Some arriving flights were delayed because of the intermittent outage, which is under investigation, according to Karen Pride, spokeswoman for the Chicago Department of Aviation.
The problem occurred when there were already flight delays of about 90 minutes because of fog, according to the Aviation Department.
At one point, a controller at the O'Hare tower tells the pilot of United Flight 6849 to "use caution."
“It seems to be the approach lights on all the runways are starting to become intermittent," the controller radios, according to a recording provided by liveatc.net. "Two-eight went out one time. It looks fine now but use caution.”
Three voices are then heard on the recording:
Voice 1: "United 68-49 clear to land runway two-eight.”
Voice 2: “Clear to land two-eight, United 68-49."
Voice 3: “(inaudible)6-4-8. There were no approach lights for 2-8."
Voice 1: “Alright, the approach lights are out on runway 2-8 for United 6849 (inaudible) heavy."
Voice 2: “Copy.”
The approach lights are the strobe lighting outside the runway apron that provide a visual aid to pilots while lining up their landing. There is also a separate set of lights closer to the runway apron that warns the pilot if the descent rate is off -- too high or too low.