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Estimate the weight of of a 747 airplane.

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Notes
This question was originally asked in MS interviews & the idea behind this asking question wasn't guesstimation but to see how creative candidates are. The question originally was "HOW would you estimate the weight of 747". There are many answers to it but here is how  I would answer it
  1. I will ask the aircraft manufacturer
  2. I will put the plane in the water, weigh the amount of water displaced and then assume that most of the plane is made of aluminum. From the water, I will get the volume and then I will multiply it by the density of aluminum.
  3. Put the plane on ship. Measure how much the ship goes down. Measure the density of water & do some maths around that.
_________
Below I have tried to do a guesstimation. Final result is in the ball-park of actual weight but I do believe that the individual calculations are not correct.
 
Assumption
  1. We will consider the long haul 747.
  2. We will consider weight of an empty aircraft with no people & luggage.
  3. All the aircraft body is made up of aluminium with similar thickness
  4. Will ignore weight of things inside the cabin like seats etc as they would be a fraction of weight of aircraft
Logic
  1. Aircraft can be thought to consist 
    1. of the cabin/fuesalage which can be considered cylindrical in shape.
    2. 2 wings which we will consider a right triangle
    3. 3 tail/stabilizers, again a right triangle
  2. Engines
  3. Landing gear which consist of approximate 10tyres & some mechanical stuff
  4. Electricals & wiring  & other stuff
  5.  We will try to find volume of the metal used for cabin & multiply it by density of Aluminium
  6. For engine, we will have to approximate the weight. We will take an assumption of weight of car engine & power it produces & take a similar ratio for plane engine.
  7. For electrical
 
Calculations
 
  1. Aircraft body/Fuesalage (Cylinder)
    1. Aircraft as far as I can guess, is about 75m long, about 20m in height and a body thickness of about half a metre(.5m)
    2. Aluminium density is approx 3kg/m3
    3. weight of body  = (Volume of outer shell - volume of inner shell)*density
    4. Weight of body = (((3.14*(20+.5)^2)*75)-(3.14*20^2*30)))*3 = 14000kg
  2. Weight of 2 wings (right triangle)
    1. Base of the triangle is around 5m
    2. Length is around 70m
    3. Thickness is .5m
    4. Weight = 2*(.5*60*5) *3 = 525kgs
  3. Tails/Stabilizers (right triangle, 3 in number)
    1. Base of the triangle is around 5m
    2. Length is around 3m
    3. Thickness is .3m
    4. Weight = 3*(.5*3*5) *3 = 34kgs
  4. Engine
    1. Car engine weighs around 150kg & produces 100hp
    2. Aircraft needs around 100,000hp (in total) to fly
    3. We assume weight/hp is similar for aircraft & car.
    4. Weight of engine = (150/100)*100000 = 150,000kg
  5. Landing Gear
    1. Tyres looks to be double in size of car tyres & I believe would be made of little more dense material
    2. Truck tires weigh around 50kg so I will assume that plane tyres would be little heavier around 75kg
    3. Total weight = 10*75 = 750kg
    4. Rest of the landing gear is hard to guess but from the looks of it it looks ot be made of metal and some mechanical gears. It doesn’t look to be very heavy in scheme of things, so will ignore it
  6. Wiring & Stuff - Will ignore as it would be very less to change the number drastically
  7. Total Weight = 14000+525+34+150000+750=165,000Kgs
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Hey this is really helpful. Thanks for posting. It will really help if you can higlight where you got some of your assumptions from. For example, how did you determine the density to be 3? Is it ok to just say you'll be assuming it's 3kg /cubic meter?
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