# How reliable is the Airbus FAC calculated gross-weight?

I've noticed some variation between loadsheet weights and FAC (Flight Augmentation Computer) calculated weight. That's of course normal, since loadsheet is calculated with standard weights and FAC calculates the weight from alpha and speed (at least as far as I know).

Question: Does anyone have any information about the correctness and reliability of the FAC calculated gross-weight?

I'm also interested in how exactly, and with what parameters, the FAC calculates it.

• Just as a little historical info, back in the pre-FAC days of the 1990s on 747-100&200 aircraft, after takeoff if we thought we were heavier than the load sheet showed, we could come up with our actual gross weight by working backwards through the tables for fuel flow, temp, mach number and altitude. This was on 747 freighters, and in third world countries they would routinely overload us. Even Japan Airlines would do it on contract freight flights by using 2.2 lbs per kilogram rather than the true value. Commented May 8, 2016 at 16:59
• I just recall seeing somewhere that a warning message appears when the difference is over 4t so clearly that is when manufacturer thinks you should start being concerned. And I read they changed it not to automatically accept the measured value, IIRC after the crash that involved frozen AoA vanes. Though I suppose you know this. Commented May 9, 2016 at 7:36

so after having done some more research this is what I have found.The A320 has two independent gross weight calculations. When the aircraft is on the ground the FMS does the GW calculation. Prior to engine start, the current GW and CG are not defined.; predictions are computed based on the pilot entered ZFW , ZFWCG, blockfuel and the FM calculates the TOW. The GW is calculated by adding FOB to the pilot entered ZFW (GW=ZFW+FOB) However I don't know what this iteration rate would be. When the A320 is airborne, the FACs independently determine the aircraft gross weight (GW) by using the aircraft angle of attack sensors. If a significant difference occurs between the FAC and FMS gross weighs (delta > 7 tons) , the message "CHECK GW" will be displayed on the scratchpad.

• You should have edited the original answer instead of creating a new one. Does not matter too much, just for the next time. Commented Jun 27, 2016 at 9:41
• … anyway, this is basically what we knew. The question is how trustworthy the later value is (the former is obviously as trustworthy as the load sheet). Commented Jun 27, 2016 at 9:45
• On the A320 NEO, the internal gross weight computation is done by the FAC based on the FMS-inserted data and not anymore based on aerodynamic data. Source: Airbus FCOM. Commented Feb 26 at 11:44

You could work out how accurate the answer is based on the fact that the normal accuracy for Vls is +/- 3kts. Following is quoted from the manual:

The principle of the speed computation is as follows:

• First, the FAC computes VS1G (stall speed). From VS1G it computes the Gross Weight (GW) which is also sent to the Elevator Aileron computers:
• When the aircraft is below 14 500 ft and 250 kt, it computes this from current angle of attack, speed/Mach, altitude, thrust, and CG.
• When the aircraft is above 14 500 ft or 250 kt, it computes this out of the GW, which it has memorized and updated with a fuel consumption model set in the FAC.
• Finally the FAC computes the various minimum and maneuvering speeds, Vα prot and Vsw.
• The accuracies of the various minimum and maximum speeds are functions of the accuracy with which the FAC computes aircraft gross weight. Normal accuracy for VLS in CONFIG FULL is about ± 3 kt.

This is old FMS. GWFK is AOA/FAC calculated, and used for Vls when landing. The GW in the lower ECAM is based on the figures you put in the FMS (fuel+ZFW). I believe the newer FMS gets Vls based on the weights you insert, and not the FAC calculated values. Also in the new FMS there is only GW and no GWFK.

the following information only concerns the A320.

The Flight Augumentation (FAC) performs the following functions:

• Yaw damper
• Rudder travel limiting
• Monitoring of the flight envelope and computation of maneuvering speed
• Yaw autopilot order
• Bite fuction of the AFS

However to my knowledge, the Flight augmentation Computers, (FACs) do not calculate the Gross-weight. In fact it's the single or dual channel weight and balance system utilizing six weight sensing elements attached to mounting surfaces in the main and nose landing gear. these transducers are driven by a digital computer (WBC). The WBC computes the aircraft center of gravity and the gross weight which is communicated to the flight crew via the Display Management Computers (DMCs) the ECAM.

• While I am not sure whether it is FAC that does this, I am sure the A320 calculates the weight after take-off from calibrated speed, angle of attack and vertical acceleration. It is that calculation this question is about. I was not aware of any system using sensors on gear; could this be a new feature of the later series? Commented May 24, 2016 at 7:52
• This kind of information is not easy to find. Most of the engineering documentation is very abstract. Therefore I will need to study it and get back to you. Commented May 25, 2016 at 22:15
• so after having done some more research this is what I have found.The A320 has two independed gross weight calculations. When the A320 is airb Commented May 26, 2016 at 23:17
• The answers are editable here. If you found more information, rewrite the answer. Also, please, add some references or at least cite the sources. Especially if the information is difficult to find, it would help others to know where to look. Commented May 27, 2016 at 4:49
• Absolutely, I'll add the references for those that are interested. Commented Jul 20, 2016 at 4:12