Adds Flightradar24 estimate of planes' positions in paragraphs 6-7, capacities of two planes, edits throughout

By David Shepardson

Feb 25 (Reuters) - A Southwest Airlines LUV.N flight was forced to abort a landing at Chicago Midway airport on Tuesday and narrowly avoided a collision with a business jet that entered the runway without authorization, the Federal Aviation Administration said.

Around 8:50 a.m. CT, Southwest Flight 2504, a Boeing 737-800 arriving from Omaha, Nebraska, abruptly pulled up and flew over a FlexJet Challenger on the runway. The Southwest plane performed a go-around, a maneuver in which it circled and reapproached the landing.

A dramatic video posted on social media showed the Southwest jet nearing touchdown then ascending sharply. The FAA and National Transportation Safety Board are investigating.

Southwest said the crew "followed safety procedures and the flight landed without incident." According to Southwest's website, its Boeing 737-800 aircraft can carry 175 passengers.

The business jet, which Flexjet says can carry up to nine passengers, had been taxiing on runway 31C before departing for Knoxville, Tennessee.

The Southwest jet had descended to an altitude of 50 feet when it abandoned its landing only about 2,050 feet away from the business jet, according to tracking service Flightradar 24.

As it abruptly rose, the Southwest plane had only reached an altitude of 250 feet when it passed over the smaller aircraft, the service said.

Over the last two years, a series of troubling near-miss incidents has raised concerns about U.S. aviation safety and the strain on understaffed air traffic control operations. There was no indication of air traffic control error in Tuesday's incident.

In October, the FAA said it was opening an audit into runway incursion risks at the 45 busiest U.S. airports.

Democrats in Congress seized on the fact that the Trump administration recently fired 352 FAA workers, including some involved in safety operations.

"Maybe not the best time to fire hundreds of FAA workers, tell the remaining workers you want to 'put them in trauma'," Senator Chris Murphy of Connecticut said on Tuesday, citing a comment from a Trump administration official.

Transportation Secretary Sean Duffy has said firings did not include any "safety-critical positions" or air traffic controllers. Duffy did not immediately respond to a request for comment.

The FAA, which has aging technology and facilities and needs billions of dollars to modernize, is about 3,500 air traffic controllers short of targeted staffing, prompting the aviation industry and lawmakers to call for action.

(Reporting by David Shepardson in Washington; Editing by Chris Reese, Marguerita Choy, Nia Williams and Cynthia Osterman)

((David.Shepardson@thomsonreuters.com; 2028988324;))