How does the "weight" computer work on a CRJ?
 

I have been moved around in a CRJ before to "balance" the load, but last Friday in FAR, with all the Pax on the aircraft, we left the gate 5 mins early. Only to sit and wait for 45 minutes on the tarmac. The pilot said he was waiting for the "company" (his words) to acknowledge the "weight computer"
He came back every 15 mins, saying that they had no reply from the messages he sent. Eventually with no moves on the aircraft we finally left 45 mins late and arrived 40 mins late in MSP. What gives?

All "weight" issues are centralized at NW. The information about number of passengers, occupied seats, # of luggage, location of luggage (forward/rear compartments in larger planes), weather and fuel loaded (and probably more information) is sent to the company and takeoff information is sent back to the crew regarding possible need to move passengers/luggage, takeoff speeds...

This is something that I really hate... I know it's for saftey, but I've waited 30+ minutes more times than I care to remember on a CRJ for the infomation to run its course and it sucks!

Agreed. On my last flight from MEM we sat on the tarmac nearly an hour while the pilot said we were waiting on this piece of information. Then he said we needed more ballast, and took us back to the gate area for more cargo and fuel. Soon we were over 90 minutes late getting off the ground. For some reason part of the wait was w/o air too. So coming back from Europe I was tired, late and soaked with sweat. Ah the joys of the CRJ.

Does anyone know what part of this calculation cannot be done on the aircraft? From what XYU said, it seems pretty straightforward and that it need not be centralized (unless there are other variables involved or unless it's for legal reasons or some such thing).

I worked for AA as a Load Agent 20 years ago ..so some of my info could be out-dated or not applicable to the NW way of doing thins and/or the CRJ (I also wrote a long explanation just a few months ago)...

None of the calculation is done on the aircraft, it is all centralized.
Way too much info to have in charts or a database on an aircraft computer:
'center of gravity' ...and...
'takeoff speeds' ...and...
'takeoff power settings'...and...
'flap setting' ...and...
'stabilizer trim setting ...
... for each and every combination of ...
'takeoff weight' ...and...
'runway length' ...and...
'wind speed/direction in relation to runway heading' ...and...
'temperature' ...and...
'air density' (think of weathermen saying 'we have a low[high] pressure...)
'runway grade (slope) ...and...
'passenger/cargo distribution by zones' ... and...
'air conditioner on/off' ...and...
'engine anti-ice on/off' ...and...
'surrounding terrain' ...and...
'performance considerations for any mechanical issues w/the plane ...and...
... the list goes on and on

You Flight Crew (pilots) know how to calculate all of this 'by hand' and 'on the aircraft' ...and... every flight can wait on a taxiway somehwere while the pilots spend an hour doing this ...or... the calculation can be done on a computer ('aircraft based' or 'gound based') in a matter of seconds.

An 'aircraft based' computer seems to be the best choice at 1st glance.
But.... does the aircraft computer have 'first hand knowledge' on....
number of passengers booked
number passengers that 'no-showed'
number of passengers that will 'mis-connect'
number of stand-by's accomodated
of the passengers on-board, how many are (lighter weight) children
what zones would the passengers be in (if they took their assigned seats)
how many pieces of luggage were loaded/ into what cargo zone
how much freight was loaded/ into what cargo zone
how much mail was loaded/ into what cargo zone
any live animals in what cargo compartment

A 'ground based' computer does have 'first hand knowledge' on a lot of this stuff. And the things it doesn't know directly are easily updated. IE: as soon as the freight department knows exactly how much cargo is being sent to the gate, they can update the computer. If another shipment arrives an hour before departure, they can add it in.

The downfall to the ground based computer is getting everyone to 'finalize' their data in a timely fashion. A ot of this info isn't known until right at departure time and may not be entered into the computer until after the flight has left the gate. IE:just because freight entered into the computer that there would be 8,000 lbs of cargo for the flight it doesn't mean that the ramp crew had the time/cargo-space to load all of it. And there may be cargo (denied) from previous flights added to this one.

So a 'departure plan (weight-n-balance) can't be sent to the airplane (via a computer radio link) until both the Gate and Ramp Agents have 'finalized' their numbers. They usually don't have time to do this until the flight has left the gate.

Some 'last minute' changes don't affect just one flight. If the wind direction changes right at departure time at a major hub, the Load Agent may suddenly get very busy updating every flights 'numbers'.

Or the radio link between the ground and aircraft based computers maybe down/backlogged.

Or the ground based computer maybe down. This would affect not just your flight, not just every flight at your airport, but every flight worldwide for the airline.

Many, MANY things could be causing the (understandbly frustrating) delay waiting for the 'weight computer'. But the calculations are much more involved than most folks would think and ...bluntly put... the flight ain't taking off until the Captain has numbers that he knows are up-to-date and correct.

Steve