View Full Version : Weight and Balance C of G and Trim
8th Jul 2012, 14:41
just a question about the above in relation to recent trips i have made as a passenger. Having done a bit with trim sheets myself and knowing how much sometimes needs to be moved longitudinally to make a difference to the whole trim C of G issue it leaves me wondering when on a 767 with only 90 pax and a full underfloor freight load why all the passengers are forced into little blocks so close together then told it is all to do with Weight and Balance, C of G and trim... That said as soon as the seat belt signs go off they then allow a free for all as people charge about trying to secure more space for themselves... Not a complaint as such but if some Purser/Loadie or dispatcher could take the time for a short (or long I really don't mind) explanation....
12th Jul 2012, 04:27
Among other parameters ( runway length, airpressure, air temperature, runway condition, wind speeds and wind direction, etc. ) the C.G. ( Center of Gravity ) is crucial for computing the stabilizer setting (if you look closely, you'll see that the horizontal stabilizers at the aircraft tail aren't fixed - the can be 'swiveled' within a certain range) , the speed at which the crew can abort the take-off run with enough runway left to bring the plane to a stop, the speed at which they can rotate ( get airborne ) safely, and the speed at which they have to rotate to avoid making a hole in the ground.
Moving about during the take-off run would muck up these careful computations , in the worst case to a point where safety would be adversely effected. Take-off speed is comparatively slow compared to cruising speed, so once the plane has reached a certain altitude and speed, the C.G. isn' t that crucial anymore - in fact it's shifting continously while in flight as fuel is burned off; the stabilizers will offset that shift by repositioning themselves.
There have been incidents with cargo planes, where cargo had torn loose during take-off, the sudden shift in C.G. causing tail strikes.
16th Jul 2012, 06:12
OK of all the views and only 1 answer i wonder if i posted this in the wrong place? Anyway thanks for your reply. It wasn't really the answer i was looking for. I do understand about trim and the setting of, for example, TPI or the equivalent, also that at higher speeds control surfaces being more responsive to the airflow across them. That said it was more in relation to how much the few passengers crammed together would affect it. If they had spread them out a bit more the trim calculations could still have been done and it seemed that the total effect or that movement wouldn't put the aircraft out of (or make it unable to be trimmed) IMHO it seemed it was being done for ease to the dispatchers/movers rather than for the comfort and service to passengers...
Also i'm not entirely sure the stabiliser/TPI settings will have a huge affect on V1 or the point from which you can abort a take off....
16th Jul 2012, 10:05
Also i'm not entirely sure the stabiliser/TPI settings will have a huge affect on V1 or the point from which you can abort a take off....
It does. A more rearward CogG reduces the amount of rear stabiliser trim required. This reduces drag. The benefit of this is two fold. Firstly aircraft will burn less fuel over the course of the flight (assuming pax don't all move to the opposite end). Secondly it affects V1 because it means more derate or flex is available (up to the regulatory max of 25% rated thrust). Less thrust equals a lower VMCG (minimum control speed on the ground with aerodynamic controls used only), and thus a lower V1.
Each takeoff is planed to have the most critical engine fail at the most critical point (V1), and the aircraft performance/ loading/C.G planned to give the best feasible handling/performance characteristics should it actually happen.
With regards to landing, the allowable trim range is actually larger forvsome aircraft. A larger wing area (flaps) and more energy into the G/A (at Vapp or Vref which are higher than V1/VR). My beast has the same limits, but as weight increases the allowable range increases.
As for each pax seat making a difference, it does. Every item has a balance arm and a weight associated. The balance arms are all known, but the weights are either known of assumed(pax/bag weight). Should the catering configuration change, or another crew member joins us, the load sheet start point of Dry Operating Weight and Dry Operating Index( the trim figure) need adjusting. As the plane is loaded up with pax and bags and fuel in differing locations, the load needs tracking to make sure it doesn't exeed any limitsin weight, or balance. For the loading of pax, the night before a department called load control will look at the booked load, and typical number of bags that get checked in. If its a light load, seat rows are blocked off to prevent a to far forward trim occurring as pax request seats "as far forward as possible". To make this task simple, seat rows are divided into bays. So when you next ask a cabin crew member to move, she may well have to check with the flight deck first. If its ok, they may well specifiy the seat rows I which you can move to. These seat rows form on of the bays.
Hope that helps.
