OMA's Sweet September

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.