CHICAGO (CBS) — For the second time in three days, a Southwest Airlines flight aborted takeoff at Midway International Airport. The latest incident happened just hours after another Southwest Airlines plane struck a piece of ground equipment after landing at Midway.

Southwest Airlines Flight 4384 was preparing to leave for Philadelphia around 5:30 a.m. Friday, when the pilot of another Southwest jet spotted signs of a possible mechanical problem, and the pilot of Flight 4384 returned the plane to the gate.

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“On initial review, there were no indications of anything abnormal, but the plane currently remains in maintenance,” Southwest Airlines spokeswoman Emily Samuels said in an email.

The 136 passengers were placed on a new aircraft to travel to Philadelphia. The flight departed Midway about two hours late

“As always, the safety of our Customers and Employees is our top priority,” Samuels said.

Several hours earlier, around 10:15 p.m. Thursday, Southwest Flight 163 collided with a piece of ground equipment at Midway, after arriving from LaGuardia Airport in New York.

Samuels said the piece of ground equipment struck the airplane, and then the jetbridge used to get people on and off the aircraft, as passengers were waiting to leave the plane.

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“All 143 Customers and five crew members deplaned via air stairs. The aircraft is out of service for maintenance,” she said.

Samuels said no one was injured.

At least one passenger reported seeing the jetbridge catch fire, but that information could not be confirmed.

The incidents at Midway happened less than two days after another Southwest flight experienced an engine fire as it was taking off for Boston. Around 10:45 p.m. Wednesday, Southwest Flight 3299 was speeding down the runway at Midway when sparks began shooting out of one engine. The plane had barely lifted off the ground when the crew spotted the fire, and halted takeoff.

That plane was evacuated, and the 143 passengers were transferred to another plane to take them to Boston. The flight was delayed three hours.

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The cause of the engine fire was under investigation, as were the latest two incidents at Midway.