... I'm sorry I don't know how to posts links. But here are a few excerpts from my posts in the NW thread not too long ago. The original post was questioning what appears to be last minute "we've know idea what's going on" in respect to airline's weight and balance planning........

I was a Load Agent for AA in the 1980's ...

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... so I can't speak for NW or the CRJs specifically.

AA has a centralized "Load Control" facility near DFW airport. Practically all flights are planned from that location. A computer does all the calculations and most communications are from the same computer.

Each Agent has a list of flight assigned. We'd start planning a given flight about 4 hours from departure. When we intialize a flight, the program would automatically query the reservation system for an estimated passenger count. From that count we would estimate the number of bags. We'd look at past history, what was denied off previous flights today, etc to determine an estimate of how much freight and mail will be on-board.

Things start cooking about 1.5 hours before departure when the Dispatchers determines the amount of fuel required for the trip. The fuel load can be a major part of my planned take-off weight. Around that time I'll get messages from both the mail room and cargo on the weight of any mail & freight sent out to the gate. A similar message comes from the "bag room" about 15 minutes before departure.

The Res system will send me automatic updates 90, 21 and 11 minutes before departure on how many passengers are actually checked in and any changes to the estimated final count.

The Load Plan can't be finalized and transmitted to the flight crew until I get the "close outs" from both the Gate Agent and Ramp Crew Cheif. They will enter into the computer the exact amount & location of the actual passengers and anything in the belly. Due to the nature of their jobs, these close outs usually aren't done until the flight has left the gate. Sometimes they get tied up in other issues (the next flight for their gate is already on the ground) and don't realize that 10 minutes have passed since the flight "pushed" and they still haven't sent their close out.

There can be many surprises in those close outs. Delta may have cancelled a flight and sent 50 folks over without having time to re-book them. I don't "see" these passengers until the closeout. Many times I've specifed on my load plan "deny all freight & mail" until to have the ramp close out specify "500 lbs freight aft compartment - human remains" (a "must ride").

Even if a substantial last minute weight increase doesn't throw my balance off or or exceed any weight limits, I still need to contact Dispatch first. A heavier plane will burn more fuel so we might not have enough fuel on-board, even tho the flight is taxi-ing out. In most cases, the increase weight won't affect "enroute fuel burn" enough for a major concern. And usually Dispatch can re-work the Flight Plan to get everything to work. The best example would be if the weather forcasts at the destination are pretty good, Dispatch can reduce the planned holding time or select another "alternate" airport closer to destination. In effect, reducing "holding fuel" and/or "fuel to alternate" to compensate for the increased "enroute burn". Any change to the Flight Plan requies Captains approval, so there may be 3-way communications going on between the flight, Dispatch and Load Control.

Other "adjustments" we can do to get all the pieces of the puzzle to fit:
1) change to a non-standard flap setting
2) restrict takeoff to "air-conditioning" or "engine de-ice" turned off
3) get a "child count". We assume all passenger sent in the close out to be adults (airlines use standard weights .. one adult weighs 170lbs and 175 lbs in the winter [extra clothing] ... I think these numbers have been increased over the years due to the unfortunate fact that Americans are gaining size ... these numbers are set by the FAA)

A simple thing like a slight wind change can wreck havoc on a load plan. If we had been takeing off towards the south all day and now all of a sudden we're going north then my original plan was based on us takeing off from a runway with a slight 1 degree downhill grade; I'm now trying to accelerate a heavy plane on a 1 degree uphill grade. That bird will need just a little more runway.

That last minute runway change may have also changed from takeing off towards the valley to headed towards the mountain ridge. I have to have that plane light enough to clear that ridge by a margin of 2000ft ... all with the assumption that one engine will fail as soon as the aircraft breaks ground.

Temperature can be critical in the summer (hot air is thin air, those wings will need the air over them faster to get the same lift that they'd get on a cold day). If a flight was borderline, you can bet I'm calling ops at the airport to get the temp as of NOW. I have called flights back to he gate to offload weight because my load plan was based on takeoff at 101 degrees and it's now 105.

Bottom line:
Many last minute things can change which seem very minor to the outsider (if they even happen to notice) but cause airline personnel to have to re-think things to ensure 100% safety of the flight.