P.s. I forgot to Mention that airlines can take a performance credit by using a more aft forward CG limit. This enables an 'improved climb' performance to be gained by using some of the runway length acquired as a result of less drag. This enables more energy to be taken into the second segment of the 'Net Take-off Flight Path' used for performance regulations/calculations. This can result in less obstacle limits or a greater climb limited weight(WAT limit). We always look to use improved climb, using excess runway for greater energy. To do this we try where not a full load, to seat pax towards the rear. The excess runway and less stab trim drag means less flap (drag) and more speed on the excess Tarmac. Thus more energy. Where runway excess length won't permit a full flex or derate, we actually use more thrust to enable less flap use and higher speeds. Obviously this can only be done to a certain limit, depending on runway length, wind, temp, QHN and dry/wet. Using full flex or derate actually burns more fuel in the long run, but lower EGTs prolong shelf life of engines and maintenance costs. It's a balancing act between the two. airlines don't just blindly seat pax at the rear to pee them off.
I did try to upload some diagrams, but computer said no. Sorry.
16th Jul 2012, 21:38
I remember a flight as SLF out of Newark in a Swissair A310 with about 40 to 50 pax only. For the takeoff everybody was seated in a bloc, even the people in C had to move to economy until the seat belt sign was off. Might be some models are more or less prone to this problem?
17th Jul 2012, 01:55
that will take some time to wade through. Some of the refs you made are unfamiliar so i will take time and work it out. I guess some aircraft must be more prone....the carriers that allow free boarding obviously take it into account and the 4 jet I fly certainly doesn't have the same issues with the moment arm of individual passengers. We traditionally were kind of free boarded but not quite in a Ryan Air/EasyJet style.... Anyway..i will be approaching the despatchers for the carrier involved for their answers to my question and see if it is as comprehensive as you have eluded to here. Thanks for your time.....
17th Jul 2012, 09:04
As one who spent some 8 years on loadsheets, some of the posts above reminded me of one fairly regular airline that diverted to Luton that always insisted on the aircraft being trimmed right on the aft limit. The aircraft was a B707 and as I thought it was an unusual request, I asked a couple of the Captains why. The simple answer was "it's easier to fly." I never did find out what the actual technical reason was, but accepted the answer and their aircraft were always trimmed to the aft limit when possible.
18th Jul 2012, 00:06
No probs at all axel. Larger aircraft that can 'containerise' using ULDs in the lower hold, are less prone to pax distribution as the majority of the weight is spread out in the cargo hold over a larger area. Thus pax distribution is less of an issue. Most bags fit into a few ULDs. The rest of the space is filled with cargo containers. Think horse sperm to tomatoes to sports cars. There is far more revenue in the under floor, champagne free zone, than in the F/J cabins. Route/Flt/carrier depending. 737/a320s typically have 4 cargo zones to load bags in loose form, or containers for some 320s. loads spread evenly over a larger area underneath tend to reduce pax trim sensitivity.
As to the 707, it was probably easier to rotate with an aft trim limit. Handling for an EFATO (engine failure after take off) is better, in general terms, with a forward trim limit. Looking at old CFS notes from the RAF Little Risslington, the dart analogy is used. A dart in sport has all the weight loaded to the forward end of the implement. This aids it's stability through the air in 'free fall trajectory', yaw moments are more easily cancelled out with a longer moment arm from the rudder. Optimum CG is a balance between fuel burn/engine wear/safe handling(limited by regulation so not to be abused my MoL et al) and aircraft limits (usually super deeded by regulation). It varies by type, operator config, runways, weather, aviation authorities.
Secondly it affects V1 because it means more derate or flex is available (up to the regulatory max of 25% rated thrust). Less thrust equals a lower VMCG (minimum control speed on the ground with aerodynamic controls used only), and thus a lower V1. For the majority of cases it has no affect on V1 primarily because the V1 is based on a fixed CG position, the only time that you will see a change is if you use an Alternate Forward CG AFM appendix.
using a more aft forward CG limit. This enables an 'improved climb' performance to be gained by using some of the runway length acquired as a result of less drag Why are you associating Alternate Forward CG position with Improved Climb? We have used Improved Climb since the B707 and that aircraft certainly didnt have an alternate forward CG.
The OP stated that the pax were in little groups, he didnt say if those groups were forward or aft.
18th Jul 2012, 11:51
For the majority of cases it has no affect on V1 primarily because the V1 is based on a fixed CG position, the only time that you will see a change is if you use an Alternate Forward CG AFM appendix.
Hi Mutt. It depends if your company permits the use of semi-permanent or 'fixed' de-rates, such as TO1/2/3 etc. for those that don't and use TO or D-TO you are correct. Some airlines believe it to be to 'risky' to use fixed derates.
As to the CG and improved climb. You are right. It's been around a long time. these days lo-co EU operators look to gain every single perf advantage and bundle them together for a higher PLTOM or full flex and min flap. Hence why I answered in that way. But yes. You can certainly do an improved climb with a regular, or even fully forward CG. You would however burn more fuel. Different operators, types, same runways, different ideas.
