The U.S. Department of Transportation subsidizes regional air carriers through their Essential Air Service program. This program basically pays all or most of an airline's cost to serve small cities that could never sustain scheduled passenger service on their own. There's a list of criteria cities have to meet, including being a certain number of miles from larger commercial airports, city government funding and promised passenger loads, etc. But these airports are usually among the smallest population areas to be served by scheduled service.
In my part of the world, KBWD (Brownwood Regional, Texas) is a good example. I don't want to knock the city of Brownwood, but it's a non-descript city with a "metro" population of around 25,000 that most people will never hear of in their lifetimes. Durango, CO
and similar cities cannot be compared to Brownwood, because they are major tourist destinations; the only people with an interest in flying to BWD are local residents or business executives; 3M
has a large manufacturing plant there, and is probably the only reason BWD has pax service at all. Current service is provided by Mesa Airlines, providing two round-trips a day to DFW
, using one Beechcraft 1900 that is also used for subsidized routes to Ponca City and Enid, Oklahoma. Passenger counts for 2003 were around 3,700 for BWD (just over 10 per day on average). BWD is only about 55 nm from ABI
and 117 nm from both DFW
As of a few weeks ago, Mesa was scheduled to pull out of BWD due to low pax loads. Corporate Airlines has submitted a bid to the U.S. DOT to provide 12 round-trips a week using Jetstream 32s.
Airfields that are part of the Essential Air Service program are probably the best examples of the smallest airports/metro areas with commercial passenger service.