A320 Altimeter baro difference between left and right

[WaterMeths]

Hi.

I was leafing through the FCOM looking for the threshold where the A320 (and no doubt all the airbus series) will ping an ECAM message when the left and right altimeters are set at different baro settings - i found reference to the 250 foot difference with the SAME setting on both sides, but can't find the bit I want.

Can anybody assist?

Merci

WM

[fredgrav]

Have a look both on 1.34.25 p.5 and FCOM 3.04.34 p6, there should be enough lights on the matter ...

Best,
fredgrav

[WaterMeths]

Ta mate. Not quite what I was after though. FCOM 3 ref is to altimeter tolerances - and I can't see a FCOM 1.34.25 ? My copy goes from 34.20 straight to 34.30.

My query is more to do with the incorrect setting of the altimeter on one side or the other - QNH - and what threshold does this invoke an ECAM message.

When I am next on the aircraft I can have a play - but that isn't until Sunday and I am very impatient ;)

[fredgrav]

Hi WaterMeths, :)
there's an Airbus Briefing Note about the Altimeter Setting:
http://www.airbus.com/fileadmin/media_gallery/files/safety_library_items/AirbusSafetyLib_-FLT_OPS-SUPP_TECH-SEQ01.pdf

Though it doesn't provide any info about the baro-setting deviation that generates the ECAM caution, and neither does the FCOM. At this point, I'm quite keen to believe the caution comes with some new SDAC's standards, maybe listed in newest FCOM's bulletins. But it's just a guess ... :bored:

[Gutter Airways]

FCOM 1.31.40 P27

Indicating and Recording systems - Indications on PFD:


11) CHECK ALT flag (amber)

The CHECK ALT flag appears, as does an ECAM caution, if the disagree between the two PFD altitudes is greater than 250 feet when QNH is selected, or 500 feet when STD is selected.
The caution and flag disappear, when the Pilot's and Co-Pilot's barometer references disagree.


From this I understand that initially the caution and flag both appear but they disappear soon after if the cause of the discrepancy is incorrect altimeter setting.

[fredgrav]

God job Gutter Airways, luckily any new SDAC's standard ! :O

[WaterMeths]

Cheers Guys.......thanks for your inputs.

Gutter Airways - yes I did read that part of the FCOM - my understanding of that was that an ECAM will be generated if the altimeters disagree with the same pressure setting. Once the settings are made different - that particular ECAM goes away (because now the a/c logic expects there to be a difference) - to be replaced by another ECAM - which this time states that one of the guys in the flight-deck is a right "duffer" 'cos he has his QNH setting wrong.

That is the tolerence I am trying to find........and it would appear more and more likely that this info is not in the FCOM.

Much Appreciated anyway chaps ...... :)

WM

[WaterMeths]

Doh !!!! Just read it again - baro references differ means one set to STD and the other to QNH. Of course.....................ahhhhhhh now I will go and give my brain a rest.

Good job GA

WM

[Microburst2002]

There is something I haven't ever fully understood regarding the baro ref.

According to the 1.34.10 p9:

ALTI DISCREPANCY
triggered when difference in Capt and FO displayed altitude exceeds 250 ft (when on QNH) and 500 ft (when on STD).

I don't know which computer monitors this discrepancies. SDACs, FWCs, ADIRUS...?
Does it have to do with the FCU channels? When one of them fails we have the BARO REF...X CHECK blue line. Does that mean that with an FCU failed the ALTI DISCREPANCY will not be triggered? Or it has nothing to do with it?
But then... Why should we be extra careful in x-checking the baro ref when losing an FCU channel?
There is not a caution for "one pilot on QNH and the other in STD", right?

Next time I am during cockpit prep I will set capt baro ref to STD and then set the FO QNH to change altitude in 250 ft and 500 ft and see what happens, and then repeat the experiment with the FO in STD and capt in QNH and both on QNH, of course.

I guess the ECAM just warns of a discrepancy, whatever the reason, which can be due to one faulty probe or system or to one incorrect QNH or one PDF in QNH and the other in STD.

[Airbus_a321]

microburst - why are you so eager to know all this detailed stuff behind :confused::confused:
Relax!! Just follow the AIRBUS PHILOSOPHY. If you have an ECAM - do ECAM action - thats all. Makes flying much easier.

[DesiPilot]

Airbus_a321,
microburst - why are you so eager to know all this detailed stuff behind

I am sorry but your comment make us pilots sounds like we are monkeys sitting there and doing what we are told/taught !!

If people stopped being curious or stop thinking or being innovative, aviation won't be where it is today. There is nothing wrong in inquiring about some good to know stuff, even if it is bordering to useless information.

[rudderrudderrat]

Hi Airbus_a321,

There are some ECAM logics which will get you into trouble. It's far better to understand your aircraft than be a mindless slave to ECAM.

e.g. Look at "Fuel L & R TK LO LVL" with an engine failure.

[rc330jj]

Hi microburst,

Since SDAC's normally take care of all the yellow cautions,my guess is they are the ones in charge of this too...

[Microburst2002]

I admit sometimes I waste too much time and energy to find out quite irrelevant things.
However, in doing so, I learn a lot of relevant stuff. And I like doing it.
I know of many very good pilots who don't know the airplane as well as I do.

In my opinion, in a computerized airplane, the system descriptions should always say which computer or hardware compares valuses, detects discrepancies, or controls automatic logics of a given system, etc...

Anyway, I think that this is a good question to ask:

Why do we have to BARO REF..... X CHECK when an FCU channel is lost?

We do it every time we change the setting, right? (as per SOP and if we use good cross cockpit communication). It is kind of insulting. Does the ECAM mean that we don't xcheck the baro settings?
Or is there another reason to be extra careful? Can the setting revert to STD on its own, or something like that?

cheers

[shortfuel]

MicroB,

ALTI DISCREPANCY will be triggered by FWCs. The difference of +/-250ft (QHN) and of +/-500ft (QNE) is detected by an external comparison inside the FWCs of the baro-corrected alti provided by ADRs.

Triggered typically when F/O and CAPT baro settings differ by more than 10 hPa (on QNH).