As we don't know the type or where they are sat in the OP's case, this is intended to be a general sweeping comment on the way some, not all, lo-cos go about arranging their aircraft for a cost saving takeoff. If your company uses CARD or ACARS for perf calcs, try using fixed derates on different intersections to see the whole range of possible V1s. A 'mean' V1 is usually settled on by the system, depending on whether it's octopus,CARD, navtech, EPOC etc etc
company permits the use of semi-permanent or 'fixed' de-rates, such as TO1/2/3 We use both fixed derates and assumed/flex..... but we dont use alternate forward CG, so regardless of the CG position, the V-speeds stay the same. Therefore i would say that the thrust has a direct impact on the V-speeds, the CG doesnt.
A 'mean' V1 is usually settled on by the system, depending on whether it's octopus,CARD, navtech, EPOC etc etc By "mean" do you mean "Balanced"? Actually i would have thought that whatever software you use would give the same answer based on the users selected parameters. As all of the GUI's have to go back to the STAS software, the answer should be the same.
18th Jul 2012, 17:46
I doubt the real reason was W&B, as most carriers would use loading zones and average pax weights anyway, with a lot of margin (actually is proven in the master W&B manual supplied by the manufacturer). However, it is much easier to get the few pax through the exits in case of an emergency as a group sitting together than scattered around the plane.
18th Jul 2012, 22:25
Thanks it seems to have opened a few thoughts all of which i find very interesting...my thoughts at the time were akin to Hunters offering. That said when i started getting answers i started to believe i was being a pessimist and there really was a good reason for it......:E
19th Jul 2012, 08:39
Airbus produced performance data is compliant with JAR-OPS 25 (ok now EU-OPS) at all times. In fact, there is nothing particularly odd about the assumptions made within the calculations. What is slightly unusual is the notion that for any given weight there are a range of take-off speeds that comply with ‘the law'. Picking the best speeds within this range is the job of OCTOPUS (the programme that uses algorithmic calculations to produce our performance data).
x OCTOPUS would like us to lift the most amount of weight (mass, actually) off the runway and it calculates Take-Off Performance Limit or TOPL not structural weights;
x It uses the excess margin to produce the highest FLEX figure possible. FLEX, or rather lack of FLEX, is so important to us (read costly!) that the Airbus Fleet uses 1 degree FLEX increments whereas other fleets only use 2 degree steps.
Pages in the performance manual are produced by OCTOPUS – they are not necessarily performance numbers based directly upon the stall speed but on a range of ratios of V1 to Vr or Vs to V2. In this way they are identical to the numbers produced by a CARD request.
If OCTOPUS cannot lift the maximum TOPL from a runway at ‘standard' speeds (i.e. a V2 defined by a fixed ratio on stall speed) then it will use excess runway to to push up V2 in order to improve second segment climb performance (essentially higher speed = less induced drag).
Obviously, this is only useful on runways that are more than long enough for the Airbus. Fortunately for us this applies to nearly every runway on the Airbus route network – therefore improved climbs are ‘the norm'.
There are a range of possible V1s before we consider the take off calculations. On the Airbus CARD and OCTOPUS always calculate the MAX and MIN V1s but we publish (and therefore use) a numerical mean value called MEAN V1.
At the upper end of the range we might be limited by brake energy or runway length before Vr becomes a limiting parameter. At the lower end we might be limited by the energy requirements of the ‘Go-case' considerations – remember from V1 we must be able to stop but we must also be able to go having suffered an engine failure.
When we are very light we can stop and go with ease, we have lots of excess power (even on one engine) and oodles of brake energy. OCTOPUS calculates the range of V1s and finds that we are limited by Vmcg (we can go) and Vr (we can stop). This range is nearly as wide as it can be and therefore MEAN V1 – the value that we use and CARD publishes is between Vmcg and Vr. A Split.
At high weight the picture is different – we can still stop from Vr (even using high, optimised speeds) because we have very powerful brakes but it is not possible to reach those high speeds with only 50% of our thrust remaining. Therefore we reach the following scenario. There is now only a small spread of speeds from which to take our average V1. In this case we are left with little or no V1/Vr split.
In summary, our spread of possible V1s is constrained by the need to go as well as stop. Our fleet policy is to calculate and publish the mean V1. This balances the risks associated with low speed GO (from min V1) against those of a high speed STOP. It is not possible to second guess CARD published V1s and therefore the V1s must be respected however small or large the V1/Vr split.
Hence the term mean V1. As to CG affect on V speeds, they don't. The CG does however affect our ASDR and thus the flex range. Hence why we input it on my fleet. Balanced V speeds are based on Balanced fields with no consideration for obstacle or climb limits.
19th Jul 2012, 15:37
B767 Forward Hold has a large door allowing a mix of containers and cargo pallets to be carried. The Rear Hold has a small door, only accepting containers. This can make for an interesting very nose heavy situation when only a light passenger load is booked. Restricting passenger seating to the rear of the aircraft for take off and landing, is often the only way to resolve this situation