Questions for Airline Pilots...

[Corey Fisher]

How do commercial airliners perform weight and balance calculations? It would seem the variables change for each flight.

How do the pilots know the takeoff weight of the aircraft before pushoff? Are there "scales" on the landing gear?

How do the pilots adjust the CG of the aircraft?




Just some questions from someone training for a PPL. Its really amazing how much I don't know about things I take for granted...

Thanks
Corey

[asav8tor]

The ops agents use load planning software, input the data and transmit it to the acft via ACARS (email)

[Papa]

Corey,
I have flown for two different airlines, and it was done differently at each. At my current airline we have a central load planning office that calculates this for us. They have the info for each individual aircraft on file, and after the baggage and cargo are loaded, they use that weight along with the passenger count and seating to give us what we call "final weights" along with our trim setting based on cg. We also use the final weights to determine our thrust settings as most takeoffs are done at reduced thrust settings.
At my previous airline we did the weight and balance ourselves. When the aircraft was loaded the ramp personnel would tell us how many bags had been loaded in each cargo pit and the flight attendants would tell us how many passengers were sitting in each "zone". The aircraft was divided into 4 different zones and all bags and passengers were assumed to weigh a standard weight, which differed between summer and winter. With this information we would enter a chart to check our cg and determine our trim setting.
As far as adjusting the cg, cargo can be moved from one cargo pit to another. Also at times we have to move passengers from one area of the aircraft to another.
Hope I answered your questions, Mark.

[John Clark]

Originally Posted by Corey Fisher

How do commercial airliners perform weight and balance calculations? It would seem the variables change for each flight.

How do the pilots know the takeoff weight of the aircraft before pushoff? Are there "scales" on the landing gear?

How do the pilots adjust the CG of the aircraft?




Just some questions from someone training for a PPL. Its really amazing how much I don't know about things I take for granted...

Thanks
Corey

Corey, The short answer is Yes. Various systems are used (and approved by the FAA) at different airlines to calculate weight and balance. The bottom line is that the Captain has to be aware of, and agree to the loading of the aircraft. Average weights are used, but you would be surprised how accurate the system is. When I was a chief pilot we did routine audits of the "average" numbers. Also, the flight crew can ask for "real" weights if there is a question. I remember one of the questions I had on an oral exam. "You are watching the passengers board and notice 10 sumo wrestlers being seated. do you use standard weights?" Of course not.

John Clark ATP CFI
RV8 N18U "Sunshine"
KSBA

[gmcjetpilot]

Originally Posted by Corey Fisher

How do commercial airliners perform weight and balance calculations? It would seem the variables change for each flight.

Yes

How do the pilots know the takeoff weight of the aircraft before pushoff? Are there "scales" on the landing gear?

Just like an RV, there is a BOW (basic op weight including the crew and number of flight attendants for the flight). Just like a RV it is weighed at some point in its basic flight configuration. Unlike little planes the BOW includes the crew which is based on standard weights (given by the FAA). For little planes its up to the PIC to either measure or estimate the weight of passengers and payload.

Standard weights are also used for passengers and their carry on. There are seasonal variation, in the winter higher weights are used for passengers than summer. The flight attendants actually count the passengers and give "the count" to flight crew, usually broken down into zones, front, mid, aft. Also there are corrections to BOW, like variation for full food and beverage or less, 3/4, 1/2 and 1/4 or ferry. Small regional planes are not going to have as many corrections since they carry just ice, drinks and peanuts.

The KEY is a lot of W&B on large planes are estimated on a average or standard weight, weight of passengers and bags are average and based on FAA regs. Exceptions are when doing charters with athletes or military. Since their weight's are not standard, actual weights are used, not the standard average. In the latter case the military gives you an actual measured weight of the personal with all their gear, including weapons. (It's kind of weird to see all M-4 or M-16's laying on the floor of your plane.) God bless the America and all the Military men and woman, and to all those who have lost their lives on 9/11 and in the fight against our enemies.

How do the pilots adjust the aircraft CG?

You can move cargo from the front or rear lower cargo holds. You some times have to ask passengers to move forward or aft when the plane is not full and too many passengers are sitting at one end of the plane.

If you are too heavy some times you have to leave fuel behind. We have the MIN FUEL for take off but often carry contingency fuel (extra cover your tail fuel). As Captain you can leave that behind but not less than MIN T/O FUEL. If you can't get the max takeoff weight down just on fuel you some times have to leave cargo behind, including checked luggage which is carried later by another plane. That typically happens on smaller airliners. I can say the B757/767 can haul the payload and fuel unless the runway is real weight limited due to short length and/or high density altitude. They just can haul a lot with lots of fuel which is nice. Not all planes can carry full fuel and all the pay load. The C-172 is an example. Its impossible to fly 4 adults in the plane with out leaving some fuel behind. With bags forget it. The C-182 is a true 4-place plane. Just because it has 4 seats does not mean its a 4-seat plane.

