Weight and balance anomalies

[Tony_EM]

In my last year of my job, I qualified as a weight and balance trainer as well as retaining my position in daily ops. As part of the intended training program, I was going to do a presentation on the potential consequences of wgt/bal and loading errors. Despite the fact that I have long since left that job, I still have a rather insatiable curiosity and a secret desire to use my experience to either write a book or produce a training aid. The real shame is that at the time, I was not aware of PPRuNe and what an invaluable resource it would have been.

So my questions are;

Have you ever experienced any control anomalies due to inaccurate CG calculation or weight discrepency?

If so, how did those anomalies manifest?

Did any of those errors/anomalies pose any threat to the safety of the flight, that is, did it produce an actual threat, or produce a situation that could have lead to a potential threat?

Did you ever notice any difference in aircraft handling, especially in pitch sensitivity, or lack thereof, due to varying CG locations. i.e. CG at forward or rear limit, and what type of effects did they produce (Note, I am aware that a nose heavy trim can make elevator response 'slushy', while tail heavy will make it more sensitive, so I'm looking for anything beyond that like stability in turbulence etc).

How often have you noticed loadsheet, DG anomalies or procedural errors? (as a percentage of segments flown)

Any other loading or wgt/bal related stories would of course be appreciated.

Your indulgence and contributions will be greatly appreciated.

Note; any information will NOT be used for sensationalist media consumption or publication, though it may possibly be used in a book regarding airline ops or as a training aid for load control, subject to permission from each individual contribution. Any information which is considered sensitive can be sent via PM or E-mail. I do realise that the gutter press try to draw out material for distortion, so no offence taken if you want to keep it to yourself.

I'll kick it off by recounting a story told to me by a 733 jock;

Apparently, the load controller had instructed the ramp to load the 2000kgs of bags in hold 1 and 200kgs of cargo in hold 3, yet somehow the bags ended up in the rear hold while the cargo went to the front. Unfortunately, it showed on the loadsheet as he originally intended with the resulting (erroneous) CG close to the forward trim limit. The flight crew set their stab position according to this which resulted in a severe pitch up at rotation. Having passed V1, they were committed to continue the take-off but only after this jock had to jam his knee behind the control column to prevent a tail strike and stall until he could wind the trim forward.

Now I know this could have been deliberately exaggerated by this pilot to keep us on our toes, but either way, it shocked a few into paying more attention and making more checks before releasing their flights, so I always recounted it in training as it was told to me. If anyone could confirm that the control column forces can reach such severe levels when the stab trim is set so far from its proper position, I'd be grateful.

Thanks in advance,

Tony Manzur

[Genghis the Engineer]

I'm not sure if this is exactly what you're after, but I've a fair bit of experience of evaluating the CG limits in light aircraft, inevitably this has meant that I've seen the same effects that getting it wrong in service would throw up. Specifics I've come across...

Too far aft CG
- Increased risk of LOC post-stall
- Pitch divergence with flaps selected.
- Increased Dutch Roll tendencies.
- Difficulty in fine pitch control, particularly following pitching manoeuvre (such as rotation)
- Tendency to inadvertently overcontrol at high or low speed ends of envelope (increasing risk of overspeed or inadvertent stall).
- Imprecise speed control.

Too far fwd CG
- Increased stalling speed.
- Inability to flare (3-point landings are not much fun with a nosewheel)
- Tendency to phugoid (porpoise).
- Difficulty in establishing trim.
- (Also structural overload of the tailplane, but this is unlikely to be noticed by aircrew unless it's got REALLY badly wrong).


Not all of these will apply to a modern airliner, but some certainly might.

G

[john_tullamarine]

This is a concern especially in the freighter community and there have been numerous screw-ups with misloading .. some leading to serious mishap... one of the reasons that many freighters have onboard oleo pressure sensing (or similar) weight and balance computers. These don't necessarily give an accurate answer but variations from the "normal" error levels provide sufficient warning to the crew to go back, pull the cans, and run them over the weighbridge again ....

Might I suggest that the moderators consider moving this thread to Freight Dogs as I suspect you will find numerous anecdotes with which to terrify your book-reading audience.

