On April 19, 1998, at 2030 central daylight time, a Boeing 727- 227, N722AA, operated as American Airlines Flight 1625, experienced an uncommanded passenger evacuation while parked at Gate L6 at the O'Hare International Airport, Chicago, Illinois. The three cockpit crewmembers and four flight attendants were not injured. One passenger suffered a serious injury, 2 passengers received minor injuries, and 146 passengers were not injured. Visual meteorological conditions prevailed and an IFR flight plan was filed. The airplane received minor damage when the over wing emergency exit hatch was thrown onto the top of the left wing during the evacuation. The 14 CFR Part 121 flight had a destination of Kansas City, Missouri. The cockpit crew was in the process of starting the Auxiliary Power Unit (APU) when passengers saw flames on the right side of the airplane. A passenger was heard to scream "fire" at which time the uncommanded evacuation was initiated by the passengers. Passengers exited the airplane via the left over wing exit(s), the rear airstair door, and the main entry door onto the jet bridge. In addition, at least one passenger exited the airplane through the L-2 door. A ten year old boy received a broken arm and two other passengers received minor leg and ankle injuries as they jumped off the wing. The captain reported that the APU "torched" as it was being started. He stated the passengers stampeded the doors, exiting the airplane through the aft airstairs and the over wing exits. The first officer reported hearing passengers yelling "fire" at which time he called for the fire equipment and assistance. He stated the captain then determined what the problem was and he attempted to calm the passengers down with an announcement over the public address system. The #1 flight attendant reported that she walked into the aisle and saw the passengers in the middle section of the airplane in "hysteria." She stated her first thought was that there was a fight onboard the airplane. She went into the cockpit and informed the flight crew that there was a problem. She reported she then looked back and saw passengers running toward the front of the airplane. She stated she ran onto the jet bridge to get out of the way. She reported, "I had no idea the people thought there was a fire and I had no idea the people were evacuating using the window exits and the aft stairs." The #2 flight attendant reported hearing someone yell "fire" during the boarding process. She stated she saw flames coming from the right engine and thought it was "torching." She stated she tried to explain the situation to the passengers, but no one listened. She reported the passengers were coming at her in the rear of the airplane so she opened the aft airstair and let them exit. She reported, "I honestly thought they [there] were [was] going to be a stampede." The #3 flight attendant reported she was near the cockpit door when she heard a "weird sound" and turned to see "half the aircraft passengers stampeding toward me." She reported hearing them yelling there was a fire. She reported she looked for signs of a fire and without seeing any she tried to slow the passengers down as they exited the jet bridge. The #4 flight attendant who was in the back of the airplane reported hearing someone yell "fire" at which time she saw a flame coming out of the right engine. She reported the passengers panicked and started jumping out of their seats and running up the aisle. She reported that in order to avoid getting trampled she turned and went to the front of the airplane where she informed the captain of the flames. She then remained in the front of the airplane until all the passengers had exited the airplane. This flight attendant reported, "I must add that the passengers took over the aircraft and it was pure mayhem. It was a very frightening experience." Questionnaires were mailed to the passengers on board flight 1625 as part of the NTSB Evacuation Safety Study. Some of the passengers reported the airplane was still being boarded when the evacuation occurred. The majority of passengers who responded to the questionnaire reported panic inside the airplane with the passengers pushing each other to get to an exit. Several passengers reported they went toward the front of the airplane and were redirected by a flight attendant to go out the back of the airplane. Of those who have reponded to the questionnaire at the time of this report, 13 passengers reported that they exited the airplane via the overwing exit(s) on the left side of the airplane. Seven of these passengers reported they jumped off of the wing. The remaining six reported they stayed on the wing for a period of time prior to reentering the airplane and exiting through either the forward or aft exit doors. One passenger reported opening and exiting the airplane through the L-2 door. This passenger reported she was lowered out of the airplane by another passenger until she was at a point about seven feet off the ground at which time she jumped.