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 plane 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.

[rammel]

On the 737 when you have a light pax and cargo load to get the a/c in trim you need to seat most of the pax down towards the rear of the a/c. The cargo doesn't get loaded on one side or the other it just goes in compartments underneath. Hope this helps with your question.

[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.

[Buster Hyman]

JT seems to have covered much of what I may have put in. All I can add is that we often used to block rows out for trim to avoid what happened to Matthewc. Confused the hell out of the pax, but was always a last resort. Never rely on pax seating for trim, always an unknown factor as they tend to move about, even if told by the seagulls not to.

Another thought was perhaps the acft had a fuel problem on one side & couldn't centre it? Highly unlikely I know & I've never loaded pax laterally for anything like that, but something perhaps to consider.

[Eastwest Loco]

Buster - manual load and trim systems never had an allowance for lateral fuel load discrepancy from DHT right through 727-276- I would be interested to hear if the computerised systems allow for this.

Load control is generally worked on a passenger by zone theory, and a good loco will ensure the forward and rear zones are filled first and the no-trim zone above the MAC filled last. I am sure todays flight editors are capable of this.

One can only imagine late cargo or a misload by the bag snatchers caused this - or the systems are not as good as we thought.

Best all

EWL

[Buster Hyman]

Hi Loco. As I said, it's not something I've come across & I never really had a fuel trim problem, but there could be, in some systems, the facility to accommodate it. The only one I recall is tail fuel on the 744. Never used it ex MEL, except when pumps weren't working on an MH 744 & the thing still had fuel up there...nasty to work around.

I agree, the electric ones worked by zones too, but I mainly worked on the widebodies, not those tiny Boeings & their like!!!

[john_tullamarine]

One would need to know the details on a case by case basis .. but to figure a computer-based trim by zones is pointless and loses the convenience of the computerised calculation. I would be very surprised to see the internal calcs done on a zone basis and, having paid a bunch of dollars for such a system, I would be dealing very severely with the software people. The output, however, might very well be presented on a zone basis either for compatibility with manual systems or just PSO/cabin crew convenience .. but that is quite a different animal.

For a trimsheet, the use of zones is only adopted to keep the physical size of the sheet and the number of trim lines under control. Every time the designer opts for zones, the envelope limits get compressed in the error analysis ..... and the operator's flexibility in loading reduces ....

[rammel]

On QF if we have to do a manual trim sheet it is done by the pax zone breakup, if it is computer generated the trim is done by rows. The people on check in normally seat everone fwd as everyone wants a fwd aisle seat. I haven't heard of lateral imbalance due to the fuel load or part of the fuel system U/S but I would imagine if the system did have a problem engineering would be required to fix it. I do know that on the 767 if ld3 (ake type) cans are used then there is an adjustment to the mtow by subtracting 150kg for each ake used, due to lateral imbalance.

[john_tullamarine]

Rammel,

Your can adjustment for the 76 has me tossed completely.

(a) don't the cans get run over a weighbridge prior to loading ? .. ie any tare adjustment is figured automatically in the procedure

(b) not having an underfloor plan to hand I am in the depths of ignorance here .... are the LD3s loaded symmetrically ? ... and why the need for a MTOW adjustment to meet a lateral requirement ?

Would be quite interested in your comments ... perhaps I might have to give one of the weights engineers a call to lift the cloud of confusion in my tiny brain .....

[Rayford Steele]

The QUBE DCS system which does the weight + balance does not care about lateral stability.
It does however check-in after using rows 3-7 first it then starts at row 23EF then 22AB then 21EF going forward. It will try to put the same weight on each side of the aircraft.
However at the end the Load Controller is presented with a Weight for 3 pax zones in the 737 based on front to back situation. There is no data for side to side seating config.
The 737-300/400 needs weight in the tail to trim it nicely - it is hard to go outside the aft CoG but is easy to go out the forward CoG.
If there are not much baggage to stow in the aft hold the only thing left for the load controller is to walk pax to rear of the aircraft.
On a light load (<30pax) I don't allow any seating in rows 1-7.
Also if the flight is running late and the loaders have stuffed up the bags by putting them in the wrong hold (happens all the time - they don't listen or understand plain english!) then the quickest solution is to move pax and get the flight out.

[Checkboard]

I was a passenger on a United Airlines flight from Hawaii to Sydney some years ago, on a 747SP.

Just before pushback, they announced a trim problem, and needed volunteers to move from economy to Business class! Being quick on my feet, I made it to business - all well and good.

Once the take-off was completed though, they dragged all the volunteers back to economy! For the landing? Yep- moved us all up again.

Now if the aircraft was out of trim for take-off, and out of trim for landing, what was happening during all those hours of cruise!!

[Buster Hyman]

DOH!!! Touche' John T...of course the computer calculates by row, I realised my error as I was thinking of the end result. The printed loadsheet displays pax loading by zones.

As for the 767, unlike the A300, you cannot load AKE's (LD3's) side by side. The hold is too narrow. From memory, I think it was LD2's only or pallets (enclosed types as well). Then again, you could've bought them with narrow FWD doors & an engineers seat, that really made them good value!! NOT! I'm trying hard to remember, but I'm unsure about lateral calculations when using LD3's. Most definately on the 74F's & Combi's, that was fun!