[Tony_EM]

Thank you guys.

As usual, good answers lead to more questions;

Genghis, some great info there.

In the section 'too far aft CG';

LOC post stall - I would assume would be the result of the CL moving forward and past the CG during the stall.
I have experimented on CG positions (often inadvertantly) with R/C model aircraft that I build. Some of the aerobatic slope soarers work best with an aft CG, sometimes causing divergent pitch tendancies and in the extreme causing such LOC once it was allowed to enter a stall. In most cases, relative size of the H/stab was a factor in LOC or recovery.

Divergent pitch during flap extension - would that be the result of the increased camber adding a pitch rotation effect to an already marginal pitch stability condition? (see F27 G-CHNL ref below)

I assume dutch roll is specific to swept wing. Do you think (some) yaw damper corrections could become inaffective in extreme conditions?

Imprecise speed control - Is that related to pitch instability or inability to maintain altitude?

Too far forward CG;
-------------------------
Increased stalling speed - do aircraft operating manuals quote different (higher) stall speeds for forward CG positions?

Thanks again.

This F27 crash (http://www.dft.gov.uk/stellent/groups/dft_avsafety/documents/page/dft_avsafety_502780.hcsp) displayed some of the characteristics you describe, yet the uncontrollable pitch up they experienced caused the cargo to shift further aft making any recovery impossible. Interesting that the condition lay hidden until they lowered flaps on approach. I heard of a similar accident involving a Beech 1900D (not the recent one at Charlotte which was probably caused by a mis-rigged elevater cable) where the pitch became divergent once they lowered the flaps, although the retained some semblance of pitch control.

John, yes, cargo ops seem to suffer more incidents, possible because the lack of dedicated load controllers in some cargo ops. The Fine Air DC8 was a combination of loading error, possible badly secured load again, and subsequent incorrect stab angle, yet it is likely that the CG was not actually beyond the aft limit, but just that the crew did not have the elevater/pitch authority to overcome the effects of the stab angle and could not reel the trim in before LOC and ground impact.

I am also familiar with the circumstances of the Valujet crash in the gladesand one of the reasons I detested our companies total lack of DG training. What I am really looking for is incidents related to commercial passenger ops where the expertise of the crew overcame the problem, either by detecting it or taking specific action to compensate. Regardless, all information, suggestions and anecdotes are gratefully appreciated.

[Black Baron]

Wasn't there a plane full of schoolkids ( over Hawaii?) that ran into trouble with an aft C of G?
Pilot got all the kids to come and look at the nice and shiny cockpit and brought the aircraft back into balance.
:ok:

[Genghis the Engineer]

LOC post stall - I would assume would be the result of the CL moving forward and past the CG during the stall.

This is generally a progressive characteristic. Stall itself is clearly a LOC in it's own right, but should nonetheless show benign and predictable characteristics. Moving CG aft gives shallower static stability in the pitch and yaw axes, which means that the likelihood of the aircraft behaving outside of acceptable departure limits (wing drop, spin tendency) increases with aft CG.


Divergent pitch during flap extension - would that be the result of the increased camber adding a pitch rotation effect to an already marginal pitch stability condition? (see F27 G-CHNL ref below)

We attributed this to the flap deployment moving CP relative to the CG and creating a static instability.


I assume dutch roll is specific to swept wing. Do you think (some) yaw damper corrections could become inaffective in extreme conditions?

DR is universal and by no means restricted to swept wing aeroplanes (I can think of one motorglider of my acquaintance that does it constantly). If you shift CG, you are most likely in my opinion reducing the directional stability whilst leaving the lateral stability unchanged. Since the roll:yaw ratio is a function of the ratio of lateral to directional static stabilties, and also a design parameter in any damping system, this can render the dampers relatively ineffective.

Imprecise speed control - Is that related to pitch instability or inability to maintain altitude?

As CG moves aft, the curve of control force or displacement versus pitch change becomes more shallow. This means that any FCMC (Flight Control Mechanical Characteristics), particularly stiction and freeplay become more pronounced in their effects upon the aeroplane. This impacts on speed, attitude and height holding - the three are interrelated. So, an aircraft within limits might be able to hold speed to within (say) 2kn, yet out of aft this becomes ±10kn and a real problem.


