Altering the Pax count by 1- how does it affect CG?

[shon7]

If the passenger count is altered by 1 pax. and revised numbers sent out after takeoff -- how does it affect the CG on a widebody?

[gas-chamber]

On a widebody you could not measure it unless you were a boffin with a very accurate computer program. On a narrow body jet you probably could, but it would not change the stab trim to any measurable degree. On a Cessna 172 it might cause a problem, but you would have to be pretty unobservant to not notice an extra or missing pax in one of those. Of more concern these days is how someone could get on board or go missing and the Captain not know about the change before takeoff. If this is happening at your airline you need to review your dispatch procedures.

[john_tullamarine]

.. make up some numbers for a widebody and see what you get .. at least that will give you a bit of a feel for the problem you pose ... for the sake of discussion, let's run with the 744 ..
(a) check out a website which will give you an idea of the dimensions .. a quick search on Google gave me this one (http://home.swipnet.se/~w-65189/transport_aircraft/b747/boeing_747_series.htm) from which I scaled off some guess numbers to use in the calculation below ... for the sake of the discussion, let's presume that the CG and datum are co-located somewhere near the inboard engines ... so we might have an aft-located seat somewhere near 25 metres aft of the selected datum.
(b) similarly for weight we could use the information in the previous url to pick, say, 350,000 kg
Simple now to run a calculation for the addition of an 80kg passenger somewhere down in the back of steerage ..

350000*****0*****0
***80*****25**2000
350080**0.006**2000

(.. one of these days I'll find out how to format numbers in columns in PPRuNe .. for the moment please excuse my lack of finesse ..)
For an aircraft of this size, a calculated CG change in the vicinity of 5mm is going to be lost in the normal error clutter so a reasonable answer to the question is it doesn't affect things to any significant extent for a widebody ....

[boeingbus2002]

As gas chamber mentioned, on a widebody, 1 pax has little influence on trim.
With other smaller types CRJs for example or when trim critical, then it could make a difference. CRJ captains will often ask for a new loadsheet with any change in Pax figures rather than LMCs.
Looking at a Manual loadsheet for any aircraft type you can work out which zone the change occurs and extrapolate down to see the move in CG.

[Old Smokey]

Ya do it in FONT "Fixedsys"
Dat's how it works J_T:::::: (Look at that great even spacing):=

350000*******0*******0
****80******25****2000
350080***0.006****2000

Doesn't quite make up for obstacle polygons John, but a small favour in return!:ok:

Regards,

Old Smokey (who likes Unbalanced Fields but Balanced Type spacing)

[Jon Lei]

If just say, for some reason, inflight, all the pax on a B744 or a B773 were 'forced' forward to occupy the first/biz class, or worst still as far away in the aft cabin as possible, will it still be possible to fly or even land the bird??
Have you'll thought bout this? Thks.

[cwatters]

On the original question.... Given that some people weigh at least 10% more or less than others....

10 heavy pax = 11 light pax

You only need a small group of ballet dancers or a slightly higher percentage of children than normal the throw the numbers out by an amount equal to several people.... so it can't be that critical.

[shgsaint]

I'm no professional pilot so i'll say this with novice ignorance.

I agree to everything that is said but what about in flight? The way I see it is if it was found half way through a flight that the pax count was out by one I wouldn't be concerned at all. Especially on a heavy where there maybe 8-14 cabin crew pacing up and down the length of the aircraft pulling and pushing trollies. What does that do to the CoG? Nothing that is noticable I imagine.

Just my 'outside of the box' thinking.

[Mad (Flt) Scientist]

Actually, accounting for inflight cg shift is certification requirement.

Being outside the expected departure loading is OUTSIDE that requirement, and additional to it.

Just because you can accomodate a trolley at the forward end of the cabin doesn't mean you can accomodate that same trolley PLUS a missing pax.

[Intruder]

Though it may not make any PRACTICAL difference, it certainly can make a LEGAL difference...

