How many miles does a Southwest Airlines plane fly in a month?

July 01, 2006

This is a very straightforward market sizing question. There are some small wrinkles in it, but they are minor. Use this as a warm-up for more difficult questions in the future.

The basic math here will be the number of segments a plane flies multiplied by the number of miles per segment. These two inputs, however, are correlated. For example, if a plane flies from Los Angeles to Phoenix for half the month and then from Phoenix to Baltimore for the other half, the number of segments per day for the first half of the month is going to be higher than the number of segments per day for the second half. So let’s make the simplifying assumption that the plane flies the same segment for the entire month. And let’s assume that this plane flies from Oakland to Phoenix.

Remember that interviewers really like assumptions that de-clutter a problem. Don’t worry too much about the accuracy of the assumption itself.

So how far is it from Oakland to Phoenix? For ease of calculation, let’s assume that it’s 500 miles. It’s probably a bit further than that, but it’ll make the math easier. Now we need to find out how many segments the plane can fly in a month.

Let’s estimate that a trip from Oakland to Phoenix door to door is about 90 minutes. And I know that Southwest is pretty quick in turning around flights, so let’s say that a plane can unload and reload in 30 minutes. So, for example, if a flight leaves Oakland at 7:00 a.m. and arrives at 8:30 a.m., it will leave Phoenix at 9:00 a.m., and arrive back in Oakland at 10:30 a.m. So a round trip takes three and a half hours. Let’s say that a plane can do three round trips in a day, which is the equivalent of 3,000 miles. Multiplying this by 30 days gives us 90,000 miles.

This answer would score you about a B. It answered the basics, but there are a couple of other factors to consider.

However, we need to think about scheduled maintenance time. A plane can’t fly all day every day. So let’s say that once a week it undergoes basic servicing. This gives it roughly 25 days of flying (again, using round numbers for ease of multiplication). Twenty-five days times 3,000 miles per day equals 75,000 miles.

Now it’s about an A-.

Finally, there are probably delays or flight cancellations that impact the mileage. Let’s assume that five times a month a round-trip flight is canceled, which will reduce the mileage by 5,000. This means that this plane flies 70,000 miles per month.

Now you have an answer that merits an A.