Incidentally, although I don't have a copy to hand, I seem to recall a few tales of handling effects of inadvertent misloading in Ernest Gann's "Fate is the Hunter" (a very readable tale of the life of an early freight pilot from the 1920s to 1960s).

G

[411A]

Departed DHA some years ago in a B707, rolled what seemed like forever...11,500 feet on a 12,000 foot runway and climbed at 300 ft/min at V2 (all 4 engines operating)...

Found out upon landing SIN that an extra 15,000 pounds of cargo had been loaded in the aft hold (not reflected in the loadsheet)...was not pleased.
Neither was the Chief Pilot...and he mentioned that it would NOT happen again.

And...surprise surprise, it did not.

Good guys in charge...later on.

[Tony_EM]

Genghis, always a pleasure and privilege to learn from a pro, thank you very much.

411A, thats a nasty one indeed. Did the difference in CG location produce significant control problems (excessive stick forces)? Was the actual CG still in the envelope? And finally, did you ever find out how the error occurred? Thanks in advance.

[BoeingMEL]

I'll be happy to share experiences Tony... email or pm me? bm

[Tony_EM]

Hi Mike.

The interesting difference between cargo and pax ops is that cargo/mail ulds and items are specifically weighed while pax and most baggage use notional weight values, so you could say that cargo type errors are occasional while notional issues are systematic. However, there are plenty more aspects that make me agree with your view.

Of course CG limits are set by the manufacturer with that and many other things in mind, but I'm talking about the times when all the fat tourists are in the back of the plane while the jockey club fills the front, 50 suit carriers of business class baggage in the front hold and the 200 bulging y/c bags in the back. The CG is already at its aft limit (according to notional weights for pax and bags), and then someone mistakingly loads 1000kg of undocumented cargo in hold 5 while the load controller goofs the stab fuel because he wasn't trained for that aircraft type or wgt/bal computer system.

We all know that accidents happen when enough things coincide and conspire to produce a chain of events. It is also true that most ops in most airlines get it right most of the time and when they do make mistakes they are spotted and corrected most of the time. This still leaves other layers of protection like the resilience of the aircraft design to such abuse and ultimately to the flight crew that operate it.

Remember also that the crew rely on the CG location to set the stab trim and weight for performance data, so the actual CG location does not have to be outside of the envelope for the pilots to experience control problems if they are given the wrong location (Fine Air DC8) or the weight to above limits to have perormance issues (SQ tailscrape).


What I'm trying to research is the performance of load control and what sort of consequences it can have from the pilots' perspectives. Having worked in load control for a few years, I know that mistakes do happen and the scenario I describe above is one I have seen. The frustrating part is that we never got much feedback from flight crews, not even for that one, while the training did not deal with it at all. I've already talked to a number of pilots about this over the years, but every pilot has a different and usually enlightening perspective, so the bigger the sample the clearer the overall picture.

[sycamore]

Tony, you might also do a search for the SIA 747 got a/b 30 kts too slow as the loading figures were put into the FMC incorrectly.(Rumours ---tail-scrape-recently).

From personal exp, flying C-130, I picked up a full load of mil. frt. in Idaho to go to GooseBay, and we did all the figures , co`s leg, new co, not MJ( don`t recall he was young!), and of course as ever, we have a bunch of Trappers, trapping Trappers trapping us, as is usual in the RAF.
We get a/b ok and climb out towards F230, but at about F150 I begin to get a feeling it`s not working out,so we level out and let Albert get up to cruise.I do a little flying and can feel it is rather sluggish. So, we all go back through the paperwork, perf etc etc, with all the trappers looking-on, and we decide we are about 12000 lbs overweight.The navigator is busy with his box of bones, and we decide to continue , gaining height where we can; fuel is ok, as normally the graphs are pessimistic.Reaching that place G-B, we offload all the freight for a re-weigh, which is not popular, but in the end it is justified by every other pallet, about 4000 lbs different from the manifest. This was because there were two lines loading freight, one US, and one UK.

