As someone who worked as a "Load Planner" (AA) in the 90's, I can tell you that the FAA has updated the standard passenger weight charts at least twice since my days. Due to both increasing obesity and more carry on luggage.
Airplanes which are a "stretch" model and/or have aft mounted engines tend to be more "touchy" on the "Center of Gravity" (CG) balance. Smaller planes are more sensitive than larger ones.
For mainline aircraft, very,
very few flights would be "border line" enough for concern. But as trained professionals, we know what to look for and would notify Dispatch (who would notify the Flight Crew) if the weight or CG might be critical. Two that always come to my mind when these FT threads pop-up :
- An MD-80 with less than 40 passengers will need closer scrutiny. Around 20 or less and things will start getting critical. Physical phone calls will be made (vs simple text notes on the load plan). Dispatch is advised to notify the Flight Crew. Confirm that the Gate Agent knows First Class must go out full and all Coach passengers are seated within the 1st "x" rows. Ground Crew Chief is called to confirm 1) everything goes into the forward hold ... no exceptions without calling me. 2) have the sandbag cart ready for ballasting the flight at departure time 3) do not push the flight out until you have confirmed with the Captain that they have called me on the radio to discuss the final load plan. Myself, I will turn off "auto" closeout of the load plan, forcing the pilots to call me before they can takeoff.
- The 727-200 was a wonderful aircraft in all aspects. Among other things, despite being a stretched, aft engine plane, she was hard to throw out of balance for take-off or landing. However, gate arrival had to be watched ... if the arrival fuel was less than 1800lbs (rare) and if there was more than 5000lbs in the aft cargo hold (rare) then the passengers can not deplane until sandbags (we'd have to calc the amount) were loaded forward.
You seem to know about the "simple version" of the FAA approved passenger weights and how they affect weight-n-balance. As I already implied, most flights are ok with the simple method and have plenty of "spread" from the limits so that there's no concern if the averages are off a bit. When the Gate Agent enters the final passenger count (and count of gate checked bags), all of those passengers are automatically counted as "adults". As the Load Planner, I have a "passenger exceptions" screen where I can adjust some of these "adults" to "kids" (lighter) or "NFL" (heavier). On border line flights, it's fairly common for the Flight Crew to know in advance that I will want a "kid count". I've never asked for an "NFL count" but have had crews call in for an adjustment.
Yes, there are last minute surprises. If a "normal" flight changed to "critical" at departure time, I won't allow the Load Plan to auto close (send the final number to the ACARS printer in the cockpit). The crew will be forced to radio me and we can discuss one-on-one any assumptions/adjustments/stipulations that I've included in the calc.
Failing all of that, the "final numbers" list not only weights and their allowable max values, but also the stabilizer trim (and flap) settings for takeoff. The pilots know their planes and can see if I'm approaching any of the limits, weight or CG. Flaps are a generic setting per aircraft model, but the stab trim is different each takeoff and implies the CG. The crew is free to call me and if the Captain isn't comfortable with an "all adult" (or whatever non standard allocation I used) passenger count, then they can tell me what counts they want me to enter.