A320 MCDU

[captainpaddy]

Hi Guys,

I seen a couple of you comment on the use of the AIDS propmt on the MCDU to determine the GW and other things. Does anyone know of anywhere that I might find out further info on this and other applications. The FCOM goes into some detail about CFDS and ACARS but leaves the AIDS alone!!

Cheers,

CP

[gyro]

AIDS- Parameters - Alpha call up (From memory only, LSK2L) for Gross wt in kg enter GWFK, or for lbs enter GWFL. This will display the gross wt as calculated by the FACs based on the principles described in Fcom 1.22.40 pg 4. It forms the basis for determining ALL characteristic speeds displayed on the PFD. It can be significantly different to the wt entered into mcdu (greatest I have seen is 3 ton on an A321).
There are in excess of 100 codes that can be entered, but most are of no real value to the drivers, eg burner PSI, status of fadec sensors.

Cheers

[captainpaddy]

Thanks Gyro!

[Jimmy The Big Greek]

where can I find the full list?

[Max Angle]

It forms the basis for determining ALL characteristic speeds displayed on the PFD.

On the two brand new A319s we have just had delivered all the PFD displayed speeds (green dot, vls etc etc.) are now generated by the FMGS rather than the FACs as before. Not quite sure why they are now doing it this way but it does away with the differences that you notice in the speeds on the the other aircraft. I have to say that in 7-8 years of flying the thing I have never once felt the need to dive into AIDS to find the "true" weight. I have long suspected that the information derived from the angle of attack vanes is just as prone to error as those derived from loadsheet weights, a tiny error in the measured AOA has the potential to generate quite a large discrepancy in sensed weight I would have thought.

[Wodrick]

It's in chapter 31 of the AMM and differs with engine type so two seperate lists, get one of your maintenance people to print it out, it goes on a bit !
I will try to generate a pdf version next time at work which is at the weekend, will post if I have success.

[SIDSTAR]

Max is correct,

The latest version of the 320 family has all the speeds computed by the FMGS. Does away with the differences between FMGS and PFD speeds. FACs remain as a backup speed computation.

[mcdhu]

Max & Sids

Do you have an Fcom reference for this change. Bulletin 819/1 has not yet been superceeded and I don't think our lot have 'clocked' this yet!

Cheers,
mcdhu

[Gnadenburg]

Originally Posted by Max Angle

I have to say that in 7-8 years of flying the thing I have never once felt the need to dive into AIDS to find the "true" weight. I have long suspected that the information derived from the angle of attack vanes is just as prone to error as those derived from loadsheet weights, a tiny error in the measured AOA has the potential to generate quite a large discrepancy in sensed weight I would have thought.

Yes. Alpha probe errors can be significant- 0.1 degree equals a three ton, 4kt error. But the voter function between the three alpha values offers system protection against gross errors.

Airbus states the errors of the FMGC speeds equals +/- 2kts and +/- 3 kts for the FAC generated speeds.

The threats as I see it are 1) pilots flying at VLS when FMGC speeds are incorrect. This is a product of a poor loading procedures- inaccurate nominal weights and incorrect load sheets. This is airline dependant. I worked for an airline where loaders took bribes and 4 tonnes overweight normal. I have also worked for an airline where similar errors occur due conservative nominal weights. The major risks in this scenario are tailstrike susceptability and hard landings.

2) Charateristic speeds being too close to VFE. When G dot equals VFE Flaps one, forewarning to slowdown early in difficult conditions useful to avoid overspeeds and unneccessary go arounds.

Checkings FAC weights through AIDS is very useful in the above cases- and good practice with some airlines depending on operational circumstances.

[captainpaddy]

Surely whether the FAC weight and speeds are inaccurate is sort of irrelevant. You have to honour and follow them regardless??

This question of inaccurate loading and non-reflective weights is a funny one. In my mind the airbus is the first aircraft I've flown that gives an indication of the size of the errors involved. I am very sure that any aircraft will have the same problem if standard weights are used for bags and/or pax. It's just that the airbus highlights it.