Any way I won`t embarass our Allies, and fortunately we were first in the trail. Fortunately, also, that Albert can take quite an overload, and still fly safely,as when we had tankers they were operated up to 20000 over normal maximum.

:ok:

[Tony_EM]

Mike, some very usefull information, thank you

Some points;

When I said occasional, it was not a comment reflecting how often I thought errors occurred, but the fact that the system used for cargo weights does not inherently produce innacuracies like the notional pax/baggage weights do. On your point that cargo errors are not occasional, I completely agree.

Free seating is something I have experience with, but in those cases, it still involved wgt/bal of the pax. This is done by the load controller boarding the aircraft and physically counting the number of pax in each seating bay. This is obviously no problem for a fully loaded aircraft and for loads over 1/2 full, it is easier to count empty seats. It is sometimes possible to ask the cabin crew to help count if their other duties allow. Not impossible and AFAIK, obligatory.

Smaller aircraft are not typically more resilient to CG errors as a percentage of MAC, and in fact are more sensitive to individual errors because they usually represent a bigger percentage of the load; Sticking 20 bags in hold 5 ISO hold 1 for a 747 wouldn't even be noticable to the stick force, while a similar mistake could make all the difference to a Beech 1900D in terms of pitch stability.

"For example, specified maximum turnround times should not be so tight that service providers can only meet them by taking health and safety shortcuts."

That has to be the most abused statement I've ever read, mainly because airlines and especially handling companies routinely provide insufficient ramp staff to complete all the tasks involved in a turnaround. 45 minutes doesn't sound too bad for a full in/full out loose loaded 757 with a typical on/off load of 300 bags and 5000kg of cargo until you see only 3 ramp staff attempt it. I've done flights with NO ramp staff at all, having to complete all the tasks myself (for which I was neither trained nor insured but was still expected to do). I would say that 75% of turnarounds I 'controlled' had insufficient staff to reasonably complete all the tasks required within the specified turnaround times, with about 10% qualifying as negligent in terms of safety.

The information on the "Data Plus" is very useful and is something I will follow up on. That CAP 642 reccomendation should be mantatory since I have seen Aviance management tell load controllers to use A300 loadplans to communicate loads for a DC10, in fact not only that, they also told them to calculate the DC10's CG using the A300 trimsheet too.
Thanks again for your input. I'll throw this topic into the Freight forum when it has run its course on this one.

Sycamore, a very interesting situation, to say the least. I have seen very similar ones which led me to allways compare the manifest weight of the total cargo (compiled by the shipper) with the actual recorded weight from the cargo shed scales. Any discrepency of about 500kg or more (depending on operating aircraft) would result in me chopping the freight or having it re-weighed if time allowed. I gather that your loads were not independently weighed, so you only had the manifest to go by. I met one ex-RAF loadmaster in my adventures and was thoroughly impressed by their professionalism and knowledge. But too many cooks.....

Your point about the Herc soaking up the abuse is a good one and I'll echo that by saying the 747 could be overloaded by 30 tonnes and still concievable get airborne, but the fun and games start when other things conspire to turn a manageable overload into the inability to maintain altitude even at sea level, like an engine out. Of course the Albert has the enviable ability of being able to drop the ramp and dump its load, something I'm sure some pilots would love to have for passenger cabins.

Thanks again for the info and pointers,

Tony

[Genghis the Engineer]

At the risk of being simplistic, it's CG that's the biggie for aircraft safety. There are usually quite large structural and performance margins (as some of the anecdotes above illustrate), but the handling margins for CG out of limits are often very small indeed.

G

And please nobody take this as a suggesttion from me that it's safe to overload an aircraft so long as you stay within CG limits. I'm just saying you are more likely to get away with it, not that you will !

[john_tullamarine]

Several thoughts ..

(a) errors are an inescapable consequence of that which we do so we, as pilots, engineers, whatever .... need to put sensible error trapping routines in place to minimise and mitigate both the errors and the magnitude of their consequences.

(b) I suggest that, overall, errors in pax ops will be more routine but of lesser magnitude than in freight ops due to the use of standard weights versus routinely running the can over a weighbridge ... but be very wary of bridge problems and errors.

