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How Do They Find The A/c Weight?  
User currently offlineRemcor From United States of America, joined Feb 2006, 358 posts, RR: 0
Posted (8 years 6 months 4 weeks 1 day 22 hours ago) and read 5206 times:

I was wondering this yesterday at LAX as a friendly old porter told me he could get my slightly overweight bag on the aircraft without me paying the penalty (but by paying him a nice tip):

how does the airline calculate the gross weight of the aircraft before takeoff?

Do they add up the weight of all the individual bags and the number of passengers?
Do they just assume a standard bag weight and passenger weight and multiply by the number of passengers?
Do they weigh the full cargo and baggage pallets (and add the weight of the passengers)?
Are there load transducers on the landing gear that tell the pilot the weight of the aircraft as it's sitting on the ground?

[Edited 2006-05-23 17:29:32]

18 replies: All unread, jump to last
 
User currently offlineB4real From United States of America, joined Aug 2003, 2661 posts, RR: 5
Reply 1, posted (8 years 6 months 4 weeks 1 day 21 hours ago) and read 5160 times:

Quoting Remcor (Thread starter):
Do they just assume a standard bag weight and passenger weight and multiply by the number of passengers?
Do they weigh the full cargo and baggage pallets (and add the weight of the passengers)?

Yes, in fact a combination of above. I always ask the captain or first office to announce the take-off weight b/c it is really intersting to me... 'Haulin' Mass' I say  Smile

But take the established weights:
Aircraft frame
fuel (should be known)

Add the dymanics:
Passengers
Cargo
Crew

Each has a multiplier, but the cargo may be exact.

In a simple explanation, that's how they do it!



B4REAL, spelled like it sounds
User currently offlineTugpilot From United States of America, joined Sep 2005, 31 posts, RR: 0
Reply 2, posted (8 years 6 months 4 weeks 1 day 20 hours ago) and read 5110 times:

Each aircraft has an operational empty weight. Some aircraft are identical to one another, but as equipment can vary, each ship number generally had it's own OEW. Number of seats, avionics equipment, catering equipment (drinks, carts, coffee cups, napkins, peanuts, etc), full lav fluid tanks, full water tanks, cabin dividers, overwater equipment (liferafts, vests), IFE equipment, etc are used to determine OEW.

Procedures have changed since I worked weight and balance, but the preliminary baggage and freight load estimates used to calculate the load distribution on the aircraft was the number of passengers times 1.5 or 2 bags (seasonal) at 23.5 lbs per bag. The final load numbers used for W&B paperwork provided to the crew was actual number of bags times 23.5 lbs and total number of bags in each underfloor cargo hold.

The 23.5 lbs per bag is obviously an average weight and if your bag is "slightly overweight", the difference tends to become negligible on a full load.

Freight was always weighed before delivery to the aircraft, palletized, containerized, or bulk so weight averaging was not used for those items. Again, location on the aircraft was used in the final W&B calculations.

At departure time, when weight and balance figures are delivered to the crew,
an acurate passenger count, crew count, fuel, cargo, catered food in addition to operational basics included in the OEW, are totaled and added to the OEW. A check of the temperature and runways in use, and voila, your basic gross TOW.

Since we did this all by hand using books and pencils and very few computer tools, it was a daunting task. I would be very interested to learn of the process today and how much it has changed!

Cheers!
TP


User currently offlinePolymerPlane From United States of America, joined May 2006, 991 posts, RR: 3
Reply 3, posted (8 years 6 months 4 weeks 1 day 20 hours ago) and read 5075 times:

This has been discussed in tech/op forum

Payload Weight Calculation (by PolymerPlane May 12 2006 in Tech Ops)

Cheers,
PP



One day there will be 100% polymer plane
User currently offlineRemcor From United States of America, joined Feb 2006, 358 posts, RR: 0
Reply 4, posted (8 years 6 months 4 weeks 1 day 20 hours ago) and read 5032 times:

Quoting Tugpilot (Reply 2):

The 23.5 lbs per bag is obviously an average weight and if your bag is "slightly overweight", the difference tends to become negligible on a full load.

