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flaps30
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How Does An Aircraft Weigh Itself?

Sun Jun 14, 2009 3:19 am

This may sound like a silly question, but I have always wondered how does an aircraft compute its final weight for takeoff? Are there scales located throughout the aircraft?
 
DescendVia
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RE: How Does An Aircraft Weigh Itself?

Sun Jun 14, 2009 4:05 am



Quoting Flaps30 (Thread starter):

For big airlines its: load planners, dispatchers, but usually both (they coordinate with each other along with input from the captain) who give ZFW (Zero Fuel Weights) and they are plugged into the box by the pilots. Then the FMC figures its GW (Gross Weight) from the data that was entered in the ZFW line in the pages.

Its not an "exact" right on number but its very close to the actual weight but its always subject to pilot error (junk in junk out). Thought these type of entries are usually verified many times so not to worry.
 
L-188
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RE: How Does An Aircraft Weigh Itself?

Sun Jun 14, 2009 4:06 am

Well I think that some of the new birds have sensors to tell how much pressure is in the struts.

I myself am old school.

Get a piece of paper
Put the empty weight of the aircraft on it.
You count every person and multiply that by 175 to get the pax weight.
Add to the piece of paper.
You count every bag and multiply that by 35 to get the bag weigh.
Add to the piece of paper
Put the weight of the mail on the piece of paper
Put the weight of the cargo on the piece of paper
Add in the pounds off fuel loaded on the airplane to the piece of paper.
Add the numbers on the piece of paper.
Have the pilot come in a sign the piece of paper. The good ones will actually double check your math. Some even bring their own pens.

Oh in case your are wondering, The aircraft is periodicly run up on a set of scales and weighed. It is also re-weighed if significant alterations have been made that would change the aircraft weight and balance significantly. The cabin fittings are supposed to be in the aircraft.

This is where the empty weight of the aircraft comes from.
 
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Starlionblue
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RE: How Does An Aircraft Weigh Itself?

Sun Jun 14, 2009 4:06 am

No weighing. It is estimated based on load. Basically you add empty aircraft weight + fuel + pax (based on average) + bags (based on average) + cargo. The averages for pax and bags mostly depend on seasonality and route (for example heavier in winter).

On smaller aircraft, a few pax can have a big effect on weight and balance so you'll often see F/As moving pax based on center of gravity calculations.
 
FredT
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RE: How Does An Aircraft Weigh Itself?

Sun Jun 14, 2009 11:07 am

Attempts have been made to create reliable on board weight and balance system, primarily using pressure sensors in the struts or (I believe, although I haven't seen such a system) strain gauges or strut compression sensors.

Never worked reliably enough to allow them to replace manual calculations. There are simply too many factors which will wreak havoc on the automated weights, such as wind influence. Airlines do not want an on board weight and balance system which will cost maintenance if all it will do is to provide a verification of the manual calculations.

In addition it was found that in actual practise, OBWBS was found to be a hindrance. Flights which were found fine on the manual calculations (using approved standard weight) were found overweight by the OBWBS, either through miscalculation or through actual weights being above the approved standard weights. A non-mandatory backup system to a system which has been found to work well, which forces unloading of cargo or cancellation of flights which would otherwise have been flown will not stay installed for long.
 
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Jetlagged
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RE: How Does An Aircraft Weigh Itself?

Mon Jun 15, 2009 5:28 pm



Quoting Starlionblue (Reply 3):
No weighing. It is estimated based on load. Basically you add empty aircraft weight + fuel + pax (based on average) + bags (based on average) + cargo. The averages for pax and bags mostly depend on seasonality and route (for example heavier in winter).

Aircraft have had weight and balance measurement systems for many years. The 747 classic had an optional one based on strain gauges in the landing gear struts, which allowed the computation of weight and CG. So yes, they can weigh themselves. However the issue is accuracy, as Fred said above.

Current aircraft with such systems offer the pilot a calculated weight to enter into the FMS before takeoff. The pilot can accept this information by selecting the adjacent MCDU LSK or input calculated weight and balance from the loadsheet in the scratchpad.
 
AirframeAS
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RE: How Does An Aircraft Weigh Itself?

Mon Jun 15, 2009 8:11 pm



Quoting L-188 (Reply 2):
The aircraft is periodicly run up on a set of scales and weighed.

We did that when we got N203FR. I have pictures but I don't know if I should attempt to put them up on here....
 
mandala499
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RE: How Does An Aircraft Weigh Itself?

