A320 Seat Assignment on A321 Causes Late Rotation

[Evan]

This flight was assigned to an A320 but an A321 was substituted. The seat assignment was not replanned however, leaving the back of the cabin empty and the CG setting was not updated. After the plane failed to respond to pitch at Vr, and continued to not respond following full aft stick, the captain (PM) added TOGA thrust and the plane rotated and lifted off. On shorter runways, this could end badly.

After landing uneventfully, it was determined the the CG during the approach was out of acceptable range. Fortunately there was no go-around.

http://avherald.com/h?article=4dd9ae59&opt=0

[3WE]

What’s it doing now?

Well for ALL OF AUTOMATION, what it is NOT doing is checking W&B which SEEMS like an ok
spot for a few sensors and computer code...

[Evan]

What’s it doing now?

Well for ALL OF AUTOMATION, what it is NOT doing is checking W&B which SEEMS like an ok
spot for a few sensors and computer code...

Not sure how a few sensors could check balance... I suppose it could compare NLG to MLG compression... something like that..

[Gabriel]

After landing uneventfully, it was determined the the CG during the approach was out of acceptable range. Fortunately there was no go-around.

I don't know where you got that CG was out of limits "during the approach".

It was out of limits throughout all the flight. Way out. Off the charts (literally).

[Gabriel]

Not sure how a few sensors could check balance... I suppose it could compare NLG to MLG compression... something like that..

You can have strain gauges in each leg effectively turning each leg into a scale from which you can get both total weight and CG in 2D.
It has been done, some models offered it as an option, which ended up being expensive and would break often so it ended up MEL'ed or not used most of the time which made the expense not worthy.

Instad of improving the system the industry took the lazy way of eliminating it.

[3WE]

...It was out of limits throughout all the flight. Way out. Off the charts (literally)....

Fortunately it was in the stabler relentless push over direction and lift is proportional to the square of speed.

[3WE]

I suppose it could compare NLG to MLG compression... something like that..

Yup, I suppose that might work...I’d use some sort of load cell, but...

[Evan]

I don't know where you got that CG was out of limits "during the approach".

The article stated that it was later noticed with regard to the approach due to the reduced fuel weight. Interestingly, it did not affect the controllability of the flight.

[3WE]

Interestingly, it did not affect the controllability of the flight.

Huh?

1. it did.

2. AND some fundamental insight is above in several locations.

[Gabriel]

The article stated that it was later noticed with regard to the approach due to the reduced fuel weight. Interestingly, it did not affect the controllability of the flight.

Two things:
1- I didn't find the part of the article where they said that it was later noticed with regard to the approach due to the reduced fuel weight (whatever that means).
2- The reason why it didn't affect the controllability of the flight during approach (or any point after take-off) is because the auto-trim compensated the FWD CG condition. That's why the crew thought that the problem with the W&B when they changed planes had led to an incorrect take-of trim setting, not to an off-limit CG.

[ATLcrew]

Not sure how a few sensors could check balance... I suppose it could compare NLG to MLG compression... something like that..

Ah, but they can! Research F/CTL PITCH TRIM/MCDU/CG DISAGREE. Also, research the "CHECK WEIGHT" MCDU scratchpad message.

The FACs are actually pretty smart.

[Schwartz]

A camera with simple image recognition could do it too. Maybe even running on an ipad.

[Evan]

Ah, but they can! Research F/CTL PITCH TRIM/MCDU/CG DISAGREE. Also, research the "CHECK WEIGHT" MCDU scratchpad message.

The FACs are actually pretty smart.

OK, I've been a bit lazy on research lately. I assume you are referring to the Take-Off Securing function. But this still depends on the correct ZFWCG entry from the loadsheet. If that number is incorrect and the THS is set on the assumption that it is correct, there shouldn't be a CG DISAGREE, right? That function is there to catch an incorrect THS setting, not a CG entry error. The FAC's are remarkably smart but they have no way to know the CG on the ground aside from what is entered on the INIT page.

I could see how the FAC's could possibly flag a discrepency between trim and performance in flight to determine a CG issue, but not on the ground.

I didn't find anything on a CHECK WEIGHT message. There is a CHECK GW message but it has nothing specific to CG.

Can you elaborate on that?

[ATLcrew]

OK, I've been a bit lazy on research lately. I assume you are referring to the Take-Off Securing function. But this still depends on the correct ZFWCG entry from the loadsheet. If that number is incorrect and the THS is set on the assumption that it is correct, there shouldn't be a CG DISAGREE, right? That function is there to catch an incorrect THS setting, not a CG entry error. The FAC's are remarkably smart but they have no way to know the CG on the ground aside from what is entered on the INIT page.

Apparently they do have a way (depending on the mod status), because my manual says the message will be triggered if there is a discrepancy between

- Actual pitch trim from THSA
- The pitch trim value entered by the crew in the MCDU.
- The pitch trim value calculated by the FAC based on the CG (emphasis mine). I don't yet know exactly HOW the FACs calculate CG (but I do know who to ask) other than to say that, according to my diagram, the FQIC is involved somehow.

Stand by for further.

[Gabriel]

Could it be that it is the "pitch trim value calculated by the FAC based on the CG entered by the crew"?

(ok, I'll wait)