Corsair2
Topic Author
Posts: 243
Joined: Thu Jan 25, 2001 11:02 pm

Weight And Balance

How do airlines that operate large aircraft (i.e. 737, M80,...) ensure that the aircraft are within range on weight and balance? The method I use as a Cessna 172 pilot involves finding the weight of each passenger and using an Excel spreadsheet with all the moment arms. I suppose this is not practical at the airlines as they do not ask passengers how much they weigh. Any info would be of interest...
"We have clearance Clarence. Roger, Roger. What's our vector Victor?"

MrChips
Posts: 933
Joined: Wed Mar 30, 2005 2:56 pm

RE: Weight And Balance

They do it just the same way you and I do.

Instead of asking passengers for weights, the FAA (or whomever) publishes minimum standard weights for passengers. There is a different weight for males, females, infants, etc. that also varies from season to season on account of heavier clothing.

Also, every airline has a figure that they use for average passenger baggage weight, which varies depending on a number of factors.

Fuel W+B is calculated just the same as any other aircraft, weight*arm.

As for passenger arms, I believe it goes one of two ways - either each class has an average point (like the arm you use for fuel), or there is one average arm for the entire cabin (the dispatchers in the crowd can straighten this out for you).

Finally, all the catering carts, crew, etc. are accounted in the OEW of the aircraft. Instead of a distance from the datum, most larger aircraft report C of G in %MAC (percentage aft of the leading edge of the Mean Aerodynamic Chord)...its really reporting the same thing in a different format. Really, its no different than calculating the W + B for a 172, just with a few more figures.
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SlamClick
Posts: 9576
Joined: Sun Nov 23, 2003 7:09 am

RE: Weight And Balance

 Quoting MrChips (Reply 1):As for passenger arms, I believe it goes one of two ways - either each class has an average point (like the arm you use for fuel), or there is one average arm for the entire cabin

It does indeed go one of two ways, but not exactly like that.

For most planes a certain seating order can be assumed. For example, the assumption for coach may be that the passengers sit window-window, filling in from front to back, then they sit aisle-aisle filling from front to back. When all the window and aisle seats are filled they begin to fill in the center seats.

So let's say coach hold 120 pax in 20 rows of 6 arranged are 3-3 on each side of the aisle. Well the arm of each row is known, so if we have 90 passengers on board, we can assume that all the window seats are filled (two passengers in each row) all the aisle seats are full (two more passengers in each row) and ten more passengers are distributed throughout the cabin in the center seats - based on some assumption about in which rows. This gives us a total weight (standard weight times 90) and a total moment based on the above assumptions.

Now the exact assumption might not be the one above, but it will be something like that.

For more CG-sensitive airplanes (the MD-80 might just be such an example) there will probably be an actual seatmap produced as part of the weight & balance.

The W&B for a larger airplane should not intimidate you in any way. The system is well worked-out and just has a few more fields in which to enter data than does your Cessna. (and it does not normally use negative numbers)

Most of us use an indexed system by now, where the last two digits, plus one decimal place are not pounds, but trim index numbers. So, for example the basic operating weight of a particular plane might be something like 72502.4 which means that the BOW is rounded off to 72,500 pounds and the empty-airplane trim setting starts at 2.4 units nose-up. Each item we add ends with the trim index value for that; for example fifteen thousand pounds of fuel might translate, or be reported to us as 14998.5 which means that while it is still 15K lb. it has the effect of (adding algebraically) giving us 1.5 units nose-down trim from where we'd be without it.

That might sound confusing but all we do is add up all these weight-index numbers and the last digit plus the decimal is the trim setting. Simple.

So there we are climbing out and two flight attendents unlatch a five hundred pound service cart and drag it eighty feet forward or aft! We sit there watching the elevator trim wheel spin.
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MrChips
Posts: 933
Joined: Wed Mar 30, 2005 2:56 pm

RE: Weight And Balance

Informative as always, SlamClick. That clears up a lot in my mind now.

As for the trim wheel, I too have seen that. Back in the days when cockpit doors were unlocked, you could watch the trim wheels of the 737 compensating for the FAs moving back and forth throughout the aircraft. On a more humourous note, if the seatbelt sign has been on for a long time, when the crew shuts it off, you will see a noticable movement of the trim wheels.