Mon Jun 15, 2009 8:15 pm

And after all that input... get a gross weight into the box... and then it can calculate it's gross weight by "initial GW - fuel burnt"... although, the calculated fuel remaining and the actual fuel remaining can differ on a long flight.

Mandala499
 
FredT
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RE: How Does An Aircraft Weigh Itself?

Mon Jun 15, 2009 8:50 pm

Quoting L-188 (Reply 2):
You count every person and multiply that by 175 to get the pax weight.

184 lbm these days, with a no-carry-on program.

[Edited 2009-06-15 13:51:20]
 
474218
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RE: How Does An Aircraft Weigh Itself?

Mon Jun 15, 2009 9:22 pm



Quoting Jetlagged (Reply 5):
Aircraft have had weight and balance measurement systems for many years. The 747 classic had an optional one based on strain gauges in the landing gear struts, which allowed the computation of weight and CG.

The problem with these on-board weight and balance systems is that the stress and strain that is put on the landing gear when landing a 200K lbs. plus aircraft is not good on sensitive electrical equipment.
 
L-188
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RE: How Does An Aircraft Weigh Itself?

Tue Jun 16, 2009 3:17 am



Quoting FredT (Reply 8):

Quoting L-188 (Reply 2):
You count every person and multiply that by 175 to get the pax weight.

184 lbm these days, with a no-carry-on program.

[Edited 2009-06-15 13:51:20]

I figured it has changed. I haven't dispatched an airplane in 5 years.

I remember when I did load planning before that it was 175 summer weight/185 winter weight for passengers.
 
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Jetlagged
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RE: How Does An Aircraft Weigh Itself?

Wed Jun 17, 2009 5:22 pm



Quoting 474218 (Reply 9):
The problem with these on-board weight and balance systems is that the stress and strain that is put on the landing gear when landing a 200K lbs. plus aircraft is not good on sensitive electrical equipment.

Strain gauges aren't exactly "sensitive electrical equipment". The system on the Classic worked, but I'm not sure how much practical use it was. There were a lot of adjustments which could be made by the F/E on ground to null errors which presumably allowed the indicator to read what the loadsheet said it should!
 
JER757
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RE: How Does An Aircraft Weigh Itself?

Fri Jun 19, 2009 12:05 am



Quoting L-188 (Reply 10):
175 summer weight/185 winter weight for passengers.

In the UK its airline specific (although its all approved by the relevant authorities obviously)

Most work off the sex of the passenger. Male 85kg. Female 75kg. Child 35kg. Infant 0kg.

Hold baggage 13kg (this depends on which type of flight however).

How accurate it is, I have no idea.
 
FredT
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RE: How Does An Aircraft Weigh Itself?

Sat Jun 20, 2009 2:29 am



Quoting JER757 (Reply 12):
In the UK its airline specific (although its all approved by the relevant authorities obviously)

EU OPS 1 Subpart J specifies standard weights for European operators, just as AC120-27E does for US operators. If airlines want to use their own standard weights, they have to tell the authorities exactly why. This means making a survey of the weights. Currently it's 88/70/35 kg males/females/children, holiday charters and <19 seats excepted.
 
JER757
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RE: How Does An Aircraft Weigh Itself?

Sat Jun 20, 2009 3:10 am



Quoting FredT (Reply 13):

Fair play, its been a while since I worked with those weights. 88/70/35 sounds right.

I remember switching a few BACON flights from those weights to Adult/Child/Infant weights (the exact figures escape me) when things got a little too heavy. It was all in their manual and was approved by ops etc.

It seemed to help when those little DHC3s were full!
 
dfwramper
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RE: How Does An Aircraft Weigh Itself?

Wed Jun 24, 2009 4:10 am



Quoting Starlionblue (Reply 3):
On smaller aircraft, a few pax can have a big effect on weight and balance so you'll often see F/As moving pax based on center of gravity calculations.

Even on large aircraft -- if it's only half full of passengers -- wouldn't some adjustment be required to keep the plane in CG limits? You can move the baggage around a bit to offset, but I'm wondering if consideration is made to which seats have been assigned and the impact to CG on partially loaded aircraft (especially if F was empty, for example)?

Wasn't there a story about a football team or something similar that were on a charter flight and thought it would be funny to have everyone on the right side of the plane quickly move to the left side (or reverse) while in flight? Naturally, after doing this (and not crashing the airplane) they were chastised by the flight crew as I remember the story. Not sure who or where this was.
 
planewasted
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RE: How Does An Aircraft Weigh Itself?

Wed Jun 24, 2009 2:39 pm

As I've said in another thread. I think it would be really easy and almost for free to weigh an aircraft by monitoring acceleration and engine thrust. mass = force/acceleration
 
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Jetlagged
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RE: How Does An Aircraft Weigh Itself?

