Rejection of AOA Sensors by ELAC on A320
 

Hey guys,

does anybody know at how many degrees of difference one AOA Sensor is rejected/"voted out" by the ELACs? I had a rather big discrepancy between the FMGC weight and the FAC calculated weight recently (which also triggered the "Check GW" message) and resulted in a difference of around 10kts between the characteristic speeds on the PFD and the FMGC (Green dot etc). The load sheet was definitely correct and I have heard from other crews have experienced similar issues on this specific aircraft in the past, so I am suspecting some kind of AOA sensor (or possibly ADR/FAC) problem. The strange thing is that both PFD seemed to be affected in the same way. According to the Alpha Callup there was a difference of around 0.6 degrees between the AOA provided by FAC1 and the one provided by FAC2 (the difference in gross weight between FMGC1 and FAC1 was in excess of 7 tons, but this only became apparent after around 2 hours of cruise flight, the difference between FMGC2 und FAC2 was just under 7 tons, so the "Check GW" message was only triggered on the left MCDU). Does anybody have a plausible explanation for this behavior (except load sheet error which I would exclude as it apparently happened on different flights to other crews as well - we also double checked the load sheet after being the "Check GW" message to exclude a load sheet error)? It seems to be hardly conceivable that independent sensors fail in a very similar way over several flights (i.e. excluding issues like freezing water within probes etc.). Is there any way to access the AOA value from the STBY AOA trough the Alpha Callout menu?

If it happened after quite some time in flight is there any chance it was a fuel amount difference between the FMGCs/FAC? I think both FMGCs take input from all three ADIRS, so all AOAs are used, and I assume you would get AOA disagree before you get Check GW if it was due to AOA difference.
Also where do you find the FAC GW, just curious to see for myself.

"GWFK" alpha parameter call-up inside AIDS on some machines.

In the wake of MCAS I tried to look for AoA values there as well, there was a difference without any apparent adverse effects. I'd say 0,4 deg but do not take my word for it without checking.

AIDS Alpha Call-Up "AOA".

Thanks..............................

Just to throw in another option:

How much of those 7 tons can be accounted for by an inaccurate fuel measurement?
I'm aware there is some automatic cross checking between the FOB and fuel used by the engines.
But as level sensors can't be super accurate i imagine especially when the tanks are half full there is a certain error that is normal.
Combined with a load sheet that is a bit off due passengers being fat.
And maybe this airframes empty weight is also off by a bit.

(All those errors adding up to 7 tons seems a bit excessive though)

Was the FAC weight more or less than the FMGC weight?

Either way there should probably be maintenance action to weigh the aircraft and check the AoA sensors.
That should give a definitive answer, should it not?
(Excluding a transient issue which seems unlikely if both sensor roughly agree)

I have a story of FMS GW being wrong because of FMS fuel value, all other indications being what they should be. Seeing FOB remaining 2,3 on one side, and 1,4 in the opposite box makes you think of this akward thing quite intensively. Does not match the description above though.

Another obscure issue are wrong SAT readings. That releases great many goblins out of the FMS caverns! ;)

The FAC weight was more than the FMGC weight, the FAC Weight was around 70 tons, the FMGC around 63 tons (Greendot on PFD was around 10kts higher than the one calculated by the FMGC). I was thinking also about the fuel thing, but 7 tons of difference due to inaccuracies in the fuel calculations seems excessive on an A320. On the flight we used around 11,5t of fuel, so 7 tons is around 60% of the fuel used on that flight. Today, on the same aircraft we observed a similar behavior but as the difference stayed slightly below the threshold of 7 tons we didn't get the "Check GW" message today. During todays flight we could not engage AP2 during the whole flight (surprisingly, it could be engaged again after landing) and got a "F/CTL Sidestick priority" ECAM after parking, but I am not sure if this is in any way related to the discrepancy in GW at all or not.

At those weight differences you should almost be able to tell which figure is correct by comparing fuel flow with a similar A320 at the same weight :)

I'm curious how is the "Check GW" message handled maintenance wise, especially if it occurs more than once?
Seems like something that needs fixing.

The FAC weight being higher isn't the critical case, if the FAC thinks the aircraft is heavier than it actually is the GD speed will just be faster.
So at least stall margin shouldn't be a problem.

(Making it less of a maintenance concern than the other way around)

Definitely sounds like an aircraft I would like to avoid.......

As to fuel accuracy issues, this incident is probably relevant.