The Boeing 777, parked at the gate at O’Hare International Airport, was extremely hot. As in 115 degrees. The plane’s auxiliary power unit, which generates electricity when engines are shut down at the gate, was broken. That killed the air conditioning and triggered a five-hour ordeal for passengers and crew.

All other flights to Denver were booked. United tried to find a replacement plane, but none was available.

United officials had a dilemma: Figure out how to cool the plane in near 100-degree heat, then board the passengers and send them to Denver; or buy them hotel rooms in Chicago.

They chose the first option.

"I couldn’t breathe; I thought I was going to faint," said passenger Sandy Ball, in seat 37C.

The plane was due to leave at 2:45 p.m.

Maintenance workers first tried to pump cool air into the empty plane at the gate. That didn’t work. Then the crew started one engine, called for a push back and drove the plane away to get the temperature to a bearable level. That helped.

About 6:30 p.m., the big jet reappeared at the terminal and a growing gaggle of United officials told remaining travelers they would be boarding. The cabin still was hot, but once an engine was started, the temperature would drop.

By 7, all were on board, and the door closed. It was hot in the cabin, probably in the low 90s. Officials said they had extra water and juice, but instead of a bottle of water on each seat, there was a blanket.

Passengers waited for the engine start – and waited some more. The body heat of hundreds of people was lifting the temperature.

Captain Michael Glawe gave an ultimatum to United officials: Get air hooked up for the start in one minute or empty the plane. He was worried about heatstroke in the cabin.

"I was right on the verge of getting everybody off the airplane," he said Tuesday. "The plane was so heat-soaked that it was going to be warm until we got to altitude."

Passengers began to use cellphones to call for help. Finally the engine was started.

Ball was at her breaking point by then.

"I was going to stand up and scream," she said. "They endangered our lives putting us on that plane."

One frustrated flight attendant told passengers to call United’s headquarters, "otherwise it will never change."

The plane took off at 8. Within an hour, it was so cool in the cabin that many passengers were grabbing the blankets.

As the jet landed in Denver, a flight attendant begged people to give United another chance. She said, "This has been a very embarrassing and unprofessional situation."

"Our crew did the best job they could to get that plane cooled as quickly as possible," United spokeswoman Robin Urbanski said Tuesday. "We’re extremely sorry."

Denver Post staff writer Jeffrey Leib was a passenger on Flight 909. He can be reached at 303-820-1645 or [email protected].