737 weight/balance question

[Matthewc]

This happened to me yesterday coming back to sydney. I was on QF1104, and extra service put on due to fog in Melbourne on sunday night, it was a 737 and me and the missus were sitting in seats 1D and 1F. After about 20mins sitting there a flight attendant came up to us with new boarding passes and said that due to weight/balance distribution she has to ask us to move to another seat. We had no problems with this as this can be expected when travelling on a staff ticket. The new seats were 13A and 13B. There was only about 25-30 people on the plane and most (90%) of people were sitting on the left hand side of the plane. Noone sat in seats 1D or 1F for the rest of the flight. Looking at the seat plan for that aircraft theres only 24 rows, so we were around the half way mark.

Could anyone shed some light on the reasons? Im presuming cargo was on the right hand side of the aircraft? Could that be why? But then again why would 2 people matter on an aircraft that was nowhere empty? Help is needed, as Im trying to understand the technical side of it.

[john_tullamarine]

I can only talk in general as I do not have the relevant loading system to consult. However, I am quite familiar with the design of 737 loading systems and there are no surprises particular to the design of systems for that aircraft Type which I can bring to mind.

The problem which you saw usually arises as a consequence of the loading system cg calculator design and practical protections built into the system.

Most larger airlines have the load control calcs built into the computer res system which permits, after commercial imperatives are addressed, the cg to be kept somewhere near the min trim drag position. The initial load distribution, including seating plan, is figured on the basis of known bookings and historical statistics (read educated guesses) as to no-shows and walk-ups. The sorts of calculations are not dissimilar to what you might do on a light aircraft.

If you were assigned boarding passes EARLY in the check-in exercise it is not at all difficult for a few no-shows to upset the applecart, especially if the load is light on and the particular aircraft cg without pax is a bit close to either limit .. in your case the load was somewhere towards the forward limit. In the situation where the final calculated loaded cg is outside whatever limits the system designer has specified as a margin on the AFM limits, the computer, when it comes to generating the loadsheet has to make some adjustments ... you and your lovely lady were two examples of such adjustments.

The situation is somewhat less flexible with manual systems such as trimsheets the design of which tends to work on aggregated error trapping using last minute change (LMC) adjustments. The only way to get around these more restrictive requirements is to redo the trim calcs.

What you experienced was not likely to have had anything to do with pax loading left or right .. that sort of concern is significant in helo loading systems but not fixed wing.

Nothing for you to lose sleep over ... just the particular airline's system taking care of itself.