Sometimes, it's just that easy. I recommended Grupo Aeroportuario del Centro Norte (NASDAQ:OMAB), known as OMA, to members of our Motley Fool Global Gains newsletter service for a couple of very simple reasons.

First, airport operators have monopolies that are rarely broken, creating a nearly bottomless moat. Next, this particular airport group had a growth story that was different from, say, more tourist-dependent Grupo Aeroportuario del Sureste (NYSE:ASR). At OMA, the potential gains would come from low-budget airlines making inroads with domestic passengers inside Mexico. The story -- which sounds a little crazy, until you ditch the "impoverished Mexico" stereotype -- was that low-cost carriers like Ryanair (NASDAQ:RYAAY) cousin VivaAerobus, could take a bite out of the big, long-haul bus business in Mexico.

And that story still seems to be coming true. This week, OMA released passenger figures that show an incredible 23.8% increase in traffic from September 2006 to September 2007. That was directly attributable to a 26.6% increase in domestic passenger traffic owing to new business from carriers Aladia, Alma, Avolar, Interjet, VivaAerobus, and Volaris.

International traffic, from carriers such as UAL's (NASDAQ:UAUA) United, Alaska Air Group (NYSE:ALK), AMR's (NYSE:AMR), American Airlines, Delta, and Continental Airlines (NYSE:CAL), contributed to a smaller, but still very healthy, 8.3% increase.

Thus, the payoff theory is very simple. Relatively fixed costs (the airports) taking in larger numbers of passengers -- who pay tariffs and spend money on things like food, parking, and car rentals -- will eventually leverage into growing margins and great cash flow.

That's why I still like OMA's shares, and hold them myself, even though they've tacked on some 30% (beating the S&P 500 by nearly 20 percentage points) since I recommended them at Global Gains. Investors looking at a long-term hold will likely do fine even from today's higher prices, but because storms and short-term system burps like airline pullouts can cause significant turbulence, it's also a good idea to keep some money on the side, and wait for shares to become an even bigger bargain.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.