737ng FMC Gross Weight Autodetection

[beorn 0]

Hallo everyone!Why does the FMC calculate the gross weight a little bit larger than it is? The empty weight of a B737-800 is 41413 kg. I boarded 20217 kg of payload. I made it through the MSFS Fuel and Payload Menu. So the ZFW should be 41413+20217=61630 kg. But when I pressed the LSK1L button (yeah, I know it's a cheat, but still...) the FMC displayed 61,8 tons. Where are these 200 kg from? And which value is the correct one -- the FMC auto calculated or the one I've calculated myself?Thanks for your assistance!

[Mats_J 2]

Michael,What value are you using for 'your' EOW? You should use the one in the aircraft.cfg file and you'll probably end up with 61,8 tons. If you want to reflect an individual NG (all have different EOW) you need to change this in the aircraft.cfg file before commencing loading.Hope it helps,

[beorn 0]

I'm using the value you're talking about. It is written in the aircraft.cfg file and displayed in the Fuel and Payload dialogue as "empty weight".Here's the Weight an Blance part of the .cfg file:[WEIGHT_AND_BALANCE]reference_datum_position= 52.786, 0.000, 1.861 // Vector of Boeing RFP to visual mode centre !! People DO NOT TOUCH !!empty_weight_CG_position=-57.770, 0.000, 0.000max_number_of_stations=50empty_weight=91300.000empty_weight_roll_MOI= 1630328.70empty_weight_pitch_MOI= 2103844.05 // was 2734997.20empty_weight_yaw_MOI= 1754301.78 // was 2403392.37empty_weight_coupled_MOI=212.00//station_load.0=27400.000, -57.450, 0.000, 1.050//max_gross_weight=173000.000CG_forward_limit=0.133CG_aft_limit=0.33091 300 lbs = 41 413 kg

[Mats_J 2]

Michael,You are right. It seems there are round-off errors (or maybe truncation errors) when converting back and forth between kilograms and pounds. It seems to be better if you go pounds all the way. (I believe FS internally calculates with pounds.) The error is marginal however. 200 kg in error is most definitely what you would encounter if you ever did weigh all payload before a flight in the real world.Cheers,

[beorn 0]

>The error is marginal however. 200 kg in error is most>definitely what you would encounter if you ever did weigh all>payload before a flight in the real world.Hm, that sounds good. Thank you. But what should I use then: the auto-calculated value or the one I get from the briefing? Which is the real one? And once more: are those 200 kg very relevant if I land with a GW of 65,3 tons, which is the MLW for -800, wheighing 65,5 tons in reality due to that 200-kg-error? Is it too bad or not?

[Mats_J 2]

Michael,Think about it. When you board a flight no-one ever weighs you or your hand luggage. What do you think the average error is on 120-180 people in all different walks of life and weights. Plus any crap the loaders opt to throw in the bulk before take off. ;-) I do not know the real margins in real world operations but would not be surprised if it is +-500 kg or more.Use whatever figure you're comfortable with. Both figures are 'right' within the margin of round-off and truncation errors. If you land with 65.3 or 65.5 is not a big difference. But you should try to land well below Maximum Landing Weight.Hope it helps,

[beorn 0]

Thank you very much for your help, Mats.