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Why don't aircraft weigh themselves?

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Why don't aircraft weigh themselves?

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Old 25th Feb 2008, 23:28
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The WABC (Weight And Balance Computer) system is ano option on most Airbus and Being aircraft. SOme of the 744s I used to fly had it. But we always ignored the WABC figures and went for the loadsheet figures. Flying out of JFK with a couple of hunderd 'average' Americans on board, with their "3kg" of hand baggage often showed the latter's futility. I have seen a TOW indicated on the WABC some 20 tonnes (yes, 20 tonnes!) greater than the loadsheet. And I have also seen the WABC say we were several tonnes over MTOW once.

The Airbus FBW aircraft measure the weight when airborme through the Flight Augmentation Computers (FACs) from data provided by the angle of attack. Our A321s are always a couple of tonnes out, always heavier. And from the way the aircraft performs, I suspect the FACs are correct. The FACs provide the protection speeds wheras the FMC provides the approach speed from the loadsheet weights input at the planning stage. Adding a few knots to Vapp is the norm.



PS. The last time I was notional weight, I was age 16. Time to change the notional weights methinks!
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Old 26th Feb 2008, 05:05
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I can get an up to date weight by accessing the appropriate page on the ACARS. type in GW, FQL,FQR,FFL,FFR and you get a current Gross weight, Fuel quantities in left and right tanks and left and right engine fuel flow
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Old 26th Feb 2008, 10:13
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Thanks.

So it's not economical, not reliable and not that useful.

Apart from that, it's a great idea!

I'm a bit surprised that it's so unattractive and difficult to make worthwhile. You're constantly moving known mass into and out of the a/c during refuelling and engine operation, giving a great opportunity for auto-calibration. Likewise, slopes are easy to detect and factor into the sums.

But I guess that the number of expensive accidents and fuel inefficiencies caused by weight discrepancies are too low to justify the expense of yet another system...

(This may change. I was talking to a chip maker about embedded supercomputers, where you effectively have a real-time physical model of your vehicle running on a few teraflops of cheap silicon inside the vehicle itself. As things change over time, the model spots non-optimal operations, models 'what-if' experiments on the fly and recommends optimising operational changes. The chip guy was of the opinion that this could compensate far better for things like control surface failures than pilots ever could. Well, maybe. But any system using such ideas would have to be very well instrumented, I'm sure to the extent that the weight and distribution would be accurately known at all times).

R
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Old 26th Feb 2008, 10:55
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I can get an up to date weight by accessing the appropriate page on the ACARS. type in GW, FQL,FQR,FFL,FFR and you get a current Gross weight
This works using the concept of garbage in, garbage out!

SLF, maybe airlines dont want to know the exact weight?

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Old 26th Feb 2008, 17:18
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I believe that when the FAC of the bus uses the angle of attack, it can only determine where is the cg regarding on the trim used to maintain that AoA at a given PA. You must take into account that during the flight the cg changes also with the cabin crew moving with the catering. For the weight, it is also involved the thrust, the speed, and the perf fac. Is it not?
I have heard that the only moment to check the cg is at the lift off. When you introduce the cg at the pre flight, you calculate a trim for the TO run. Someone told me that this cg must correspond with the one that the ap uses during the 2 segment. I don’t know if that is true.
I have been searching for a perf table within the OM to check the cg in flight. The only reference I have seen is under abnormal procedures, not reliable speed indications. Sorry if I missed the translation.
I would like to know if using the correct weight to the flight means some benefit to the airline. I suspect it doesn’t. A Spanish mayor airline, several years ago used to count the pax within the plane to check total with the loadsheet given by the gate desk. As more than 70% figures didn’t match (because bad count of the gate desk or crew) delaying the flight, the airline nowadays accepts the loadsheet as counted only at the gate. Does BA and other mayor airlines count pax OB?
Good flights
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Old 26th Feb 2008, 19:54
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STAN System

STAN system
wigglyamp says,
The system on the Merchantman worked by measuring the hydraulic pressure in each undercarriage strut using a transducer mounted at the top of the leg. The main inaccuracy came if the strut wasn't charged correctly or was leaking. The system was tested by the crew and gave a fixed weight and C of G value determined from set resistors in each transducer - this ensured the continuity of all the system wiring. It was still in use at AirBridge in the late 80's.

