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A321neo Pitch Warning

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A321neo Pitch Warning

Old 21st Jul 2019, 10:07
  #21 (permalink)  
 
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Heavier, stronger engines, higher MTOW?
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Old 21st Jul 2019, 14:30
  #22 (permalink)  
 
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Originally Posted by Less Hair View Post
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)
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Old 21st Jul 2019, 22:12
  #23 (permalink)  
 
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Originally Posted by Station Zero View Post
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.

I’ve 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 doesn’t 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'.
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Old 21st Jul 2019, 23:48
  #24 (permalink)  
 
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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:
  1. 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.
  2. The earlier, ELAC101 apparently deals with the pitch problem per Station Zero's Post #11.
  3. 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).
  4. 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
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Old 22nd Jul 2019, 04:39
  #25 (permalink)  
 
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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.
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Old 4th Aug 2019, 16:09
  #26 (permalink)  
 
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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 °nose down over a period of 8 s. Consequently, to flare the aircraft, a gentle nose-up action by the pilot is required.
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Old 4th Aug 2019, 16:14
  #27 (permalink)  

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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_1.

Last edited by FlightDetent; 4th Aug 2019 at 18:26.
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Old 4th Aug 2019, 16:17
  #28 (permalink)  
 
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Originally Posted by I-NNAV View Post
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 °nose 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".
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Old 4th Aug 2019, 20:49
  #29 (permalink)  
 
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Rotation Mode

Originally Posted by S speed View Post
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....
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Old 4th Aug 2019, 20:54
  #30 (permalink)  
 
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Thumbs up 737 Pitch

Originally Posted by pattern_is_full View Post
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....
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Old 5th Aug 2019, 12:49
  #31 (permalink)  
 
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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.
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Old 7th Aug 2019, 21:13
  #32 (permalink)  

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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.

Last edited by FlightDetent; 17th Sep 2019 at 11:02.
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Old 9th Aug 2019, 11:55
  #33 (permalink)  
 
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Originally Posted by neilki View Post
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.

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.
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