What happened

Shares of Boeing (NYSE:BA) airplane-parts supplier Spirit AeroSystems (NYSE:SPR) stock are up a strong 8.5% in 1:55 p.m. EDT trading Thursday -- and if I had to take a guess, I'd say they're up for the same reason Boeing itself is up today: Hope that airlines are getting healthier.

In a column earlier today, Barron's magazine pointed out that American Airlines saw 71% sequential growth in passengers between April and May and is planning to expand capacity on domestic flights to about 55% of "normal" by July.

International capacity should grow to about 20% by July as well.

A Boeing 737.

Image source: Getty Images.

So what

That's the headline that's getting the most attention today, for the simple reason that more airplanes flying means more money Boeing can make from servicing those airplanes through its $18.5 billion Global Services business. More airplanes flying also means, though, that more airplanes will eventually get worn out and need to be replaced.

That's where Spirit AeroSystems comes in. According to its most recent 10-K filing with the SEC, Spirit AeroSystems depends on Boeing -- and in particular Boeing's 737 line of airplanes -- for "approximately 53% of our net revenues." Simply put, therefore, what is good for Boeing is good for Spirit AeroSystems. (And the converse is also true.)

Now what

And now here's the rest of the good news: Last week, Boeing announced that it has restarted production of its 737 MAX airplane -- the very airplane upon which so much of Spirit's business now depends.

Although the 737 MAX hasn't yet been cleared by the FAA as safe to resume flying, the simple fact that Boeing is building the airplanes again suggests both that (a) Spirit can expect to get more revenues now and (b) Boeing is confident it will receive certification for the plane to resume flying pretty soon (hopefully in just a month or so) -- so Spirit can expect even more revenues later.

With Spirit stock now trading below 12 times trailing earnings, it's no wonder investors are excited by this news.