Flight 866 developed the problem soon after departure.
The plane landed safely and passengers are being unloaded.
Allegiant released the following statement on the incident:
"At Allegiant, the safety of our passengers and crew is always our number one priority. Flight 866 with scheduled service from St. Petersburg-Clearwater International Airport (PIE) to Pittsburgh International Airport (PIT) departed at 4:08 p.m. local time and returned at 5:07 p.m. due to a maintenance issue. Our maintenance team is evaluating the aircraft at this time. We are working now to determine next steps for rescheduling the flight and to accommodate passengers."
This is the second time in a little over a week that an Allegiant flight leaving the St. Petersburg-Clearwater Airport and had to return due to an issue with the plane.