Yes, but any number of reasons can cause this average to change drastically. For instance, because my fiance and her aunt were going home they loaded up their bags with all sorts of stuff they bought here. Hence they practically hit the limit with their bags. One bag 70+ lbs, another one ~50 lbs, plus a heavy carryon. I'd bet a lot of other people were doing the same thing.

If the economy of a country changes or the demographic of passengers change, then I'd imagine that the average bag weight could change drastically.

I hope they don't just stick to 23.5 lbs per bag and hope for the best during takeoff.


User currently offlineTugpilot From United States of America, joined Sep 2005, 31 posts, RR: 0
Reply 5, posted (8 years 6 months 4 weeks 1 day 19 hours ago) and read 4968 times:

Absolutely any number of factors can affect the "real" average weight of a bag.

Also, a passenger has an "average weight" based on where they sit within the airplane (F vs. Y). Since I probably weigh as much as the three poker playing grannies across the aisle combined, and I carry only a laptop computer vs. their two 50lb bags apiece, our total combined weight including checked baggage is still is pretty much within the average. The 23.5lbs per bag is an expectation based on all passengers checking bags, regardless of whether or not they actually do. It's hard to believe, but it comes pretty close to that seemingly bogus average each time. The caveat here is international baggage is calculated differently. Those are all weighed, but not for the aircraft weight, but rather for liability in the event of loss or damage. The 23.5lbs was domestic weight only.

Also, airlines don't like to accept bags that weigh over 70lbs because there's going to be a human being having to pick it up and load it. That's strange because the bulk cargo that is hand loaded onto the narrow bodied a/c can be many times that weight. I think there may actually be an OSHA limit that dictates this, but we all know that gets ignored unless someone wants to make an issue out of it. Many times you can buy forgiveness for a heavy bag with an excess baggage or overweight baggage fee being paid.

Yes, the demographics of the origin and destination can dramatically affect the the weight of baggage that is checked. A flight between ORD and LAX would yield one type of baggage while a flight from JFK to SJU would yield a totally different type. Suitcases, duffles, and overnight bags on one flight and big, heavy boxes tied with ropes and bungees on another. I actually saw a 'fridge, complete with contents inside and wrapped with duct tape to keep it closed, rolled up to the counter to be checked as baggage (destination LAX). More than 23.5 lbs, even without the beer in it Big grin

Anyway, with 100 passengers, some with and some without baggage, the weight really does stay pretty close to that goofy number when averaged across.

TP!


User currently offlineTugpilot From United States of America, joined Sep 2005, 31 posts, RR: 0
Reply 6, posted (8 years 6 months 4 weeks 1 day 19 hours ago) and read 4950 times:

Oh, BTW, the fridge didn't go, but the contents did. Repackaged into 8 smaller boxes and excess baggage fees paid, they disposed of the fridge at curbside!

TP!


User currently offlineStarlionblue From Greenland, joined Feb 2004, 17166 posts, RR: 66
Reply 7, posted (8 years 6 months 4 weeks 1 day 18 hours ago) and read 4916 times:

Quoting Remcor (Reply 4):
I hope they don't just stick to 23.5 lbs per bag and hope for the best during takeoff.

The larger the aircraft, the more leeway you get:
- 1% of the weight of a 777 is at least a couple of tons and you're still within CoG parameters even if that 1% moves a meters one way or the other. You won't see that much variation within any given bag load inside, say, all the containers standard container in front of the CoG. On the other end of the scale, on a King Air 50 kg might make a difference.
- The fuel weight on long haul is a much greater proportion of the total weight, meaning the payload is a smaller proportion. This also gives leeway.
- On larger planes there are more data points (pax and bags), meaning that the distribution will tend to be closer to the statistical norm.

That's why when you get on a regional they might ask you to move, and you never experience this on widebodies.


The "average bag" and "average pax" weights change seasonally, and are regularly reviewed.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineB6JFKH81 From United States of America, joined Mar 2006, 2902 posts, RR: 7
Reply 8, posted (8 years 6 months 4 weeks 1 day 15 hours ago) and read 4803 times:

They put it on a really big scale.  Big grin


"If you do not learn from history, you are doomed to repeat it"
User currently offlineStarGoldLHR From Heard and McDonald Islands, joined Feb 2004, 1529 posts, RR: 1
Reply 9, posted (8 years 6 months 4 weeks 1 day 15 hours ago) and read 4784 times:

Why cant an electronic scale be built into the landing gear ?

