and
Flight controls "Flare Mode" activated and
Flaps Full and
Large pitch up sidestick inputs (during a GA for example)

gearlever
19th Jul 2019, 13:55
Does anyone have a definition of reduced control?

MCAS ...?;)

sheppey
19th Jul 2019, 14:10
Talked to a former Boeing 737-200 pilot who flew for Silk Air. He was cruising in IMC one night in that part of the world and due to a mix-up in radio frequencies was unaware he was trailing a Boeing 747 freighter about ten miles ahead. Suddenly his aircraft was affected initially by mild turbulence followed by an extremely sharp wing drop to 90 degrees angle of bank. The AP disconnected at the same time. He was able to unload and rapidly regain control and roll wings level. Discussed this incident with a current 737-800 pilot who had a similar experience when being vectored behind an A330 ahead. On this occasion the 737 rolled rapidly to more than 60 degrees angle of bank and the AP disengaged. Weather was VMC and the pilot rolled back to wings level.
When practicing unusual attitude recoveries in the simulator some instructors restrict UA manoeuvers to be within the legal definition of an unusual attitude which among other parameters includes bank angle more than 45 degrees. The perceived reason being fidelity is not assured beyond those figures.

If real aircraft affected by wake turbulence behind a heavy aircraft have finished up at extreme bank angles well beyond official UA parameters, it strikes me as the ostrich head in the sand approach to pretend it will never happen and thus let the unfortunate crew and passengers wear it on the day. Prompt and ideally flawless recovery on instruments is the mark of the true professional pilot. There is no shortage of examples where unusual attitudes have resulted from a wake turbulence encounter. Yet because simulator fidelity may not be guaranteed, we choose not to equip the crew with the instrument flying skills to recover from an extreme wake turbulence encounter. I think that is wrong in principle. No doubt there are other points of view.

Question: With Airbus aircraft, does the inbuilt protection prevent instant roll beyond a specified figure regardless if the aircraft is being manually flown or on autopilot at time of jet upset due wake turbulence?

AerocatS2A
20th Jul 2019, 02:44
Question: With Airbus aircraft, does the inbuilt protection prevent instant roll beyond a specified figure regardless if the aircraft is being manually flown or on autopilot at time of jet upset due wake turbulence?



No. If something induces a roll beyond the authority of the ailerons then no protection will prevent it, how could it? Airbus will go to abnormal attitude law if necessary, this is similar to alternate law.

physicus
20th Jul 2019, 02:57
given sufficient aileron authority, the abnormal attitude will not be reached in an Airbus FBW system, as the envelope protection is trying to level the aircraft while the wing is being dropped. In a Boeing, the AP will disconnect as soon as the AP detects insufficient authority, similar to giving the control column a good yank with AP engaged. It'll disengage rapidly. A wing drop due to wake turbulence in an Airbus thus will be slowed down compared to a Boeing. Difference in philosophy.

capngrog
20th Jul 2019, 03:03
From the source cited by Longtimer: "It (Airbus) has not elaborated on the situation beyond stating that "excessive" pitch could occur under certain conditions and "during specific manoeuvres". Isn't that sort of pitch up problem why MCAS was developed by Boeing? What's going on here? Does the A321neo also have a problem with pitch instability?

This is the first I've heard of a possible pitch stability problem with the A-321neo. Let's hope that no lives are lost before the problem can be rectified.

Cheers,
Grog

Station Zero
20th Jul 2019, 06:14
Trying to compare flight control systems between certain aircraft types is very flawed as some are trying to do.

Anyway, this AD affects L102 STD ELAC software only. Spoke to a Tech Services colleague and if you want to recover the full operating envelope ie not have to apply the AFM TR requirements then you can request approval for the L101 STD software to be reloaded (de-modding the aircraft), not a big deal on the software loadable ELACs which would likely be all that are installed on A321neo aircraft.

Longtimer
20th Jul 2019, 18:17
Here is the goto: https://www.flightglobal.com/news/articles/a321neo-operators-alerted-over-excessive-pitch-ano-459718/

capngrog
20th Jul 2019, 19:28
Here is the goto: https://www.flightglobal.com/news/articles/a321neo-operators-alerted-over-excessive-pitch-ano-459718/

Thanks for posting that, but your "goto" doesn't tell us anything new; as a matter of fact, it is word-for-word the same as the article posted by the Original Poster, Longtimer.

