At 14 : 24 Central Standard Time, Flight 553 was cleared by air traffic control to Midway Airport on an approach to runway 31L . The localizer approach for runway 31L used an Outer Marker Beacon ( OMB ) named " Kedzie, " located prior to the threshold of runway 31L . Under the published landing procedures, the aircraft was to maintain a minimum altitude of until it passed the OMB, at which point the flight was allowed to descend to a minimum altitude of.
