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A320 FAC/MCDU Speeds

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A320 FAC/MCDU Speeds

Old 10th Apr 2017, 02:36
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A320 FAC/MCDU Speeds

Hi All
A few perplexing A320 questions. We know the FAC computes min and max speeds such as VSW, VLS, F speed etc.
1. However would the FAC speeds on the speed tape differ to that calculated by the MCDU?
2. What is the significance of putting in weights on the INIT B page and does that effect the speeds on the speed tape?
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Old 10th Apr 2017, 04:44
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Hi,
The FACs use MCDU weight for its flight envelope calculations, as long as an MCDU is operative. FACs also calculate the GW and CG independently for ELAC, for backup in case the MCDUs go out and also for the FMS' gross weight cross check. FACs calculate the GW and CG based on live aerodynamic data from the ADIRS. If there is GW discrepancy in flight between what FACs calculated and what FMS is programmed, then "CHECK GW" MCDU message will appear to let you know.

This is from DSC-22_40-30 Airbus FCOM.
My question is then, will the envelope speeds be based on FAC calculated GW and CG if there is a discrepancy between the two?
It seems there are two possibilities here.
1: FAC will display envelope speeds on PFD based on its own calculated GW and CG and ELAC will follow it. Which means VLS indicated on PFD and MCDU PERF pg will be different.
2: FAC will display envelope speeds on PFD based on MCDU GW and CG but ELAC will follow different speeds than what is shown on PFD.
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Old 10th Apr 2017, 13:03
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It's #1. The Vls on the PFD is almost never the same as on the PERF page, albeit the difference is usually pretty small (2-3kts, in my experience).
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Old 10th Apr 2017, 13:58
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The MCDU is what we think the airplanes weighs, the FACs compute what the airplane thinks it weighs. Thus we have the stipulation to maintain increased minimum approach speeds with respect to VLS as computed by the FACs when operating in icing conditions (ice build up on unheated surfaces or worse ice building up on heated surfaces) In these cases the FACs can determine an increase in aircraft weight caused by ice buildup that the MCDU can not. (ACA - 457 incident - roll oscillations on landing led to this section of the FM)

Conversely improper functioning of the AOA vanes have caused at least one serious incident (LH-1829) and one haul loss (GXL-888T).

I believe the MCDU message "CHECK GROSS WEIGHT" should be a red flag to every Airbus pilot. If we see that message we have made either a serious error during FMGC programming, or we have a serious undetected (by us and the fuel system) fuel leak , or we have accumulated a tremendous amount of ice or we have a serious problem with our AOA system.
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Old 10th Apr 2017, 14:09
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Another example of the discrepancy is, of course, Green Dot speed, which is defined on the A320 and A319 as [GW in tonnes x 2] + 85 (knots).

In my experience the PFD displayed Green Dot significantly higher than the MCDU, which simply calculates it on the above formula, using the crew-entered ZFW plus the fuel remaining. The PFD value, IIRC, uses a gross weight calculated by the FAC from parameters including EAS, AoA and normal acceleration.

Presumably the possible implications of a difference between the two figures is that either the loadsheet understates the ZFW, the FQI is under-reading (very unlikely), or the aircraft performance is degraded.

In our operation, any difference greater than 10 kt (implying a possible mass discrepancy of more than 5 tonnes) had to be reported.

PS
The above was posted before reading CaptainMongo's posting. Airframe icing would obviously be one possible cause of performance degradation.
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Old 10th Apr 2017, 15:23
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Older aircraft:
- characteristic speeds displayed on PFD from FAC based on sensor data

New aircraft:
- characteristic speeds displayed on PFD from FAC based on crew MCDU data

EZmunks Q1 and Q2 hard to answer.
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Old 10th Apr 2017, 16:58
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On this side of the pond, in our operation we see differences between MCDU speeds and FAC speeds, sometimes significantly. We believe that at least some of the difference is due to the FAA's "standard passenger and carry on baggage weights."

The FAA told the airlines to increase the allowance for the average weight of female passengers and their carry-ons from 145 pounds to 179 pounds in the summer, and from 150 pounds to 184 pounds in the winter.

The average weights for male passengers with carry-ons were increased from 185 pounds in the summer to 200 pounds, and from 190 pounds to 205 pounds in winter.
For children ages 2 to 12, the weight estimates were raised slightly, from 80 pounds for both summer and winter to 82 pounds in summer and 87 pounds in winter.


