Estimate the weight of an aeroplane. How would you go about calculating it?

Is the aeroplane filled with passengers?
What kind of aeroplane are we talking of? - Commercial, cargo, military jets, private jets, or something else.

Assuming that it's a mid-sized commercial aircraft in the question and the aircraft is empty (including the fuel tank). The following would be my approach to estimate the weight of the aeroplane.

Estimating the length of the aircraft - Assuming that the main seating area has 30 rows of seats and each seat gives a meter of leg space, that adds up to 30m. Furthermore, we can add an additional 10m of length taking in the cockpit and other miscellaneous lengths into consideration.
Estimating the height of the aircraft- Taking 2m the height of the seating area and around 3m for the luggage compartment. The net height can be taken as 5m.
Estimating the thickness of the aircraft skin- Assuming this to be 1cm.
Estimating the density of the material- Since water is 1000kg per cubic meter, it would be fair to assume the aircraft material would have a density of 2000kg per cubic meter.

Net volume of the aircraft = 6.29 cubic meters

Hence, the weight of the entire aircraft would be - 2000 x 6.29 = 12585 kg

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Bijan

Platinum PM

Jun 1, 2020

Things you did well

Clarifying Questions: You started by asking a few good clarifying questions
Narrowed down scope of the project well: You described well what’s included in your estimation and what’s excluded
Assumptions: You did a great job describing your assumptions and why they were reasonable assumptions

Areas of Improvement

Provide your formula first: I would explain in the beginning how you'd plan to estimate the number (provide your main formula right at the beginning). This puts the interviewer at ease and helps them confidently watch you go through your estimations
Sanity check: It’s a good idea to do a quick sanity check to ensure that your numbers make sense and you’re not off by an order of magnitude
Answer format: This is just a feedback for the format of your answers so people can easily read them and post more feedback. I suggest using the editor to have a heading with a bold and larger font for each section of your answer:)

I hope it helps.

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Sayan Sahay

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Jun 4, 2020

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Krishna

Platinum PM

Jul 28, 2024

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P · Answer 2 · 2020-07-14T20:10:03+0000

First, I'd like to ask a few questions:

What kind of airplane: military/commercial, small/large, model airplane / life-size airplane? Let's assume commercial, typical life-size airplane. Let's take the 747 as an exmaple, wich is one of the highest selling airplanes int he world, which I believe has two engines on its wings and fits about 250 passengers in it. Around 200 ft long
Is the fuel tank empty? How about the cabin, any people inside it? And the cargo compartment? Let's assume it's completely empty but includes all typical airplane parts.

Ok, now I would like to estimate the weight of the airplane by breaking it down into parts:

Weight of airplane =

(1) Weight of fuselage

+ (2) Weight of wings

(1) Weight of fuselage = 100K lbs
weight of frame = (80 + 108 ft^3) * 200 lbs / ft^3 = ~40,000 lbs	Assuming light metal like reinforced aluminum: ~200 pounds per cubic foot. 4 lenghtwise beams volume: 4 * 180 ft long * 1/3 ft width * 1/3 ft height = ~ 80 ft^3 Orthogonal metal circles every 5 ft (about 36 of them): Length of each circle: 8 ft radius * Pi * 2 = ~ 50 ft Volume of each circle: ~50 * 1/4 ft height * 1/4 ft width = 3 ft^3 Total circle volume: 36 * 3 = 108 ft^3
+ weight of floor	negligible
+ weight of seats	negligible
+ weight of cabin	negligible
+ weight of windows	negligible (each window is small)
+ weight of shell = 200 ft^3 * 300 lbs/ft^3 = ~ 60,000 lbs	Assuming strong metal heat proof: ~300 lbs per cubic foot Diameter of fuselage: ~50ft (from previous) Length of fuselage: 200 ft Thickness: 1/4 inch (1/48 ft) Total volume: ~200 ft^3
+ weight of wiring	negligible

(2) Weight of wings = 75k lbs
weight of engines = 64 ft^3 * 300 lbs / ft^3 = ~20,000 lbs	Assuming metal like reinforced steel: ~400 pounds per cubic foot. Let's compress the engine into a metal cube: 8ft length2 ft height2ft length: 32 ft^3 Total volume: 2 engines *32 ft^3 = 64 ft^3
+ weight of shell = 100 ft^3 * 300 lbs/ft^3 = ~ 30,000 lbs	Triangle (* 2x for top and bottom * 2x for two wings): Area of each triangle: 80ft * 30ft /2 = 1,200 ft^2 Thickness: 1/4 inch (1/48 ft) Volume = 25 ft^3 Total = 2 2 25 = 100 ft^3
+ weight of frame (1:2 of shell weight): 15,000 lbs
+ weight of wiring	negligible
+ weight of wheels (assume 1/2 of engines): 1/2*20,000 lbs = 10,000 lbs

Total weight = ~175k lbs

I did not add up all the weights of additonal equipment assuming it was negligible but it might add up to a lot. The second thing I would consider triple checking is the weight of the metals. And the third, the thickness of the shell, given that it has such a big impact on the weigth calculation.