One thing about the smaller aircraft versus the mainline jets I planned:
The passenger balance is a little more critical. My close outs from the Gate Agent would simply be counts in First Class and in Coach. Internally, the system would distribute the Coach passenger weight based on seat assignments. If you and your partner actually moved yourself to seats farther forward, the flight crew really isn't going to notice anything. But it can be critical on the smaller planes. You'll see the Flight Attendant or someone coming down the aisle and counting/making notes. They're getting a passenger "count by zone" (ie: 8 pax in rows 1-4, 6 in rows 5-8, etc.) So that last minute close out is a little more involved than "12 in First, 103 in Coach". Perhaps this slows things down a little on a CRJ

Steve
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Takeoff versus cruise

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Quote:
Originally Posted by BOS-NWA
If two pax weighing 250 lb each sitting on 3AB go to the lav, that changes balance by 500 lb, no?



Well, if you want to get technical, yes the center of gravity would change a good bit....



Quote:
Originally Posted by kathnunley
I think most weight and balance issues only play into take-off, don't they? So once in the air, I believe both of those passengers are welcome to move.



... but no one is going to care about those passengers moving around during the flight. The autopilot will adjust a "trim tab" to compensate and it will be so minor that the pilots may not even notice.

Why so critical at take-off and not during cruise
Speed brother. Ever drive a small ski boat ??
In cruise, that bird is slicing thru the air at 600mph. At that speed, it only takes a minor deflection of the control surfaces to turn, climb, adjust for a change in the center of gravity, etc. In fact, for your 2x250 lb pax in 3AB to go to the aft lav at the same time will not even cause the elevator (flight control surface the governs pitch) to move; just a small trim tab will do the job. Ont the other hand; while the 140mph lift-off speed may sound fast to you, it's sluggish for that airliner so she won't be as responsive to control input. Those pax moving around at that slower speed would require a bit more movement of the elevator to maintain that pitch. And the desired pitch for takeoff is nose-up. The combination of the nose-up pitch and slower speeds means it's much more possible to hit the elevator's travel limits at take-off than cruise. I think in most cases, it's the flight control deflection limits that control the max allowable forward/aft center of gravity.

Steve
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Overweight on a short flight ??

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Quote:
Originally Posted by geoffco
I'm still baffled as to why a Comair CRJ200 was weight restricted to 43/50 on a CVG-DTW at a departure temp in the mid 70's



Hey geoffco,

That's a very common comment.

As the Load Agent, I'm concerned about keeping the flight at/below the "max allowable takeoff weight". The key word there is "allowable".

Every plane has a certified "max takeoff based on structure". That's what the plane can phyisically hold without risking damage. It's a set number for that plane. We very rarely see that as our governing limit.

More common is the "max takeoff based on performance". This is where wind, temperature, runway grade, surrounding terrain etc etc etc come into play. This is the heaviest I can be and garuntee I can get the flight off safely. It can very easily be a number lower than the structural limitation.

What your flight probably encountered (and is the most confusing) was "max takeoff based on landing". It's very common on the short flights. A 727-200A could be certified to 178,000 lbs takeoff. Given todays conditions, we need to hold the weight down to 150,000 lbs for performance and safety. But we then call the Gate Agent and have them solicit for "oversales" so we can get the weight down to 130,000 lbs for takeoff. What the heck
The flight is DFW to San Antonio. The max structural landing weight is 110,000 lbs (these are all theoretical numbers). The flight plan calls for 20,000 lbs of "enroute burn" fuel. It makes no sense for us to leave DFW above 130,000 (130,000 departure weight minus 20,000 lbs fuel to fly to SAT = 110,000 max landing weight) pounds even though we can safely do it. If we did then the Captain would have to circle SAT for an hour to burn more fuel (there's that "holding fuel" and "fuel to alternate" again). And what if you don't have an hour of contingency fuel ?? This catch 22 can happen.

The "max allowable takeoff weight" is the lowest of: "max based on structure", "max based on takeoff" and "max based on landing".

Another possible scenario on bad weather days:
As indirectly mentioned by myself and others, the flight must have the fuel on-board to get to an alternate airport (unless the weather at destinationis forecast to be perfect). Airlines can't just pick a close by airport as the alternate. It has got to be an airport where the weather forecast basically garuntees we'll be able to get in (the specific qualifications are part of the FAA regs). That requirement can really hurt. I remember a 727-200 DFW to Toronto. We had to have the flight restricted down to around 60 passengers (capacity was 150 seats). The entire northeast was having a winter day and the closest "legal" alternate was Nashville. The weight of the fuel to fly up there, hold for "x"minutes, fly to BNA and land there with 30 minutes fuel still in the tanks was tremendous and left very little "weight spread" for revenue.

Steve