As for me, I even notice trim changes in the 182 I fly, only mine relates just to fuel burn, not flight attendants moving between rows 1 and 2...
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UAL Bagsmasher
Posts: 1839
Joined: Wed Sep 08, 1999 12:52 pm

RE: Weight And Balance

How appropriate. I just found a "Weight and Balance Procedures Training" guide in my mailbox at work today. It deals with W&B, aircraft weighing, etc. It came with a nice little study guide, and of course a test. Waterlines, Buttock lines, I feel like I'm back in school  Just remember....WAM! (Weight*Arm=Moment)

tinpusher007
Posts: 911
Joined: Wed Feb 09, 2005 2:03 am

RE: Weight And Balance

I might also add that for most large airlines (I can only speak for DL) the procedure is automated and done on a computer.
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Goldenshield
Posts: 5017
Joined: Sun Jan 14, 2001 3:45 pm

RE: Weight And Balance

Many airlines have sheets where they can do weight and balance on the fly should the computers go down. They are made by some MAJOR math wizzes, and make it so that anyone familiar with the sheet can do a complete weight, balance, and trim, all within a minute or two.
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QANTAS747-438
Posts: 1712
Joined: Mon Jan 22, 2001 7:01 am

RE: Weight And Balance

As a Southwest Ops Agent, W/B is what we do all day. We take the OEW, and add any 4th flight attendents or cockpit riders and add the weight of the pax and the cargo weights of both aft and forward bins. All cargo weights are converted into bag weights. X lbs is equal to Y number of bags. And, Y number of bags is equal to an adjusted weight. Once you add these together, you get a number. All weights have trim units on the end of them. For example - 84,945 lbs is actually 84,900 lbs with trim units of 45. Once we get that weight, we make sure that the last two numbers (the trim units) are withing range. If they are, the aircraft is in trim. It's often a range of 42-61 or something like that.

Add fuel, and you get the take off weight. Again, make sure all trim units are within range. We do use a computer system to do it, but WN has only been doing that since mid-April. Previous to that, we've used manual Loading Schedules where we do it all by hand, in the last 2 minutes before pushing back. It was quite a rush!

If there are any irregular operations (if the aircraft is out of trim or overweight) then you can do such things as using a Loading Rule which says how many pax to move aft or forward of the wing depending on how bad the situation is.
My posts/replies are strictly my opinion and not that of any company, organization, or Southwest Airlines.

Jetlagged
Posts: 2564
Joined: Sun Jan 23, 2005 3:00 pm

RE: Weight And Balance

The 747 has an optional weight and balance system which computes the weight and CG from inputs from the landing gear. On the classics the flight engineer had a panel which gave a continuous readout of gross weight and CG (but assumes the self loading cargo doesn't move around much). This calculation was based on strain gauge data inputs from the landing gear struts before takeoff, together with computed CG changes from fuel usage. On the 747-400, if the system is installed, computed weight and CG are displayed as prompts on the FMC CDU during pre-flight which the pilot can accept or override with a calculated value from dispatch.

Other large CG sensitive aircraft probably have similar systems.
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Corsair2
Topic Author
Posts: 243
Joined: Thu Jan 25, 2001 11:02 pm

RE: Weight And Balance

Thanks for all the good posts. What happens if a bunch of people above the standard weight (i.e. a football team) board an airplane? How would this be compensated for in weight and balance.
"We have clearance Clarence. Roger, Roger. What's our vector Victor?"

FredT
Posts: 2166
Joined: Thu Feb 07, 2002 9:51 pm

RE: Weight And Balance

There's a lot more to say about this subject... but anyone who's interested in W&B should read FAA AC120-27E to get a good start.

It has the answer to most common questions asked in a Q&A format, including the last one regarding football teams etc.

I suggest that the first person who actually takes the time to read the AC comes back and answers the question for the others. Nothing beats going to the source(s), especially when it is that easy.
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SlamClick
Posts: 9576
Joined: Sun Nov 23, 2003 7:09 am

RE: Weight And Balance

Last time I taught weight & balance at an airline I used a military charter as a sample problem. It is the same drill as a football team. (I flew many college football team charters during my career.) The athletic director, and the officer-in-charge of a military movement know when they charter an airplane that the special W&B situation they impose has to be addressed for the operator.

Most of the ADs I worked with knew their team weight and all their team equipement weight. We used estimated weights for the non-team personnel who went along, but it was always higher than "average" weights because it is almost 100% adults, and usually about 75-80% male.

AMC has standard weights for combat-equipped or non combat-equipped troops. Most portable military equipment either has the weight placarded on it, or it is easily found in the manual. At any rate, this information is gathered up well before the airline crew has to deal with it.

Add to that, the fact that a high percentage of charters are flown by a small number of crews who bid them regularly and I would feel that a charter flight W&B is, if anything, more likely to be correct than a scheduled, revenue flight.

The last part of the answer: The W&B manual for the airplane includes tables for non-standard conditions. So if a "standard" weight of 180 lbs. is being used in scheduled service, and we seat 120 passengers in coach, that would be a maximum weight for coach of 21600 pounds, or perhaps a weight-index of 21625 meaning that it adds 2.5 trim units. Well the "special" passenger weight tables might go up to 25800 lbs, to reflect the realities of transporting a football team. (remember not all 120 are players) That index might look like 28531 showing 28500 pounds and 3.1 trim units. Remember those are just sample numbers, but the answer will be something like that.
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