Chicago (CBS) – A Southwest flight from Baltimore to Chicago returned to the gate shortly after departing due to a medical situation on board.

Southwest representative says while Flight 314 was at the gate, the pilots reported an issue with the aircraft’s auxiliary power unit. Mechanics were called out to fix any issues with the aircraft. The issues with the auxiliary power unit were fixed and the flight was cleared to return to their service to Chicago. The flight arrived two and a half hours behind schedule.

Southwest says the auxiliary power unit powers various systems while on the ground and say there were never any concerns with the aircraft’s engines.

This incident comes three days after a Southwest flight from New York to Dallas had to make an emergency landing after an engine exploded, shattering a window that killed one passenger and wounded seven others. Southwest Airlines is apologizing to its passengers who were on board that flight earlier this week. The airline sent a letter of apology to its passengers, including a check for $5,000 and a $1,000 travel voucher.