The calm voice of a helicopter pilot announcing a Mayday situation moments before he crashed his air ambulance into a grassy field next to Interstate 57 Saturday night sheds light on a quick decision that spared the lives of all four people aboard.

Audio of the emergency communication — transmitted over public airwaves — was recorded and posted to liveact.net, a site where enthusiasts share air traffic communications.

“Mayday. Mayday. Mayday,” the pilot announces.

“What can I do for you, sir?” an air traffic controller asks.

“I need to find a place to land,” the pilot says.

“Can I give you any assistance?” the controller asks.

“Not at the moment. We’re going down.”

“Do you know where you’re going to be landing at?” the controller asks.

“In a field,” the pilot says — his last transmission before the crash.

A few moments later, an air traffic controller asks the pilot of a nearby Chicago Police helicopter if the helicopter “went down hard?”

“Yea, absolutely. It looks like it’s on fire,” the police helicopter pilot responds.

(To hear this audio, click here and go to 22:40.)

The red helicopter crashed into a grassy field about 9:15 p.m. just feet off the roadway near the junction connecting Interstate 57 and the Bishop Ford Freeway.

The air ambulance pilot, a flight nurse and a paramedic all were able to exit the helicopter after the crash. Chicago firefighters removed the fourth occupant, a medical patient who was being transported to a Chicago-area hospital, authorities said.

Two people were taken to Christ Medical Center in Oak Lawn, while two others were taken to University of Chicago Medical Center, a Chicago Fire Department spokesman said Sunday. They were all listed in serious to critical condition; the conditions of three of the people have since been stabilized.

The helicopter’s rotor blades remained intact as the aircraft landed on its belly, according to Chicago Fire Department Deputy District Chief Lynda Turner.

The aircraft was smoking, but it didn’t catch fire, Turner said at a news conference held Saturday night.

“The pilot did an excellent job of landing a helicopter that was in an emergency situation,” she said.

Radio traffic indicates the air ambulance entered Illinois from Indiana and was headed to Advocate Christ Medical Center in Oak Lawn.

Multiple calls to Elmhurst-based Superior Ambulance Service, which operates the air ambulance, were not returned Sunday.

Records on the FAA’s website show the helicopter — a Eurocopter EC135 — is 20 years old.

The cause and circumstances of the crash weren’t immediately known, according to the National Transportation Safety Board (NTSB), which is investigating the crash.

The NTSB will issue a preliminary report in about two weeks that will lay out the facts and the circumstances of the accident, NTSB spokesman Peter Knudson said. He expects a final report detailing the investigation into what caused the crash to be released in six to 12 months.

The weather at the time of the crash: clear skies, light winds and 73 degrees.

“I was told the weather was not problematic, it was not challenging,” Knudson said. “But we look at everything.”