How would you weigh an aeroplane

3.5 stars

A common brain teaser used in some job interviews... how would you weigh an aeroplane without scales?


Unless you're going for a role which requires you to weigh aeroplanes, this question is likely to be more about evaluating your 'out of the box' thinking.  

Ideas include:

  • Put the aeroplane on a ship and see how far it submerges into the water.  Mark the point on the side of the ship, then take the plane off and put weights on the ship and see how many are needed before the ship submerges to the same point
  • Use the tire pressure, tire surface area touching the ground, physics formula Pressure = Force / Surface Area; etc
  • Newtons second law

Any other ideas?  Let us know in the comments.

Difficulty
  • Hard
Type
  • Logic
  • Mathematical
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Comments

1. SHORTCUTS: Check the mfr's website for a spec sheet. Failing that, call their PR department. Failing that, call their sales department and tell them you want to buy one or two, but first you need to know the weight to make sure your private runway you're having built can handle it.
2. MATHS: Sum of its replacement parts - works only if the catalog or parts/maintenance manual is 100% complete and you completely empty the fuel tank(s).
3. MECHANICAL: A pulley or lever set up with counterweights (if those dont count as scales.)
4. CYNICAL: Convince a smartphone manufacturer that the shape of the cockpit window violates a design patent they hold; download the subsequent lawsuits' transcripts.
5. SUBVERSIVE: Email the mfr's security chief a do-not-reply memo stating new, urgent Dept of Homeland Security policy requiring immediate online disclosure of all aircraft weights and measures. Note: Do NOT use FAA logo - the DHS is still too disorganized to have effective regulation capture, but wields far more power than the NTSB.
6. WHIMSICAL: Drop it off the Leaning Tower of Pisa and count how many coins bounce up in the air when it impacts. You won't learn the weight, but hey, it's a free trip to Italy and the video will go viral.
- QuezaraT (19 Dec 2013)

-At the moment a plane takes off, calculate the surrounding air constants, pressure, temperature, etc. as well as the angles of the wings and the speed of the airplane. With all this information, it is possible, not for me personally, but, someone with a background in this can calculate the force at liftoff which should be equal to the force keeping it on the ground, divide by gravity and you have the weight of the plane.
- Poppo (7 Jan 2014)

Submerge the plane in water, preferably in pure water, then attach small inflatable devices evenly until the plane begins to rise. Once it rises and just comes out of the water, you can calculate the volume and buoyant forces and essentially go through the same process as if on a boat.
- Poppo (7 Jan 2014)

Find out the make and model of the plane and then use Google to research it online.
- id36 (10 Jan 2014)

Gay-Lussac's Law: The Pressure Temperature Law. A chamber that is compressable (to withstand very high force of gravity) containing a gas will increase in temperature. P is proporitonal to T.
- arthur pletcher (2 Feb 2014)

Fill large container with water, float aeroplane in water, collect water that overflows, and measure its volume in liters. This yields mass in kilograms. Variation: Use a cylindrical tank but don't fill to top; multiply rise in water level by pi and the square of the tank radius to obtain volume of water displaced. Note: If aeroplane won't float in water, use a denser liquid like mercury. Another idea: Transport it to the top of Mount Everest and see how much the rotation of the earth slows down.
- Ron (3 Jul 2014)

1) I will weigh some scales.
2) I will add the "scales" to aeroplane and weigh it.
3) from Step 2 deduct Step 1 to arrive at the weight of aeroplane without "scales"
- Prashanth (16 Mar 2015)

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