The system is still fitted to G-APEP at Brooklands & still works, unsure how accurate it is though!!

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Old 26th Feb 2008, 20:07
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read this article:

http://www.nlr-atsi.com/downloads/NLR-TP-2007-153.pdf
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Old 26th Feb 2008, 22:06
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Likewise, slopes are easy to detect and factor into the sums

not necessarily... one of the problems with a lot of transducers relates to sideloads on the cell. This is a big problem with routine maintenance aircraft weighings .. and can account for (an unpredictable) several hundred kilos per cell ...
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Old 26th Feb 2008, 23:52
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Good paper that. I remember when working for a cargo airline, Metro II, Metro III and Merlin IV. Dozens of times when I arrived to the plane the nose wheel was almost in the air and when I asked to the ramp agent, he always was sure the airplane was perfectly loaded. Reading the article, I can see that the heavy opposition comes from cargo operators. Another curiosity is that FAA requires a 1% of accuracy to the system. Therefore. FAA is assuming that only a 1% of error could be found on 100% of flights or that only a 1% of the flights do not comply with the loadsheet?… Boy, we don’t live in the same world. Probably all of us have seen those small tractors to pushback the bus, remote control commanded that only lifts the right or left main gear. It could be a good weighting device by only applying measure units to the strength needed to lift the wheel. Sequentially the 3 wheels, of course. If such a cheap thing or the nitrogen pressure of the wheels, could be used to random control of the airplanes and FAA only finds a 5% of error then I fly for free the rest of my life. Is it not the same difficult to weight the fuel on board? That weight could be used to check the onboard weight system. Ok I know I am going too far. Only trying to give some thoughts just in case someone of the FAA or the CAA read my post.
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Old 27th Feb 2008, 17:05
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John Tullamarine. I ask you as a moderator. Pprune has a very good reputation all over the world and there are thousands of persons involved in the aviation that comes here daily. Could we initiate a thread to get the attention of authorities, Boeing and Airbus to include the on board weight system in all the basic models? The title could be something like “To the attention of CAA, FAA, AAA, Boeing and Airbus. OB wgt & bal sys”. And the first post could be something like. “All the people posting in this message would like the onboard weight and balance system to be mandatory to all airplanes of MTOW 25000 kg or more. If you agree with this send an empty post to this thread”. Would be also very interesting if people from outside pprune could send a mail just to count on.
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Old 27th Feb 2008, 19:27
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ppppilot

Have you considered that maybe airlines dont want to know the exact weight?

With a 400,000 kg airliner, a 1% tolerance equals 4,000 kgs, so what do you think an airline is going to do, offload fuel, cargo or passengers and cause total chaos, or behave like an ostrich?

Mutt
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Old 27th Feb 2008, 19:49
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Aircraft Weigh Themselves?

I have been told that wind effects on the wings make measurement through the landing gear (by pressure, strain gauges or other means) unsatisfactory
 
Old 27th Feb 2008, 20:24
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Hard landings and the system is no longer very accurate.
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Old 27th Feb 2008, 20:46
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You mean it's a sensitive measure of hard landings too ? And if it shows 'out of calibration' next time someone wants to use it, that's a good clue of how hard it landed last night ?

Then surely it is a 'must have'
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Old 27th Feb 2008, 21:13
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Could we initiate a thread to get the attention of authorities

Not much point, I suggest. You are looking at a design standard sort of thing and that is processed at a much higher level than PPRuNe. More importantly, while on-board systems have their uses, at the end of the day, certification is not an exact science and involves weight tolerances. It would be quite unreasonable to look to "precise" (whatever that might mean) weight data for line operations when the flight test regimes are, at best, no better than routine line ops weight data ...

that's a good clue of how hard it landed last night ?

well proposed, that man ..
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Old 28th Feb 2008, 17:21
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not necessarily... one of the problems with a lot of transducers relates to sideloads on the cell. This is a big problem with routine maintenance aircraft weighings .. and can account for (an unpredictable) several hundred kilos per cell ...