In the 747F (200 or 400 -- doesn't matter except for specific numbers) all jumpseaters/upper deck occupants must be accounted for. At some point between 0 and 5 extras, the CG index will change by 1 unit. If the load is such that the airplane is at a forward CG limit, it can make a difference between a legal and illegal load.

In the situation in which I was involved, the CG was the limiting factor, not the weight.

[john_tullamarine]

OS,

Proportional font misalignment isn't the problem .. so the monofonts don't help .. rather multiple characters DEC 32 default to a single character .. so, for instance, if I put, say, 10 characters after the asterisk, * , it presents as a single character as shown by the displaced comma in the final post .. while the edit page definitely shows the actual typing .. presumably something to do with the way the engine behind PPRuNe-style boards strips "unwanted" characters .. maybe I should go ask the whips in the Computer forum ...

.. and you can buy me a beer or ten for the polygon technique in due course ..

boeingbus2002,

.. will often ask for a new loadsheet with any change in Pax figures rather than LMCs ...

The whole idea of LMCs on a trimsheet is to avoid having to redraw the sheet. The sheet designer fudges the sheet envelope limits so that the errors are constrained within the AFM limits .. the errors in presented CG and stab trim for TO increase. The only reasonable reason for redoing the sheet is if a pilot desires a more accurate stab setting .. and this is rarely worth doing for a small LMC situation.

Looking at a Manual loadsheet for any aircraft type you can work out which zone the change occurs and extrapolate down to see the move in CG.

That's OK if the sheet isn't based on zone loadings (ie individual rows) .. if one uses zones, the presented CG values are in error to a greater or lesser extent due to the use of zone centroid arms. In the error analysis, the designer will usually constrain the presented envelope limits to account for the zone calculation errors. The end result is that a reverse calculation from the sheet and just looking at the particular zone trim line will not give an accurate moment change in most cases .. and the resulting CG (whether read from the MAC/CG overlay or calculated from the moment and weight) will be in error to a greater or lesser extent.

Jon Lei,

If one were to do such a thing .. then one ought to do the sums first to determine how far the CG is likely to shift .. the result, depending on Type, loading and a few other things, could well be very sweaty ...

cwatters,

.. a small group of ballet dancers or a slightly higher percentage of children than normal the throw the numbers out ....

The standard weights used for a given population should consider variation and the trimsheet designer should incorporate such things into the error analysis done as part of the sheet's design .. although I am well aware that many such designers do no such thing ... a bit of a worry at times ..

shgsaint,

... if it was found half way through a flight that the pax count was out by one I wouldn't be concerned at all ..

If the trimsheet incorporates LMCs (or one does a quick retrim) the effect can be assessed very rapidly and, should it be significant, the passenger distribution can be adjusted inflight to account.

Especially on a heavy where there maybe 8-14 cabin crew pacing up and down the length of the aircraft pulling and pushing trollies. What does that do to the CoG? Nothing that is noticable I imagine

Can have a significant effect on CG and, sometimes, the folk in the sharp end will amuse themselves watching the autopilot move the trim back and forth to maintain the trim speed or, if one is handflying, it can be a right pain for the pilot .. however, the loading system used should account for extreme passenger and crew movement so that the ACTUAL CG stays within the AFM limits during flight .. regardless of whatever the system presented CG values might say is happening.

MFS and Intruder tell it like it is .... doesn't matter what aircraft one is considering, one is not allowed to alter the inflight load configuration willy-nilly without accounting for the effect on the CG ... however, is in not just a matter of keeping oneself legally nice .. move too far on the CG front and one can easily embarrass oneself ...

Intruder,

At some point between 0 and 5 extras, the CG index will change by 1 unit

Just as a point of style, it is worth noting that the IU for a given load is a quite arbitrary value totally dependent on the design of the particular trimsheet .. however, the philosophy remains correct.

...the CG was the limiting factor, not the weight

This often is the case .. and it is a point of concern that many have little idea of the use limitations of trimsheets and other loading systems .. with the result that the CG can be caused to end up out of limits at some stage of the flight.

Not often that we get such interest in loading questions .. good to see ..