As one who has designed more trimsheets than I care to remember, I am always surprised to find a trimsheet which DOESN'T have "limits" different to the TC.
The designer should (I would prefer to say "must", but too many either don't bother or don't understand the need) run an error analysis on the sheet design and contain the errors so determined by whatever means is appropriate for the particular sheet. Most times, this is conveniently done by constraining the sheet "limits" so that, if the trim is calculated to be on the sheet limit then, with the error included, it is still not outside the TC limit.
Why an error analysis ?
While a well-designed trimsheet gives an answer as accurate as a longhand calculation for practical loading purposes, there are various errors involved in loading calculations which ought not to be ignored and which will bite people on the tail during the court case .....
Some examples of these errors ..
(a) weighing data ... poor aircraft preparation, non-axial cell loading during weighing, mis-measurement of geometry during the weighing etc. He who believes the fanciful empty weight numbers often generated to the tenth of a kilo and millimetre .. is fooling himself ... the scheduled empty weight data can only ever be approximate due to unavoidable errors in the weighing .. we do, however, do our best to keep them under reasonable control.
(b) use of standard figures - fuel SG, pax weight, baggage weight, etc
(c) use of presumed seating occupancy if the trim is based on seat zones rather than rows. Similar concerns with presumed centroidal baggage and freight loading.
(d) CG movement with inflight fuel burn .. this one sometimes is done very poorly by the designer fraternity.
(e) movement of crew and pax during flight.
JT
Last edited by john_tullamarine; 26th Jan 2005 at 22:37.