To add to this problem my company was the leader in charging for checked baggage. So what do customers do? Attempt to stuff everything in the overhead.They would bring grand piano's on board if they could find a place to stuff them.205 is barely enough to cover the actual weight of many of our "super-sized" folks.The new rules are a set in the right direction but we Yanks are fat. Let's face it; as a nation we have high obesity rates and that 205 number on a 180 seat airplane doesn't always get it, especially with the heavy bags.

Last edited by Zaphod Beblebrox; 10th Apr 2017 at 22:29.
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Old 10th Apr 2017, 21:45
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Originally Posted by FlightDetent
Older aircraft:
- characteristic speeds displayed on PFD from FAC based on sensor data

New aircraft:
- characteristic speeds displayed on PFD from FAC based on crew MCDU data

EZmunks Q1 and Q2 hard to answer.
We fly older Bus' 1994-2003. What is the changeover date (Or is it corrrelated to MSN)We are getting some newer ones off lease (2006/7)

Thanks,
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Old 11th Apr 2017, 01:23
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Thanks all for sharing such illuminating insights. I am a newbie on the bus, learned a lot from the posts.
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Old 11th Apr 2017, 05:41
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As part of our differences training course, we're told that on the NEO airplanes, the PFD speeds come from the FMS. Can't figure why Airbus made that change. The original system had some redundancy built in. Does anyone know why they changed it?
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Old 11th Apr 2017, 07:48
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Originally Posted by CaptainMongo
We fly older Bus' 1994-2003. What is the changeover date (Or is it corrrelated to MSN)We are getting some newer ones off lease (2006/7)

Thanks,
It only changed a couple of years ago
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Old 11th Apr 2017, 08:02
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Originally Posted by Check Airman
As part of our differences training course, we're told that on the NEO airplanes, the PFD speeds come from the FMS. Can't figure why Airbus made that change. The original system had some redundancy built in. Does anyone know why they changed it?
For consistency between the MCDU and the PFD
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Old 11th Apr 2017, 10:08
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That would seem odd, given that the original architecture allowed a valuable gross-error crosscheck between two independent sources of data.
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Old 11th Apr 2017, 10:36
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"For PFD/MCDU display consistency and accuracy purposes"

PRO-SUP-10 for relevant aircraft
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Old 12th Apr 2017, 01:40
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If it's purely for consistency, it seems like a step back- as Chris Scott noted, it's a pretty good safety net. More than once, on approach, I'd find myself increasing Vapp by 2-3kt to account for the difference.
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Old 12th Apr 2017, 16:33
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The aircraft give you a 'check GW' message if there's a substantial difference between the FMGC weight and FAC calculated weight so the safety net is still there
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Old 12th Apr 2017, 23:07
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Good. IIRC we used to get that message on the 20th-century A320/A319 models when the Green Dot discrepancy reached 10 kt (representing 5 tonnes), so that aspect may not have changed.
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Old 13th Apr 2017, 10:35
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Calling up the alpha params in flight to see what the aircraft thinks it weighs can be interesting. Weekend break flights with little luggage and Asian passengers often come in a ton of two under.
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Old 14th Apr 2017, 09:59
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Originally Posted by CaptainMongo
We fly older Bus' 1994-2003. What is the changeover date (Or is it corrrelated to MSN)We are getting some newer ones off lease (2006/7)

Thanks,
In my fleet, the changeover is between MSN 5500(ish) to MSN 6300(ish)

Looking at airfleets.net, a 2006/2007 model should be around MSN 3000, so probably before the change was made.

Last edited by Dupre; 14th Apr 2017 at 12:43.
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Old 20th Apr 2017, 17:07
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The difference

I fly A320-214/232. Different speeds on mcdu and pfd are quite common. The MCDU always uses the ZFW you enter. Its calculations are based on that ZFW throughout the entire flight. On the approach phase, speeds like F, S and VLS are estimated speeds. On the other hand, PFD shows FAC calculated speeds which are actual depending on your configuration at the time.
During climb, FAC uses certain thresholds (>14500 feet, >250 kt and some more) to calculate speeds on PFD depending on the aerodynamic model. Then it uses fuel consumption model and during descend it reverts back to aerodynamic model again.
(Sensitivity of these speeds depend on the sensors' accurate working of course)
So if you have a different speed on PFD than the one on the MCDU (on a cyrstal clear day, to eliminate the ice accretion affect on the matter), say different VLS then you are either heavier or lighter depending on the speed being bigger or smaller. If sensors are working properly then it means you're at a different weight than you think you are. We need to be careful on that because CHECK ZFW message appears at 7K! So if there is a difference below 7000 kgs, you will have no alert messages. That can be a big enough difference to think on adding speed to target speed on PFD which uses Vapp on the MCDU!
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