There is about $18 billion in municipal airport revenue bonds outstanding, according to Securities Data Corp. How will the weakness of the airline industry affect that debt?

A recent Standard & Poor's corporate credit review of the airline industry shows just how bad off it is.

Of the 17 major domestic airlines or their holding companies that are rated, only four have a credit rating as high as A. Three have a BBB rating and the rest have junk bond ratings.

The airports are better off. Of the 58 airport bonds S&P rates, none is classified as junk.

A starting point for evaluating an airport is the overall demand for air travel there. How much travel does business and tourism generate?

Also important is whether an airport's business results from "origin and destination," which means that the region generates the business itself, or whether it is a hub facility, such as Atlanta or Pittsburgh, that deals mostly with people passing through.

A hub operation can be affected if it is served by two or three airlines that are weak and one seeks bankruptcy protection, as Eastern did.

Fortunately, Eastern's troubles did not affect the major airports where it had a big presence, Atlanta and Miami.

The passenger demand was too great there to really jeopardize their credit standings, and the other airlines were able to pick up Eastern's slack.

The message here, as always, is to know what you are buying. The capital expenditures of airports will remain large, and so will the amount of airport financing ($1.44 billion in this year's first quarter).

Know something about the passenger demands and the airlines that serve the airports.