"halfofrho" elaborates somewhat in his Post #5; however, some of the acronyms he's cited, such as "ZFW/TO/LDG" are not meaningful to me. To my old, "back in the day" understanding, "TO" was short for TakeOff, and LDG was short for LanDinG. I'm embarrassed to admit that I have never encountered the acronym, "ZFW", and the only meaning I've found for it on the ol' interweb was "Zero Fuel Weight". I'm sure that's incorrect, since one would not sustain powered flight for very long at "Zero Fuel Weight". I'd appreciate some help on the meaning of these acronyms since I need all the help I can get ... in most circumstances.

In his Post #11, Station Zero seems to imply that the alleged (by EASA) pitch up problems of the A321neo are due to a software glitch, not to an aerodynamic problem. Has that been confirmed by EASA/Airbus?

Cheers,
Grog

gearlever
20th Jul 2019, 19:35
I understand halfofrho's post that way the CG has to be below 34% for Zero Fuel Weight, takeoff and landing.

AerocatS2A
20th Jul 2019, 19:38
ZFW does mean zero fuel weight. Although it is not normally a weight you’d be flying at it does still need to have a CofG within certain limits.

halfofrho
20th Jul 2019, 19:42
I'd appreciate some help on the meaning of these acronyms since I need all the help I can get ... in most circumstances.

You got them all right!

Preemo
20th Jul 2019, 22:02
Airbus response to Leeham enquiry:
https://leehamnews.com/2019/07/19/bjorns-corner-airbus-a321neo-has-pitch-up-issue/

"The issue is an A321neo landing configuration at extreme aft CG conditions and below 100ft only issue, discovered by Airbus and reported to AESA. Violent maneuvers in for instance a go-around in these conditions can cause a pitch up which the pilots can counteract using their side-sticks. No FBW nose downs or similar is commanded, itís just the FBW doesnít neutralize the pitch-up (like FBW using the Airbus style flight laws are supposed to do), the pilots have to do it. Airbus has assisted AESA in issuing the AD which restricts the aft CG used in operational landings until the ELAC software is updated."

Longtimer
20th Jul 2019, 23:05
the goto was posted so all could have access to the original plus any updates.....

Station Zero
21st Jul 2019, 00:24
This is most certainly software rather than Aerodynamics, read the second page of the AD. Group 1 aircraft with L102 ELACs fitted the AFM TR has to be placed in the manual with 30 days. Group 2 aircraft without L102 ELACs fitted the AFM limitation is only applicable once the L102 is installed. This alone means it affects just one ELAC standard.

Iíve not seen the TR but if is below 100ft it will be something to do with the landing aspect of the flight control software. The A321neo from my understanding doesnít have flare law compared to a CEO, its been replaced with something more akin to direct law where the sidestick input is proportional to output during that flight phase.

S speed
21st Jul 2019, 05:27
I wonder what the reason for the change in the flare mode/law and the takeoff law was on the NEO series? For over 30 years it has been working perfectly fine.

If it ain't broke, don't fix it...

Less Hair
21st Jul 2019, 10:07
Heavier, stronger engines, higher MTOW?

S speed
21st Jul 2019, 14:30
Heavier, stronger engines, higher MTOW?

True, but surely they could have tweaked the software to make it feel the same as the CEO planes? The only reason I can think of is that they did it to match the feel of the new Airbus models. (I have only flown the 320 series)

b1lanc
21st Jul 2019, 22:12
This is most certainly software rather than Aerodynamics, read the second page of the AD. Group 1 aircraft with L102 ELACs fitted the AFM TR has to be placed in the manual with 30 days. Group 2 aircraft without L102 ELACs fitted the AFM limitation is only applicable once the L102 is installed. This alone means it affects just one ELAC standard.

Iíve not seen the TR but if is below 100ft it will be something to do with the landing aspect of the flight control software. The A321neo from my understanding doesnít have flare law compared to a CEO, its been replaced with something more akin to direct law where the sidestick input is proportional to output during that flight phase.

Interesting - kind of reverse of what Boeing is going through. If I read your post correctly, the potential exists for taking automation out at a critical phase of flight and putting the crew into 'more akin to direct law'.

capngrog
21st Jul 2019, 23:48
I've read an re-read the posts on this thread as well as EASA AD No. 2019-0171 and now believe I have a basic understanding of the situation. My understanding (such as it is) is thus:

The A321neo has bit of a pitch instability problem in certain narrow portions of its flight operating envelope. This problem could possibly be due to the larger nacelles of the new engines shifting the center of lift forward at high angles of attack.
The earlier, ELAC101 apparently deals with the pitch problem per Station Zero's Post #11.
The ELAC102 does not automatically handle the potential pitch up problem as is the case with other Airbus control laws; however, the sidestick can be moved manually to counter the pitch up problem (Preemo's Post #17).
Since modification of the software to more closely follow existing Airbus control laws will cure the problem, it is as Station Zero stated in his Post #19, ultimately a software problem.