Checkboard, cruise is not as demanding on the aircrafts trim as takeoff & landing. Balance is everything during those latter two, however, in cruise, it's just a matter of setting the stab trim to accommodate the CoG. In your case, I'd say they were burning more than efficient amounts of fuel on that leg as it would've been tail heavy & required plenty of trim.

[Eastwest Loco]

Buster

You are correct with the can types.

One of the main reasons TN accepted the A300B4 over the 762 was they could carry LD3 cans side by side where the 762 was limited to LD2's.

This gave them an immediate advantage with International freight forwarders as cans could be transhipped with no repacking or double handling, and we all know freight pays better than passengers. Wise move too. They served long and well and are now converted to freighters for Fedex I believe.

Still have a "Now there's a big difference" T shirt somewhere in the polishing rag bag.

Best all

EWL

[Dog_Bones]

There are a number of issues here, and some good material is available if one is interested in the subject. The 733/4 is loaded by zone and there is little to be gained by swapping a few people foward/aft except in take-off & landing modes. With the greater flap settings the forward CoG can become critical. Higher landing speeds with less flap will be required when forward limits are exceeded; additinally, with fuel burn considerations, landings into short strips can be interesting. Cruise is therefore not as critical.
Early days in the larger airplanes were also interesting as it was found that an aft CoG was able to provide an extra couple of knots in cruise - all flying elevators gave a little more lift than calculated and the airplane sat up & was quicker than predicted. This was useful in getting better economics when used correctly.
We all remember that while the DC3 had a CoG of 23% MAC they used to fly quicker at 18% - & a couple of knots in this beast was more noticeable than in a modern airplane.

[john_tullamarine]

Dog_Bones ...

(a) one hopes that people don't operate the aircraft outside the envelope

(b) moving a few bodies from the back to the front or vice versa makes more than a moderate change to the total moment and loaded cg

(c) provided that the aircraft is kept within the approved limits there ought to be no handling considerations of any note ... apart from the tail scrape potential on the 734 if mishandled the two are pussycats ...

(d) the speed vs cg consideration is principally to do with tail trim drag ... minimising balance loads helps the beastie along ...

(e) only had a few hours in the DC3 .. but can't recall it being faster at forward cg ?

[Balanced Moment]

It would be interesting to know how South West gets around the issue of trim as they have a very liberal seating policy. Unless things have changed recently they allow passengers to seat themselves - a "first in, best seated" policy if you like. No doubt they've found a happy medium between buffering the CG envelope and allowing flexibility in seating.

On the issue of lateral imbalance, the 762ER, 763ER and 744 aircraft all have lateral imbalance limits. For the most part the limits only apply when operating in a very small region of the CG envelope near the MTOW. The region can be so small in fact that some operators choose to exclude it from the envelope altogether, and forgo any issues associated with having to consider it.

If operating in the lateral imbalance region of the envelope and the lateral imbalance limit is exceeded, there is a variable weight penalty applied to the maximum taxi weight. The limit itself isn't imposed for aerodynamic reasons but rather for taxiing reasons - and hence the penalty to taxi weight and not take-off weight. My understanding is that FAR25 aircraft are certified to fly with one wing tank full of fuel and the other empty. Any lateral imbalance due to loading would be insignificant compared to the above ... one would have thought.

[Lurk R]

Out of curiosity, I can see that somebody being transferred from row 1 to row 13 naturally knows they can't be too upset when they're travelling on a staff ticket. What about when it's a regular punter who has forked out the money to pay for a row 1 seat? Do the bags get moved instead of the passenger?

[john_tullamarine]

Balanced Moment,

Thanks for that ... I'm now briefed re the lateral limits and MTOW ... makes the earlier comment quite understandable.

I'm not aware of any requirement for such a severe design case as you suggest ... the normal limits are associated with lateral handling qualities .. particularly for takeoff and landing .. to ensure that there is enough aileron authority in crosswind conditions.

As for SW ... as you suggest .. if one restricts the limits and, if necessary, ballasts the aircraft to give a reasonable passenger flexibility .. so be it ....

I would be more interested ... with part loads ... what do they do to give the pilots some indication of stab trim for takeoff ? I guess that there is a cabin crew protocol to figure a pax distribution from which the tech crew can figure a reasonable cg... only a concern with a severe mistrim .. but the case of a very aft cg with a trim set for forward cg makes for an interesting departure in a 73....

[Balanced Moment]

Luke R,

Passengers don't actually pay for their seating position. They pay for their class of travel ie. first, business or economy. If your scenario was to occur (assuming a two class cabin) passengers seated in the first row(s) of economy would be asked to move aft. If the problem still existed passengers in business might be asked to move to the back of their class, or alternatively (as Checkboard discovered in reverse) into economy. They would sit there for take-off and landing but spend the majority of the flight in their ticketed class.

J_T,

I've always been curious as to how Southwest structured their loading system but have never been able to find anyone who could cast light on the subject. The stabtrim issue is an interesting one - I'm not sure how they've solved it. Maybe another posted could enlighten us.

[john_tullamarine]

Balanced Moment,

I have no knowledge of how SW operate their aircraft on the line but I ran up a trimsheet system for the proud operator of a couple of ex SW aircraft several years ago .... the original SW system was entirely conventional ... and would have been fine ... except the new operator's other aircraft had a stock Boeing system and they wanted the same presentation .... I guess my quote was better than Boeing's ...