Just some questions from someone training for a PPL. Its really amazing how much I don't know about things I take for granted... Thanks Corey

An airliner is just like any plane just a lot longer and bigger numbers. To make it easy airlines like Southwest flight crews have laptop in every plane, allowing crews to just punch the basic numbers in, while the computer fills in many standard numbers you normally need to look up, like the BOW or takeoff performance limited weight for that runway.

Other wise all airlines use or have a W&B form with "index numbers" which is a short hand method calculating Wt x Arm. The form allows easier organization of numbers and has all the reference numbers for that airplane on the form. Regardless it takes a fast FO about 2 to 3 minutes to fill in a blank W&B from in from scratch. That is when you hear the Captain say, we are finishing up paper work, they are doing the W&B. They have to weight to the last minute to get the number of bags in the belly and cargo, passenger count (by zones) and fuel. Usually its the last thing done, and as soon as the W&B is done (in duplicate or triplicate) one copy goes to the gate agent and the door is closed. Airliners must leave records behind and of course carry a copy on board.

There are still lots of numbers to add and check. Large aircraft have other limits, ZFW - zero fuel weight, max taxi, max takeoff (varies with every runway and every condition winds, temps) and any reduction due to contaminated runway or MEL (min equip list & deduction for an inop item). It is more complicated than a Cessna but really its just the same thing, CG = sum(weigh x arm) / total weight. Once you do it 1000 times you can do it in your sleep, still mistakes can be made so take your time and do it right.

[David-aviator]

Originally Posted by Corey Fisher

How do commercial airliners perform weight and balance calculations? It would seem the variables change for each flight.

How do the pilots know the takeoff weight of the aircraft before pushoff? Are there "scales" on the landing gear?

How do the pilots adjust the CG of the aircraft?
\
Just some questions from someone training for a PPL. Its really amazing how much I don't know about things I take for granted...

Thanks
Corey

The short-short answer is it's all done by computer, the crew has little to do with the actual calculations.

Shotly after push back, ACARS prints a final weight and balance on a piece of paper about 3 inche long. The crew does check that the aircraft is not over a predetermined max gross weight for the take off and landing runway, the stabilizer trim is set for the cg, sets decision, rotate, and take off speeds on the ASI, and whether a reduced thrust take off is permitted.

Before computers, the crew performed all those functions with charts and graphs, but after doing it for a while it did not take long. Co-pilots got real good at it as some Captains taxied fast.

In post computer times when the computer is down, the flight does not take off. That happens now and then and the whole country goes nuts with delays.

[RV7Guy]

Originally Posted by David-aviator

The short-short answer is it's all done by computer,

Thanks David for the SHORT answer.

[Corey Fisher]

Very interesting everyone! I knew I'd get great answers.

I must admit I'm surprised to find that its still based on averages. It would seem that the average weight of 170lbs is not correct anymore, and that 25lbs of luggage should also be suspect.

Anyway thanks again.

-corey

[David-aviator]

Originally Posted by Corey Fisher

Very interesting everyone! I knew I'd get great answers.

I must admit I'm surprised to find that its still based on averages. It would seem that the average weight of 170lbs is not correct anymore, and that 25lbs of luggage should also be suspect.

Anyway thanks again.

-corey

If the public is unhappy with delays today, they would really be unhappy if everything had to be weighed....the 8 am flight will leave at noon tomorrow.

Truth is, the big 121 operators do not have the equipment to weigh people, they can weigh bags but not people unless folks will jump up on the bag scale. I once had the Penn State football team show up on a scheduled flight out of Reno, we should have weighed them as they did not average anything near 170 lbs. Sometimes, when a flight is over its max weight, the agent will go through and count kids, they go in the system at something less than the average weight. Part 135 operators do weigh people when loading smaller airplanes like the twin Otter. I've seen that operation in Alaska.

Life is a big fudge in many ways....

[BuckWynd]

Originally Posted by gmcjetpilot

Other wise all airlines use or have a W&B form with "index numbers" which is a short hand method calculating Wt x Arm. The form allows easier organization of numbers and has all the reference numbers for that airplane on the form. Regardless it takes a fast FO about 2 to 3 minutes to fill in a blank W&B from in from scratch. That is when you hear the Captain say, we are finishing up paper work, they are doing the W&B. They have to weight to the last minute to get the number of bags in the belly and cargo, passenger count (by zones) and fuel. Usually its the last thing done, and as soon as the W&B is done (in duplicate or triplicate) one copy goes to the gate agent and the door is closed. Airliners must leave records behind and of course carry a copy on board.

I've never seen or used a W&B form like this in 12 years of airline flying, so not all airlines do it that way, apparently. Our gate agents are not involved in the process in any way. At my airline, when you hear the Captain say "we're finishing up the paperwork," it's usually a maintenance deferral, or he's waiting for a fuel sheet from the fueling crew. Or he's looking for his pension.