(c) the loading system designer (of which I am one) routinely does, or should, build in some conservative fat to the system CG limits to account for reasonable error analysis considerations ... this involves a reasonable review of the operation's realities.

.. and, at the end of the day (or night) when it all turns to custard .. we rely on the driver to save the day. This is one reason why severe misloaded takeoffs ought to be an occasional sim exercise ... to allow the pilot to get a feel for the very unusual stick loads etc ...

[Tony_EM]

At the risk of being simplistic, it's CG that's the biggie for aircraft safety. There are usually quite large structural and performance margins (as some of the anecdotes above illustrate), but the handling margins for CG out of limits are often very small indeed.

Which is one of the reasons I am soliciting feedback from pilots on the consequences they have experienced. While the consequences of 'overweights' are relatively understood by most load controllers I worked with, the effects of CG beyond limits was not, even by those with more experience than average and notably by those that trained me and continue to do so. BTW, the simplistic view can often be the clearest. Thanks Genghis.

.. and, at the end of the day (or night) when it all turns to custard .. we rely on the driver to save the day.

This really cuts to the chase and again is a prime reason why feedback is so invaluable.

Regarding point (a); I used to judge load controllers by the way they spotted and rectified their mistakes more than how many they made, but ultimately on how many mistakes made it through to the flight deck.

Point (b) is duly noted. I only have pax ops experience. Although I am familiar with the technicalities of cargo ops, I can only go by the experience of others.

(c) My concerns arise from the fact that some airlines and especially handling agents are not reviewing operational realities since they are hell bent on covering them up at low/mid-management levels.

Many thanks for the input John

[Tony_EM]

While it is a requirement, it certainly does not mean that CG calcs are done by all. Turkmenistan springs to mind where the flight crews took no notice of any information we gave them, always setting the TO trim to the same mark regardless of what we gave them. They would just ask that each hold (front and back) shared whatever load weight and they'd literally 'wing' it from there.

As for our LCA's, to be honest, I really don't know, but I'm sure they use some method to safeguard from being out of CG limits, whether its ensuring that pax are evenly distributed on 1/3 to 2/3 loads (since very low or very high pax loads will give fairly predictable CG effects), whether they get the cabin crew to count the bay they are responsible for they or whether they use some other ingenious method would all be news to me. I would say that highly experienced flight crews with intimate knowledge of their aircraft type would be able to glance down the cabin and guesstimate the CG effect with some degree of accuracy.

The advantage of knowing where your pax will be seated is that it makes it possible for a load controller to place the hold loads so that a favourable CG location (in terms of fuel efficiency) can be attained where possible. Someone has obviously decided that the general fuel savings are insignificant compared to the shorter ground times, which mean lower airport charges and higher aircraft utilisation.

[Wot No Engines]

This may be worth reading (http://www.dft.gov.uk/stellent/groups/dft_avsafety/documents/page/dft_avsafety_502087.hcsp).

[Stud3]

Hi There.

Easyjet use a free seating policy because of the way 'us brits' work i.e we dont like being bunched up together, so we'll go where we'll get as much space as possible, usually as far away from others, so unknowlingly to them they'll distribute evenly throughout the cabin.

Weight and balance with easyjet is always done by a loadcontroller who will be passed information from the outbound crew, or if the crew were already on board the aircraft they'll pass that to the dispatcher who'll relay the information to the load controller such as the block fuel, burn, taxi, crew, DOW, DOI and any performance restrictions such as MTOW's etc.

Baggage is always placed into aft hold 4 (up to 3469kgs)on easyjets 733's, then any excess goes into forward hold 1.

Also, with regards to the headcount, i always get a headcount from the number 1 cabin crew before the doors closed, as a few weeks ago a whole flight had to get their boarding cards out to be checked because one of the gate staff had allowed someone through with a boarding card, so they thought. Turned out to be a calculation error with the gate staff as the number 1 had the correct number, but the gate staff didnt. :rolleyes:

Hope this helps

[Tony_EM]

Thanks WNE, looks like the pilot was on the ball during the incident but was apathetic to the CG conditions prior to the flight (attempt). This is fairly typical of GA ops where most PPLs assume that the aircraft is built to operate under any condition it is loaded under. Wgt/bal awareness should be imposed rather than suggested IMO.