In the same way you can calculate weight by torsion (i.e. having a spring, and pulling on it), the stretch in the spring will demonstrate the torsion, hence the pressure and calculate the weight bearing on it ?


With the landing gear this would be pretty simple, just reverse it, so the torsion is the weight bearing down onto the spring which would be mounted upside down ?



So far in 2008 45 flights and Gold already. JFK, IAD, LGA, SIN, HKG, NRT, AKL, PPT, LAX still to book ! Home Airport LCY
User currently offline474218 From United States of America, joined Oct 2005, 6340 posts, RR: 9
Reply 10, posted (8 years 6 months 4 weeks 1 day 15 hours ago) and read 4759 times:

Quoting StarGoldLHR (Reply 9):
Why cant an electronic scale be built into the landing gear ?

Its been done and was a maintenance nightmare. Aircraft landing gear take a real beating and transducers and load cells don't like to be handled roughly.


User currently offlineUAL747 From , joined Dec 1969, posts, RR:
Reply 11, posted (8 years 6 months 4 weeks 1 day 14 hours ago) and read 4742 times:

I've always wondered more about how the aircraft is balanced rather than total weight of the aircraft.

I mean, okay first class = front = less passengers + heavy seats

Business = middle = less heavy seats + more passengers

Economy = rear = lightest seats + most passengers


Then there's cargo balancing, and we've all seen some big mistakes with 747F and MD-11F's tipped on their tails on the cargo ramp.

Once, on a UA 737 headed from ORD-OKC, there were hardly any passengers, so we all sat up in front of economy, but the pilot came on and told us that some of us had to move closer to the rear of the aircraft. So obviously there must be instruments that tell the weights at certain parts of the aircraft, or no?

UAL


User currently offlineAeronut From Canada, joined Jan 2006, 138 posts, RR: 0
Reply 12, posted (8 years 6 months 4 weeks 1 day 14 hours ago) and read 4723 times:

Quoting Remcor (Reply 4):
If the economy of a country changes or the demographic of passengers change, then I'd imagine that the average bag weight could change drastically.

not just the bag weight, but the weight of the PAX themselves is affected by demographic changes. I have heard that new assumptions concering PAX weight had BIG impact on the overweight issue on the a380. After all, an extra 10 lbs on 800 PAX is a big number, and you can't charge PAX more money if they are fat!


User currently offlinePrebennorholm From Denmark, joined Mar 2000, 6534 posts, RR: 54
Reply 13, posted (8 years 6 months 4 weeks 1 day 14 hours ago) and read 4693 times:

Quoting Aeronut (Reply 12):
and you can't charge PAX more money if they are fat!

Some years back http://www.cph.dk had a picture from inside their terminal building around 1930 where a passenger (a slim woman) was standing on a weight scale. Passengers paid per pound.

Maybe that system should be introduced again. At least that might teach pax to pee before the flight instead of everybody queueing up at the lav as soon as the fasten seat belt sign disappears.



Always keep your number of landings equal to your number of take-offs, Preben Norholm
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17166 posts, RR: 66
Reply 14, posted (8 years 6 months 4 weeks 22 hours ago) and read 4546 times:

Quoting StarGoldLHR (Reply 9):
Why cant an electronic scale be built into the landing gear ?

In the same way you can calculate weight by torsion (i.e. having a spring, and pulling on it), the stretch in the spring will demonstrate the torsion, hence the pressure and calculate the weight bearing on it ?


With the landing gear this would be pretty simple, just reverse it, so the torsion is the weight bearing down onto the spring which would be mounted upside down ?

Apart from the mentioned weight nightmare, I imagine any strong winds would throw the measurements off.

Quoting UAL747 (Reply 11):

Once, on a UA 737 headed from ORD-OKC, there were hardly any passengers, so we all sat up in front of economy, but the pilot came on and told us that some of us had to move closer to the rear of the aircraft. So obviously there must be instruments that tell the weights at certain parts of the aircraft, or no?

