FlaspOne,
Apologies if I appeared to be argumentative .. for that was not my intention. Perhaps my original comment could have been couched in gentler terms ?
However, having spent a lifetime in engineering (including a lot of weight control) and airline flying (where I could see the front end problems caused by unhelpful loading systems), the general level of misunderstanding in the pilot ranks has always caused me some concern.
Unfortunately, the typical pilot training for weight control rarely extends beyond how to complete the loading system .... in general the detailed knowledge in the pilot ranks of how it all works tends to be limited and, in my view, it is a desirable aim to improve that level of knowledge.
the original question was not worth asking
.. not what I said ... the question is well worth asking if the end result is a bit more knowledge. However, it is "pointless" in that the LMC numbers are quite arbitrary and direct comparison between one loading system and another is of little value. Quite different considerations.
Maybe he/she was just interested in knowing the numbers
If so, then fine.
just interest as to why they might be different.
Ah .. it appears that we are on the same path, my friend ... so let the discussion continue
The figures used by my company are not arbitrary
Perhaps we are at loggerheads because of semantics. By "arbitrary", I mean that the LMC number chosen is so chosen as the result of whatever considerations went into the particular loading system .. ie, there is nothing at all sacrosanct in the particular number which ends up in your sheet .. for the same aircraft, one can readily generate a number of different loading systems each with a different LMC number or, indeed, several numbers .. this makes the sheet a little more complex but is useful on occasion.
It is just a matter of matching the number desired to fit the convenience of the loading system against the manner in which the error analysis and drafting of the trimsheet is done.
They have been researched by the Perf dept
Precisely my point .. the performance department operations engineers (which is my principal background) traditionally have responsibility for weight control matters. As part of the design of the loading system, those good folk have determined what figure is suitable for the particular operation .. and then arranged the design of the sheet to suit that figure. However, it is important to realise that they could just as easily have determined that a different delta was to be the LMC figure ..
the effect on trim will not have a significant performance effect
At the risk of inflaming the discussion, your view here is misplaced. The LMC, per se, has naught to do with performance in general. Rather the LMC defines, within the error analysis for the trimsheet, the maximum calculation misloading (in that the load now differs from the calculation) which will not cause the actual CG to be outside the certification CG envelope .. no more, no less.
the take off figures are always recalculated
Absolutely .. however LMC, while usually couched in terms of weight deltas, has little to do with weight as such and everything to do with IU (as an indirect measure of CG).
if the index change is greater than +-6 index units
Denti is onto the thrust of LMCs .. we are concerned with the net effect of a misload on the CG.
If the trimsheet is well designed (which, in general, includes choosing a datum which gives a reasonably boxy envelope shape) then it may be practicable to give the LMC in simple IU delta terms. In the more general case, the designer looks at the plotted effect of a specific weight change .. if effect, one chooses the desired LMC weight delta and then redraws the envelope limit so that this level of misload constrains the actual CG (as opposed to the calculated CG) to remain within the envelope limits.
FlapsOne .. please don't think I have any interest in being contrary for the sake of it .. there is a useful purpose to be served by continuing this discussion for others. Please do respond with specific questions and I am only too happy to address them ..
You might also consider sitting down with one of your ops engineers and having him/her run you through how they designed your particular trimsheet .. I would be VERY surprised if the story differs from mine in any material regard ...