Transcript for Pilot Saved By Quick Thinking Air Traffic Controller

that pilot flying over texas, sounding fine, suddenly an air traffic controller could detect slurring in his speech. The pilot had no idea himself. This was not alcohol. This was something else. Reporter: Nearly 5,000 flights go through her airspace each day. But, it was the twin propeller plane on its way from dallas to michigan that alarmed louella hollingsworth. Can you start a descent, can you do that for me? Reporter: All she heard on the other end -- heavy breathing. That's the sound of an unresponsive pilot, soaring at 27,000 feet, in one of the country's busiest airways. There's just nobody saying words. Then after 20 or 30 miles, i kept trying to call him. Reporter: The pilot tries to respond. Listen to him again slurring his words. Another pilot hears the exchange. I don't know if you can hear that guy but he does not sound good. I didn't know if he's answer me again. She realizes he could be suffering from hypoxia or lack of oxygen. A dangerous condition at high altitudes pilots and passengers can lose consciousness in a matter of minutes. It's what happened on board famed golfer payne stewart's jet in 1999, which would lead to tragedy. Louella repeatedly urges the pilot to descend. Reporter: Finally a clear response. Five, zero, one, papa mike. Papa mike you're sounding a little better. Reporter: She guides him down to safety. Reporter:13,000 feet in 15 minutes. Thanks for the help. You were saying he was at 27,000 feet. At 35,000 feet it gets very dangerous. You may only have five to ten seconds before you completely black out. And for that air traffic controller, big medal? The medal of safety. When we come back, a simple question.

This transcript has been automatically generated and may not be 100% accurate.