That sounds like a design flaw - or an inappropriate choice of transducer. A while ago, I did some work with accelerometers and strain gauges (albeit dealing in fractions of a newton rather than a 747's worth of tin and blood), and we worked in three axes without any particular difficulty. I'm sure that if you don't design for this, it won't work well - and a bit surprised that it's a big problem in an area where I'd expect the technology to be mature and well-understood.

Is there some reason why this is so problematical?

R
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Old 28th Feb 2008, 19:05
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That sounds like a design flaw - or an inappropriate choice of transducer

Just the way things are .. (caveat - there may be better units available these days). Consider that design is for axial loading, as is calibration. I would need to refer the present state of play question to a colleague who designs and manufactures such animals ..

To give you an example, many years ago I was tasked with sorting out the dreadful weighing history on a fleet of GIIs. Turned out that, due to the jackpad geometry, any rotation of the hull to level accentuated lateral loading of the jacks (and load cells). Consecutive weigh cell variations could easily be in the order of several hundred kilos (might have been pounds - can't recall after nearly 30 years). When we tried locking the oleos and levelling prior to jacking ... the problems all promptly disappeared and the weigh variations were negligible. This is why some of us insist on level and locked for jackpad weighings ..
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Old 29th Feb 2008, 00:31
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MK Airlines Accident in Halifax

As a result of the investigation, the TSB issued a rec (A-06-07) to worldwide regulators for development and requirement of an on-board 'performance monitoring' system which would alert crews of a weight or cg problem...

