Shares of Spirit Airlines Inc. tumbled Friday after the company warned that a key revenue figure would fall this year.

The Florida airline said Thursday that its revenue per available seat mile, which measures how much money the airline makes to fly a passenger a single mile, will fall between 2.5 percent and 4.5 percent. Besides the recent impact of Hurricane Isaac on bookings, Spirit also blamed a tax reprieve in August 2011 for its lower forecast results. Last August a congressional standoff led to a period where some government taxes on airline tickets weren’t collected. Airlines including Spirit took advantage by raising fares by an amount similar to the discontinued taxes, thereby padding revenue. Raymond James analyst Savanthi Syth