They do have an instrument. It's called F/A with weight and balance crib sheet.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineAeronut From Canada, joined Jan 2006, 138 posts, RR: 0
Reply 15, posted (8 years 6 months 4 weeks 13 hours ago) and read 4454 times:

Quoting Starlionblue (Reply 14):
Quoting StarGoldLHR (Reply 9):
Why cant an electronic scale be built into the landing gear ?

In the same way you can calculate weight by torsion (i.e. having a spring, and pulling on it), the stretch in the spring will demonstrate the torsion, hence the pressure and calculate the weight bearing on it ?


With the landing gear this would be pretty simple, just reverse it, so the torsion is the weight bearing down onto the spring which would be mounted upside down ?

modern landing gear are filled with fluid/gas shocks. why not use a pressure transducer. pressure in the cylinder is related to load on he gear (a/c weight)


User currently offlineStarlionblue From Greenland, joined Feb 2004, 17166 posts, RR: 66
Reply 16, posted (8 years 6 months 4 weeks 12 hours ago) and read 4429 times:

Quoting Aeronut (Reply 15):
modern landing gear are filled with fluid/gas shocks. why not use a pressure transducer. pressure in the cylinder is related to load on he gear (a/c weight)

I still think wind would have too much of an effect. But I may be wrong...



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineGemuser From Australia, joined Nov 2003, 5806 posts, RR: 6
Reply 17, posted (8 years 6 months 4 weeks 11 hours ago) and read 4390 times:

Quoting UAL747 (Reply 11):
I've always wondered more about how the aircraft is balanced rather than total weight of the aircraft.

It's easier to look at the sitution with a light aircraft first, as it is simpler AND the principles are EXACTLY the same with an airliner.

Let take a Cessna 172 as our generic model. Single engine, fixed gear, high wing, 4 seat (2x2), small aft cargo compartment.

Balance is calculated from the "datum", which in our example is 100" forward of the engine firewall. The location of all items in the aircraft is given as inches behind the datum (+ numbers) or forward of the datum (- numbers). On the Cessna all locations are behind the datum, hence positive numbers.

We start with the aircrafts empty weight, CG position and Moment Index, from the Flight Manual or other approved documentation. (MI=weight x distance from datum) and add the consumables/variables. The first consumable is oil, its weight and MI is added to our calculation. Then the pilot and front seat pax weight & MI is added, then fuels weight & MI, then rear seat pax weight & MI and finally cargos weight and MI.

At each stage the CG location can be checked by dividing the total weight into the MI. This will give you the actual CG location. The CG is checked at each stage and compared to the aircrafts certified limits for that weight. Then the fun starts! With full fuel, especially if the aux tanks are fitted, you will start running into both aft CG & MTOW problems as soon as you start to fill the rear seats. The PIC has to juggle things around so the aircraft is legal. (Cessnas 400 series twins are even worse, if it has main, aux and nacelle tanks fitted and you fill them, you can add a pilot and maybe his lunch, and you are at MTOW).

Airliners are exactly the same EXCEPT that airlines use, approved, average weights and "seat zones" where it is assumed that all pax sitting in the zone are at the zones position from the datum. (That is those forward of the centre of the zone equal those aft of the centre of the zone). From a sometimes bad memory a B747 has 8 zones. Cargo is dealt with the same way, although actual weights are used.

Hope this helps, at least somewhat!

Gemuser



DC23468910;B72172273373G73873H74374475275376377L77W;A319 320321332333343;BAe146;C402;DHC6;F27;L188;MD80MD85
User currently offlineChqdispatch From United States of America, joined Sep 2004, 50 posts, RR: 0
Reply 18, posted (8 years 6 months 4 weeks 4 hours ago) and read 4300 times:

Each Part 25 aircraft has a Basic Operating Weight aka BOW aka OEW. It includes everything EXCEPT Passengers, Cargo and (usable) Fuel. When those three arrive, the crew simply adds them to the BOW to get their take off weight. To get their landing weight, they just subtract the fuel they burn while enroute.

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