Wed Jun 24, 2009 3:19 pm



Quoting PlaneWasted (Reply 16):
As I've said in another thread. I think it would be really easy and almost for free to weigh an aircraft by monitoring acceleration and engine thrust. mass = force/acceleration

But the force involved is actually:

thrust - drag

How are you going to measure thrust and drag?
 
planewasted
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RE: How Does An Aircraft Weigh Itself?

Thu Jun 25, 2009 12:23 am



Quoting Jetlagged (Reply 17):
But the force involved is actually:

thrust - drag

How are you going to measure thrust and drag?

There is no exact way of measuring anything. But at low speeds drag is very low compared to thrust.
The method has for long been used to calculate engine power in cars and it works.
 
tdscanuck
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RE: How Does An Aircraft Weigh Itself?

Thu Jun 25, 2009 1:04 am



Quoting Dfwramper (Reply 15):

Wasn't there a story about a football team or something similar that were on a charter flight and thought it would be funny to have everyone on the right side of the plane quickly move to the left side (or reverse) while in flight? Naturally, after doing this (and not crashing the airplane) they were chastised by the flight crew as I remember the story. Not sure who or where this was.

That's apparently an urban legend attached to the Stanford University Band:
http://archive.southcoasttoday.com/daily/10-97/10-17-97/c02li139.htm

Tom.
 
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Jetlagged
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RE: How Does An Aircraft Weigh Itself?

Thu Jun 25, 2009 10:02 pm



Quoting PlaneWasted (Reply 18):
There is no exact way of measuring anything. But at low speeds drag is very low compared to thrust.
The method has for long been used to calculate engine power in cars and it works.

You can measure airspeed, rpm, EPR, EGT, etc quite adequately. Very hard to measure thrust without installing a complex system of strain gauges in the engine mounting, and it's not worth that complexity for anything except test airframes.

Aircraft don't move at low speeds. Even on the takeoff run aerodynamic drag is significant. But thrust is hard to measure in any degree of accuracy without special test equipment, that's why engine power is presented as fan rpm or EPR to the flight crew. If thrust is not known with any accuracy and drag is ignored then using F=ma will not give you a usably accurate figure for aircraft mass.

Car engine power can be measured by a dynamometer on a rolling road. That ignores air resistance as you say, but for cars this is acceptable as you are interested in shaft output only. Car engine manufacturers use dynamometers attached to the output shaft, eliminating the rolling road altogether.
 
planewasted
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RE: How Does An Aircraft Weigh Itself?

Fri Jun 26, 2009 5:49 pm

While aircraft don't move at slow speeds all the time, it's easy to measure when starting the take off roll, say at 50 km/h. Then the drag should be really low. Also, it's easy to compensate the measurement for the drag.
To calculate the the weight to a precision where it's is possible to detect very big mismatches between set weight and true should be possible, I believe.

The car systems I think about uses no rolling road. You just enter the mass of the car and put a unit with an accelerometer inside the car. Then it calculates the accelerating power from the engine. There's actually an application for the iphone that does this with the internal accelerometer. It's the same principle but instead of the mass the force is calculated.
 
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Jetlagged
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RE: How Does An Aircraft Weigh Itself?

Fri Jun 26, 2009 8:47 pm



Quoting PlaneWasted (Reply 21):
While aircraft don't move at slow speeds all the time, it's easy to measure when starting the take off roll, say at 50 km/h. Then the drag should be really low. Also, it's easy to compensate the measurement for the drag.
To calculate the the weight to a precision where it's is possible to detect very big mismatches between set weight and true should be possible, I believe.

You still haven't said how thrust would be measured. How is drag going to be easily compensated for? What about rolling resistance? What if there's a crosswind?

At best such a system could produce only a very approximate mass figure. Of no practical use to the crew, who will have had to enter weight into the FMC before the engines were even started.

Quoting PlaneWasted (Reply 21):
The car systems I think about uses no rolling road. You just enter the mass of the car and put a unit with an accelerometer inside the car. Then it calculates the accelerating power from the engine. There's actually an application for the iphone that does this with the internal accelerometer. It's the same principle but instead of the mass the force is calculated.

Sounds like an excellent way for "boy racers" to have accidents.  Wink
 
metroliner
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RE: How Does An Aircraft Weigh Itself?

Fri Jun 26, 2009 9:17 pm



Quoting PlaneWasted (Reply 21):
no rolling road

Thank God, or we'd have 2H4 and his impossible diagrams all over again...  Wink

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