Stud3, some very useful info, many thanks.

From your post; can I assume that specific wgt/bal calcs are not completed for each flight?

Some observations regarding LCA 733 ops;

The aircraft was designed and built with maximum ustilisation and ease of balance conditions in mind. That is to say that with a maximum cabin load, both holds could be used without causing extreme CG locations. However, the assumption was that part of the forward cabin would be business class with either bigger seat pitch, wider seats and/or typically less passenger volume. So when lo-cost airlines pack em in at the front just as many others do at the back, and the fact that the high density seating will effectively produce more pax forwrd of the CG postion than behind it, the policy of filling the rear cargo hold first makes sense.

Compare this with an MD87 where the front hold is virtually unuseable when the cabin is full despite there being weight available below MTOW, even if hold 4 has max weight.

I'm just wondering whether there are any procedures in place to prevent the 1 in a 1000 event where most of the pax sit in the back with lots of small but heavy bags in the rear hold? Or, at least some way of advising the pilots that they have an aircraft with the CG at the rear limit.

The 733 is a great aircraft and can take a lot of abuse wrt wgt/bal, but that is hardly an excuse to become complacent IMO.

[Stud3]

Hi Again.

A load sheet is produced before EVERY easyjet flight. This is checked by the load controller, the dispatcher has 11 checks that must be completed and the flight deck also check through this before the aircraft can push. These checks are:

From/TO/ Flight Number

A/C Reg - because each aircraft has its own DryOperatingWeight which is passed on so that it can be verified by the load controller and the flight deck crew

Crew - E.g. the norm is 2+3 on a 733

Date

Weight of Bags and Distribution

Passengers - Weight of them, whether its standard or actual

Traffic Load - total pax and bags

DOW

Take off Fuel or block fuel

Trip Fuel or Burn

DOI

These checks are done by the dispatcher, load controller and the flight deck crew.
The on board computers work out the trim etc. based on the above information given in the loadsheet electronically.

However if there are computer failures there are manual loadsheets which are completed by the dispatcher or captain. These manual loadsheets are used frequently with the likes of flybe's dash 8's, wideroe and eastern airways.

Basically a series of lines which come down to show if the aircraft will be in trim limits or if it will be unsafe. It is, however, very easy to go out of trim with a dash, than a 733 for example.

I'm just wondering whether there are any procedures in place to prevent the 1 in a 1000 event where most of the pax sit in the back with lots of small but heavy bags in the rear hold? Or, at least some way of advising the pilots that they have an aircraft with the CG at the rear limit.

Well im not sure to be totally honest, but im sure there was something about this brought up a while back with Ryannair, as if there is under say 70 pax then rows X and X cannot be used. However i am not totally sure on this but will try to find out for you.

[Tony_EM]

Thanks again Stud3, very helpful stuff.

Passengers - Weight of them, whether its standard or actual
Of course the biggest effect on CG location is the pax distribution so this aspect is the one that I am most interested in regarding LCA ops.

[Stud3]

I knew it was out there somewhere. Here's the thread with a bit more detail about how cabin crew deal with the aircrafts loadsheet saying its going to be out of trim

http://www.pprune.org/forums/showthread.php?s=&threadid=7965

:ok:

[Tony_EM]

Thanks Stud3, that clears it up perfectly.

[Tony_EM]

Maybe no specific information passed to the despatcher, but with set procedures, the lack of any notification would leave the assumption that each bay is loaded evenly, I guess.

It would be nice to think that companies do proper risk assessments and know that they rae not taking iresponsible risks while saving money in such ways.

It would also be nice to think that the authorities do more than just take their word when 'ensuring' that regulations and general standards are being maintained.

I know a couple of companies where dishonesty and deception has replaced responsibility and compliance with regulations. But until we know what the possible consequences of such loading/CG errors are, any risk assessment would be speculative at best.

Thanks again Mike.

[barit1]

CV440 taildragger (http://www.ntsb.gov/ntsb/brief.asp?ev_id=20050927X01539&key=1)