http://www.tsb.gc.ca/en/reports/air/...4/a04H0004.pdf
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Old 29th Feb 2008, 12:06
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OK Gents. What are you afraid of?
JT. A thread like I am proposing it would be vary in relevance from as much as a small grain of sand to nothing, depending on the interest shown by our colleges. Not too pretentious. Only trying to be a sign of our interest on to not forget a safe and useful device like OBWABS.
Actual OB weighting systems can be expensive or inaccuracy mainly because no one gives a penny to investigate. The fuel metering device, at its beginnings was a cork and a lever with a variable resistor.
Don’t all of you agree that weight procedures are archaic? How can anybody accept to load a 747 with the weight given by the person that pays for that weight? When I flew cargo I remember to discover at the arrival very dangerous goods camouflaged into a printer package. If the sender do that what is he expected to do with the weight?
How can anybody accept at the actual stage of technology, lets say?... Ok women weights in summer 81 kg and 83 in winter and men 91 kg and 93… And transsexuals? Same at Congo than at Minnesota? Very complicated, best 84 kg for all. Sign it like that and lets celebrate with scotch all night long.
Is there any regulation on checking the weighing equipment at the check-in desks or the pallets scale since the day they were installed there?
But all of us accept go to the limits with performance, using software programs to use the last millimetre of the rwy for TO.
Mutt. If you are talking about behave like an ostrich by the time their pilots are dealing with an overloaded airplane, in my opinion those airlines shouldn’t to exist. Same opinion than CAA, FAA, AAA, and all the A’s over this world and the others. But if you are talking about serious airlines I believe that of course they do like to know the exact weight on the plane, because that weight means money. Money out or money in. I am sure that I am carrying thousands of kg for free. Nobody pays for them, because nobody knows that they are there. That’s money out. The extra fuel for that extra weight is money out. Serious airlines don’t play with its future. I have seen to my company very few times going to the limits and for example when I had to TO into a “tropical storm becoming a hurricane”, they gave me a million of analysis for TO with max pax possible. I took the more conservative and from there a couple of tons down, just in case. Specially because I am not comfortable with the weighting systems at those funny destinies. That is money for the company. I know, and far away also my money. None of my bosses told me a word, because they would acted the same. They do want nobody playing with their future.
In my experience the bigger the airplane the most usually limited at ldg. The smaller at TO. Probably that depends on the plane and the range of flights. Mine is 275t MTOW, 178t MZFW, 190t MLW. That makes 97t of fuel from where I have TOW limitation and that means over 15 to 16 flight hrs. My flights are maximum 11 hrs. In that situation only will be a security margin to performance calculation. That would be the primary use I would like to use it. Always using the bigger numbers from the loadsheet and WABC. My company use to give me 10 tons margin at 190t Ldg. I adjust it to 3t. That is a more than 1,5%.
I understand you are saying that some airlines doesn’t want to have such a system in their airplanes because of maintenance cost and inaccuracy? I better would say that companies would like the OBWBS but at a state of no significant maintenance cost and a level of accuracy of 0,1%. What they do not want to pay for is the cost of investigation to obtain such a system. Those are economical facts. Therefore airlines are defending money. As a pilot I am talking on behalf not only money but my life also. That’s safety and what I am trained for. So Mutt my opinion is, we both pilots and companies must have the same relevance on designing the airplanes and their systems. I think both positions are important and direct related. There it goes a couple of examples that I personally know.
At my very beginnings I flew in an aerial photography company. The most beautiful job I have ever done. I became Director of Operations and then I was all the time fighting with the boss (owner).- Fly lower, photos are better…No I don’t.- That other company has bought the engines half of the price than you did… That other company stopped that plane one month later burning more oil than fuel. Etc. I was 8,5 years and we broke all benefit records without a single incident. Then I decided to leave that beautiful job to live the beautiful live the airline pilots do. They took a pilot well experienced 5 years flying the same job and same planes. Unfortunately less than a year later he crashed one of the planes into a roof half an hour from TO full of gas. The fire was so heavy that not a piece of the pilot and photographer bodies could be found, except teeth. 3 years later that company didn’t exist anymore. Boss or pilot guilty? Both in my opinion.
The second example, all of you have been talking about it here.
IB overrun at Quito. Crazy airport. 92000 ft elev. Surrounded by 15000 ft mountains. Bad weather, worst ATC. 2600m LDA downhill slope, nobody knows the real elev since last time they took off the rubber of the rwy. ILS displaced from PAPI. 182 gs at touch down with high pitch attitude. A340 Flap limited at 20000’, select 1 at 15000, 2 at 13000 and then turn 200º to final app. Better maintain selected speed at least 10 knt faster at the beginning of the turn… Crazy.
Everybody knows what you read or you hear, but you also must know that initially IB was flying the A340-300. When the plane began to be full and the company was loosing pax they decided to fly the A340-600. On the papers it is perfectly possible. But Spanish pilots are not better nor worst than the rest and our maximum aspiration is as yours, get to retire with no incidents. 250 tons at ldg with those conditions is going to the limit. So IB pilots recommended many times to the company not flying the 600 there. Daily frequency, thousands ldgs with no problem, but everyone can have a bad day and better not be at the limit then. The rest all of you knows. Today the 300 is back flying to Quito.
By the way, about good clue of how hard it landed last night. Do you know there is an Airbus maintenance program that any airline may subscript? The bus ACMS reports via ACARS to the company exceeded parameters, included G at LDG. If you are in the program that ACARS is also sent to Airbus. At airbus they investigate the causes, the solution and if the company has the spare parts in the stock. It is supposed that only if you contract that. But I know a guy, flying for a company out of that program, who landed an A330 somewhere at South America the same sweet manners than a rugby player giving the welcome to the opposite having the ball. The crew didn’t say a word of that and when the plane arrived back to its base, Airbus phoned to the company to stop that plane.
Hey, this seems closer to be one of those disregarded accident analysis than a post. It is too long to read it again for mistakes. I hope all of you will send corrections for me.
Tailwinds
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Old 29th Feb 2008, 12:26
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Fraud prevention

My company operates 747-400 combi aircraft which are fitted with a W&B system.
Our pilots must make a logbook entry when the indicated ZFW differs more than 10 tonnes from the loadsheet ZFW.
Many times our aircraft took off 10 tonnes heavier than what we were paid for.
Because that´s the trick: 10 tonnes of cargo having a free ride.
Company detectives paid a visit to that specific station (JNB) and voilá, no loadsheet discrepancies anymore.
The W&B system pays for itself I think. Not to mention the safety advantage of a well monitored center of gravity.


Regards,

Ballpoint.
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