Luggage is typical assigned an assumed average. The number will be in the airline ops manual. It can and does vary, if memory serves it was 35lbs, but I used to work for a comany that used 40lbs for all luggage except duffle bags which weighted in at 60lbs. This was becasue of the makeup of the typical passenger load at this particular airline. Lots of fisherman and people who did their shopping in town and brought it back as baggage.
And assumed average is also used for the containers that may be used. If memory serves at that other airline flats where 225lbs, igloos weighted in at 500 lbs. Again this numbers may vary based on what the ops manual was certified with.
The weight is determined by the count of the number of bags and is either transmitted to the Dispatcher or the load control person depending on the airlines procedure who then works the numbers into either the wieght and balance sheet or a load plan. At that a typical company I worked for the load plan was prepared by the ramp lead which on Mondays was...Me.
If it goes to the load plan it goes to either to the dispatcher to be entered on the weight and balance sheet, or the flight crew. The latter is atypical but was the SOP at that company I used to work for. In that case when at an outstation the CG was figured out by the copilot.
Working for a 135 now, The pilots leaves with a manifest which he signs a leaves a copy at the office, it also has the CG numbers on it. And he gets the WX both at launch/destination and points en-route. And I set him up with a filed flight plan.
Since I can't take responsiblity for the flight. Either the captain or the copilot will generate all the CG numbers and fuel loads. Which I don't mind. Usually it is the Captain that does this.
I don't know the real reason why though, it is either because they are in charge of the flight to they are responsible for it, Or more likely they would rather be inside in -20 weather doing paperwork the preflighting the A/C
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