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Estimation - Number of Airplanes in Air

Product Management Interview: Guess Estimate the total number of Airplanes in Air

My PM Interview

Aug 21, 2021

Before jumping in,

Important Read:

My PM Interview - Product Manager Interview Question Answers

PM Interview: How to Answer Estimation Questions?

2 years ago · 3 likes · My PM Interview

Now, let go through the solution,

Step 1: Clarify the scope of the question

Are we referring to the whole world or are we referring to any particular country like the US?
A) You can consider the US for this question.
There is a huge variation between the number of airplanes depending on the time of the day (e.g. 2 PM or 3 AM), is it okay to estimate the number for a peak time such as 3 p.m or 6 p.m in the evening?
A) Yes
Are we talking about all types of planes? like Private planes, Commercial passenger planes, military planes, and Cargo planes?
A) It’s up to you.

It makes sense to say that, you'd like to focus on commercial passenger planes for now since they represent most of the airplanes in the air and it narrows down the scope of the question for you further.

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Step 2: Create the Main Equation

The key insight to keep in mind is that,

The number of airplanes at any point in time = The number of airplanes that have taken off at some point in the past and have not landed yet.

Let’s say, the average time of a flight for a domestic flight is two hours and for international is five hours. Since the average amount of time is different for domestic and international flights, we would break down the equation into two parts so your main equation would be,

Total number of flights in the air = Total number of domestic flies in the air + Total number of international flights in the air

Step 3: Break down the unknowns in the equation into simpler equations

Total number of domestic flights in the US air = Total number of airplanes that have taken off from airports of all sizes (small, medium, large, mega).

Total number of departures from each airport size,

= Total number of domestic departures from one airport in that size group X Number of airports in that size group

Total number of departures in last 2 hours in one airport,

= Total number of lanes X Total number of airplanes departed per lane per hour X 2 hours