I find it hard to believe that loaders would be bribed to go 4 tonnes overweight (not saying you're lying gnadenburg!!). I have regularly seen the A321 at least 5 or more tonnes over the estimated weight and the A320 4 or more tonnes. No bribes required!!

[FlapsOne]

Hi Max

Have to agree with mcdhu.

Looking at our kit (brnd new 319s) there is often a slight (1 or 2 kts) diff between the fmgc and pfd figures suggesting that, as per the fcom bulletin, the FAC is doing it's own stuff.

Would really appreciate a reference for the change.

[Gnadenburg]

The system has changed on new aircraft. Old aircraft still the same ie: FAC & FMGC independant.

The new system "FAC computes speeds based on FMS gross weights. Consistant PFD & FMGC speeds. In parallel, the FAC computes it's own GW & CofG, based on aerodynamic data which is used by ELAC & FMS for for GW cross check, and as a backup for speed computation in case of dual FMS failure".

If your aircraft has the new system, this will be the paragraph in the FAC FCOM section 1.22.40.p4.

[FlapsOne]

Very interesting. Can't find that para in my fcom and our newest 319 is literally days old. No bulletins or notices advising of any change.

Anyone in the know why this might have changed??

[Gnadenburg]

Originally Posted by captainpaddy

Surely whether the FAC weight and speeds are inaccurate is sort of irrelevant. You have to honour and follow them regardless??

Not true. If you get a Check Gross Weight message on the MCDU ( difference of 5 or 7 tonnes depending on standard between GWT FAC & FMGC ) you need to establish the error source. If loadsheet error, update Fuel Pred page and if FAC error you must ignore FAC character speeds and get them from QRH.

I have had the FAC error myself. First thing to remember is the system uses an ELAC failsafe where the aircraft can have inaccurate speeds however the ELAC only considers alpha information and hence, offers protections- stall warning or alpha prot depending on laws. A small alpha error of all probes results in a significant speed error, however, this small error is negligable in critical angle of attack information for protections considered by ELAC. So you use QRH and trim sheet weights for speeds accordingly.

[captainpaddy]

Gnadanburg,

Point taken. I should have qualified the last by only including errors within the warning threshold. Our machines obviously have the greater FMGC limit.

Cheers!

CP

[Max Angle]

Here is the section of the FCOM that refers.

FCOM 03.04.10 Page 1

The characteristic speeds displayed on the PFD are computed by the Flight Augmentation Computer (FAC), according to the FMS weight data (for PFD/MCDU display consistency and accuracy purposes), and aerodynamic data as a backup.

VLS (of normal landing configuration : CONF 3 or FULL), F, S, and Green Dot speeds are also displayed on the MCDU TAKEOFF and/or APPR pages.

These values are computed by the FMS, based on the aircraft gross weight (which is computed according to the entered ZFW and the FOB), or the predicted grossweight (for approach or go-around).


This only applies to our newest A319s, what it is really saying is that the FACs are still doing the calculation but are using different data.

[FlapsOne]

Originally Posted by Max Angle

On the two brand new A319s we have just had delivered all the PFD displayed speeds (green dot, vls etc etc.) are now generated by the FMGS rather than the FACs as before.

but then........................

Originally Posted by Max Angle

The characteristic speeds displayed on the PFD are computed by the Flight Augmentation Computer (FAC), according to the FMS weight data (for PFD/MCDU display consistency and accuracy purposes), and aerodynamic data as a backup.

Now I'm confused!

[manuel ortiz]

Some FACīs are now using FMGS GW instead of aerodynamic input for speed computations. Aerod. inputs remain as a backup.

[Max Angle]

Yeah, me too. The second post is a direct copy and paste from the FCOM, the first was just my take on it having read the memo that was issued to us. The FACs still do the calculation but based on FMGS entered weights instead of sensed weights derived from the AOA vanes. Simple really (not).