What happened

Heico (HEI 1.33%). Wabtec (WAB 0.39%). TransDigm (TDG -0.21%).

As makers of aircraft components, train locomotive components, and ... more aircraft components, you might expect all three of these stocks to suffer somewhat on the day after President Trump introduced an air travel ban into a market where folks weren't all that interested in traveling to begin with. And you'd have been right to think that.

Shares of all three of these companies tumbled more than 10% in early trading Thursday. Heico ended the day down 10%, TransDigm 11.9%, and Wabco a (relatively) modest 5.1%.

Red arrows going down and crashing through the floor

Image source: Getty Images.

So what

Was there an actual reason for this sell-off happening Thursday, though?

Surprisingly, no, there was not. Sure, it probably didn't help matters that the president went on national TV Wednesday night and instructed "immigrants and nonimmigrants" from the European Union to stay away from America for the next 30 days.

That's definitely going to crimp the revenue streams at some of America's biggest airlines. But I rather doubt it is going to immediately affect sales of necessary plane parts by Heico and TransDigm, much less affect the fortunes of Wabtec, which caters more to the train industry than to the airlines. (Last time I checked, we didn't have many trains running Trans-Atlantic routes.)

Rather, selling shares of Heico, TransDigm, and Wabtec today feels like it was a panic move, plain and simple.

Now what

Recognizing that, it's now more important than ever for investors to keep a cool head, and don their thinking caps -- to not sell in a panic, but also not buy blindly because stocks are merely cheaper today than they were yesterday. (Cheap-er is not the same thing as cheap). Instead, it's time to sharpen pencils, examine valuations, and buy only those stocks that have fallen far enough to become clearly cheap.

Is Wabtec, for example, a bargain you should buy at its current valuation of nearly 34 times earnings? (Perhaps not.)

How about Heico, now on sale for the low, low price of just ... 41 times earnings? (Um...)

Then perhaps, TransDigm is screamingly cheap at 50 times earnings? (Excuse me, but I think you're counting in the wrong direction.)

Suffice it to say that as scary as today was for many investors, many stocks -- including these three, in my estimation -- are still not cheap enough to buy. As much as many stocks have fallen already, I still see room for them to fall some more.