I can't help but compare the A321neo situation to that of the B737MAX. The initial pitch instability problem was similar in both aircraft, but the two manufacturers, A and B, dealt with the problem differently. Due to Airbus' extensive experience with FBW and it's related automation, their system was far superior to the Boeing MCAS, which relied on a single sensor and could not be easily/intuitively overridden by the flight crew. Well, anyway, that's my take on the situation. Am I close?

Cheers,
Grog

pattern_is_full
22nd Jul 2019, 04:39
Possibly repeating capngrog and b1lanc:

1) as mentioned, the A321neo apparently has a balance problem similar to what the 73M8 would have had with no MCAS - a tendency to overpitch-up in some rare scenarios.

2) the A321neo is tail-heavier than the A321, due to mass added behind the center of lift and the previous CG - larger sharklets, reinforced (heavier) main gear and wing, and in some configurations, additional (2-4) seat-rows/pax-weight extending further back (aft galley and toilets reduced in size/number), and some doors moved further aft (I make the assumption that doors with hinges and seals and frames and locking mx are more massive than standard fuselage frames).

3) Airbus did flight-testing and tweaked many things (including the control software) because of the changes - but apparently they have now discovered something overlooked. A patch is on the way, but until then, they are advising that steps be taken to avoid pushing the CG even further aft. And be careful with pitch-up control inputs.

4) Part of that is the "mystery phrase" which (to me) simply means - "Avoid a CG of 34% MAC or more when: calculating ZFW, or for TO and LDG." If the calculator spits out "CG >34% MAC" - move some weight forward.

I-NNAV
4th Aug 2019, 16:09
From the FCOM
FLARE MODE A321 NEO
When passing 100 ft RA, the THS is frozen and the normal flight mode changes to flare mode as the aircraft descends to land. The flare mode is essentially a direct stick-to-elevator law without auto trim, with some damping provided by load factor and pitch rate feedback. The flare law provides full elevator authority.

The flare law has no compensation of the ground effect nor the thrust effect.


FLARE MODE All the others
When the aircraft passes 50 ft RA, the THS is frozen and the normal flight mode changes to flare mode as the aircraft descends to land. Flare mode is essentially a direct stick-to-elevator relationship (with some damping provided by the load factor and the pitch rate feedbacks).

The system memorizes the aircraft's attitude at 50 ft, and it becomes the initial reference for pitch attitude control.

As the aircraft descends through 30 ft, the system begins to reduce the pitch attitude to -2 įnose down over a period of 8 s. Consequently, to flare the aircraft, a gentle nose-up action by the pilot is required.

FlightDetent
4th Aug 2019, 16:14
pattern_is_full I have read the Flight Operations Telex (FOT FOT-999.0059/19) sent out by Airbus which prescribes the CG restriction to affected airlines. Their explanation is clear, n.b. the 1) 2) and 3) above are nowhere near the real thing.

Admittedly for the A320, this is the relevant AD https://ad.easa.europa.eu/blob/EASA_AD_2019_0189.pdf/AD_2019-0189_1.

sonicbum
4th Aug 2019, 16:17
From the FCOM
FLARE MODE A321 NEO
When passing 100 ft RA, the THS is frozen and the normal flight mode changes to flare mode as the aircraft descends to land. The flare mode is essentially a direct stick-to-elevator law without auto trim, with some damping provided by load factor and pitch rate feedback. The flare law provides full elevator authority.

The flare law has no compensation of the ground effect nor the thrust effect.


FLARE MODE All the others
When the aircraft passes 50 ft RA, the THS is frozen and the normal flight mode changes to flare mode as the aircraft descends to land. Flare mode is essentially a direct stick-to-elevator relationship (with some damping provided by the load factor and the pitch rate feedbacks).

The system memorizes the aircraft's attitude at 50 ft, and it becomes the initial reference for pitch attitude control.

As the aircraft descends through 30 ft, the system begins to reduce the pitch attitude to -2 įnose down over a period of 8 s. Consequently, to flare the aircraft, a gentle nose-up action by the pilot is required.


