Wall Street is increasingly worried about the direction of the economy, and as a result investors are selling off companies that could be most exposed to a downturn.Shares of American Airlines Group (AAL -4.23%), arguably one of the most vulnerable of the airlines, traded down 5% as of 1:30 p.m. ET.Headwinds on the horizonAirline stocks tend to be cyclical, doing better when times are good and falling off when consumer confidence falls. Households are more likely to forgo a vacation than a grocery store run ...