Just like a RV just more
Corey Fisher said:
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.
