What methods are used to achieve that?

According to my search :

  • For old planes they use a plumb bob and measuring tools.
  • On new and big planes, the process is computerized. Built-In Electronic Weighing like the Boeing 777
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    $\begingroup$ The calculation just takes a few minutes. It's the setting up that takes the time. Usually, though, this is done (or at least used to be done before I retired in 1999) as part of a maintenance check so it's hard to separate out the time used just for the weighing part. Also, if you brought an airplane in just for that, the elapsed time required would vary depending on how many people you put on it. I once watched them put a 747 up on jacks to cycle the gear. That took a little over an hour from start to finish. You'd want to add a little more time to that for recording readings. $\endgroup$ – Terry Oct 24 '15 at 19:46
  • $\begingroup$ thanks :) i hope to know the time including the equipment setup ? and is the process automated now or no $\endgroup$ – geo Ema Oct 24 '15 at 20:03
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    $\begingroup$ I'm 16 years out of date now, so I can't address how automated it is now. In my previous comment, I forgot to say that you'd also have to add in the time required to empty the fuel and remove items not to be included in the empty weight. However, if you knew accurately how much fuel you had and the weight and location of items, you can account for them that way. $\endgroup$ – Terry Oct 24 '15 at 20:12
  • $\begingroup$ The frontier between old and new aircraft is not that clear. The B777 is not really a new aircraft as its maiden flight took place more than 20 years ago (in 1994). You may also specify what "big planes" means for you (I could be airliners designed for more than 50 passengers) $\endgroup$ – Manu H Oct 25 '15 at 18:49

Having recently weighed my plane I can tell you what's involved for determining the Basic Empty Weight for a small aircraft:

  • The aircraft equipment list is checked and updated.
    This is kind of important as the equipment list includes the weight and arm of everything installed on the airframe. If this list is not accurate future weight and balance updates based on adding/removing equipment using the weights and arms on that list will be incorrect, and your computed weight and balance will start to diverge from the actual weight and balance until the plane is weighed again.
  • The aircraft interior is cleaned to "factory empty" status.
    Pens, pencils, 3-year-old charts, note pads, and other things not on the equipment list are all removed, and things like tow bars are placed in their appropriate location/holder.
  • The aircraft is serviced with "fluids" per the manufacturer's procedures for weighing.
    For most small planes this means de-fueled, full engine oil, & full hydraulic/brake fluid.
  • The shop sets up and zeros their scales.
  • You tow/push the aircraft onto the scales.
    (Or lift it on jack stands with integrated scales designed for this purpose.)
  • The aircraft is leveled on the scales.
    The plane must be leveled laterally (left-to-right) and longitudinally (nose-to-tail), otherwise the computations made later will be inaccurate.
  • The numbers on the scales are recorded.
  • The Weight and Balance is determined:
    • Add the numbers from all scales used. This is your Empty Weight.
    • You do some math on the scale numbers to determine the Center of Gravity.
      (Since you know the station/arm of each scale you can figure out the CG based on the weights)

As Terry mentioned, setting up took more time than the actual weighing and calculations. In my case I spent about a day straightening out the equipment list, and the shop that did the weighing spent about 20 minutes verifying that I hadn't missed anything. Defueling the aircraft adds a significant amount of time to the process as well. Moving the plane onto the scales and leveling it took approximately 15 minutes, and doing the math took maybe another 5 minutes.

On larger aircraft the same general process can be used with larger scales (or jacks) - load cells integrated into a taxiway or ramp area are another option for doing this.

I'm not sure if aircraft that can determine their operating weight (by load cells installed in the gear assembly) can also determine their own empty weight - I know the weighing report I received includes calibration information for the scales used, so presumably weighing a large aircraft using its built-in load cells would require those cells to be properly (traceably) calibrated prior to using them for this purpose.

  • $\begingroup$ thanks for sharing your experience that's a great answer :) $\endgroup$ – geo Ema Oct 24 '15 at 21:14
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    $\begingroup$ Incidentally at least for the PA-28 there's no plumb bob involved for leveling: you use a regular bubble level. There are two screws on the fuselage that come out for leveling it longitudinally, and for leveling laterally you put the same level on the spar box (the maintenance manual includes pretty pictures). Procedures for other aircraft may vary, but they'd be detailed in the maintenance manual. $\endgroup$ – voretaq7 Oct 24 '15 at 22:48
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    $\begingroup$ Related: How does a commercial airliner measure its weight/mass?. $\endgroup$ – mins Oct 24 '15 at 23:52

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