Interesting, the NEO flare mode looks like the A330, but it is missing the part where it mentions "at 50 ft a slight pitch down order is applied".

neilki
4th Aug 2019, 20:49
I wonder what the reason for the change in the flare mode/law and the takeoff law was on the NEO series? For over 30 years it has been working perfectly fine.

If it ain't broke, don't fix it...
There was an (overridable) protection added to the neos to reduce the likelihood of tail strike on Rotation. Theres a thread about it in tech log..
I'd suggest that while not 'broken', there was room for improvement.... Flare and Rotation incidents; especially on the 321 we not unheard of, and part of every PF before takeoff brief....

neilki
4th Aug 2019, 20:54
Possibly repeating capngrog and b1lanc:

1) as mentioned, the A321neo apparently has a balance problem similar to what the 73M8 would have had with no MCAS - a tendency to overpitch-up in some rare scenarios.

2) the A321neo is tail-heavier than the A321, due to mass added behind the center of lift and the previous CG - larger sharklets, reinforced (heavier) main gear and wing, and in some configurations, additional (2-4) seat-rows/pax-weight extending further back (aft galley and toilets reduced in size/number), and some doors moved further aft (I make the assumption that doors with hinges and seals and frames and locking mx are more massive than standard fuselage frames).

3) Airbus did flight-testing and tweaked many things (including the control software) because of the changes - but apparently they have now discovered something overlooked. A patch is on the way, but until then, they are advising that steps be taken to avoid pushing the CG even further aft. And be careful with pitch-up control inputs.

4) Part of that is the "mystery phrase" which (to me) simply means - "Avoid a CG of 34% MAC or more when: calculating ZFW, or for TO and LDG." If the calculator spits out "CG >34% MAC" - move some weight forward.
Homesick Angel mode wasn't limited to the 737 Max. Didn't Thompson get one down to 20 or 30 KIAS on qa go around per the AAIB? The 737 and A321 are very different beasts. I dont think ELAC software hides any unpleasant aerodynamic characterisitcs. I'm sure all the Airbus drivers here have flown in direct law, and after 30 seconds to get your head around her, Fifi flys just fine, even on just the Blue Hydraulics....

NWA SLF
5th Aug 2019, 12:49
In simplified terms, Airbus has been analyzing the A320 NEO and discovered under analysis and testing of the NEO flight control laws there are conditions a pilot can experience increased pitch. Temporary revisions to the A320new flight manual have been made to address the situation. Changes in the CG envelope and loading recommendations. Both the Leap-1A and P&W versions are affeted. EASA has opted for a quick implementation of an AD requireing the changes within 30 days of 14 August. They follow the problem found on the A321neo elevator and aileron computer which affected AOA protection. Airbus is working on a software fix for the A321new to become available next year.

Like the MAX but no fatal crashes yet.

FlightDetent
7th Aug 2019, 21:13
a) it was discovered on a software test rig, no in-service events or complaints from SIM rides
b) pre-condition is a significant deceleration, e.g. from 160 to Vls -10 kt (which is down to Vapp -15 kt on a normal day)
c) if at that point TOGA thrust would be applied
.
.
d) the aircraft might pitch up and follow the thrust moment.

Which is not supposed to happen in the normal control law, where uncommanded attitude changes should be attenuated and contained. The A/C will still, they insist, normally respond to a nose-down command on the stick to overcome the pitching tendency.

In a sense, the AD is issued because it might behave like a conventional aircraft - and that is not supposed to happen.

Airbanda
9th Aug 2019, 11:55
Homesick Angel mode wasn't limited to the 737 Max. Didn't Thompson get one down to 20 or 30 KIAS on qa go around per the AAIB?

G-THOF, a 737-300 on approach to Bournemouth. AAIB report 3-2009 (https://assets.publishing.service.gov.uk/media/5422eab5e5274a131700001f/3-2009_G-THOF.pdf).

Autothrottle disengaged but pilots didn't immediately notice - event masked by speeds being as expected in an approach with engines at idle. Speeds dropped below target on application of more flap. Captain initiated a go around but power/pitch couple gave a nose up pitch that exceeded elevator authority in a machine trimmed AND. Power/pitch couple is, AIUI, present in all aircraft - see DC10 at Sioux City. Difference with MAX8 is that nacelle aerodynamics can add nose up pitch over/above that associated with addition of thrust.