Is that a reason why they will leave seats empty sometimes?
I have been wondering how they do that for awhile.
Yes, they do.
My airline ops manual states that when 50% of a passenger group appears to be outside of the standard averaged criteria (like a football team), they will use actual weights for the weight and balance. Weights are obtained either verbally or in writing from the gate agent. An additional 10 lbs is added to that for their carry ons.
On a "full" flight with empty seats, the aircraft has already reached it's maximum allowable zero fuel weight/ZFW (the weight of the plane, bags/cargo & passengers). With a heavy cargo load or a alot of extra fuel for weather, this weight can easily be met long before the planes seats are all full. This routinely happens on our flights. We almost always have empty seats yet can't take any more people or we would be over our ZFW weight.
BTW- our pax weights are 190 in the summer (May-Oct) and 195 in the winter.
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The reason of the empty seats is simple they could not sell any tickets for those seats. about the weight I can tell you that the passenger weight has nothing to do, they actually compensate it with the cargo , and if by any reason the consider excess of weight the airline will move some of the luggage's to another flight.
They use an average weight of 175 pounds then add or remove fuel as needed lot of room for error on say 737 as compared to a cessna 182
they use weight standards depending on the season
Curiously enough the passengers' assumed weights depend on the time of year. 160 lb each for adults May through October. 165 lb each the rest of the year when they're wearing heavy coats for the winter.