CROMWELL, Oklahoma - The National Transportation Safety Board blames maintenance personnel for the crash of a medical helicopter near Cromwell on January 2, 2013.

The Eurocopter EC130 B4 had just taken off from the Seminole Regional Airport at 12:42 p.m. when it lost engine power.

It was heading to Okemah to pick up a patient at the hospital there.

The pilot told investigators that as the helicopter was climbing through about 1,000 feet of altitude he heard a sound as though something had struck the helicopter and the engine stopped producing power.

He performed an autorotation -- a maneuver in which a helicopter uses the airflow around it to spin its rotor blades and produce lift -- and landed in a field, narrowly missing power lines and a barbed wire fence.

He and his three crew members were seriously hurt; the helicopter sustained major damage.

According to the NTSB's Probable Cause Report on the crash, the helicopter lost engine power because the engine ingested ice. Investigators found damage to four of the blades on the axial compressor.

"For 3 days before the accident flight, the helicopter was parked outside without its engine cover installed and was exposed to light drizzle, rain, mist, and fog. The engine inlet cover was installed the day before the accident at an unknown time. The helicopter remained outside and exposed to freezing temperatures throughout the night until 2 hours before the flight," the NTSB concluded.

The NTSB says maintenance personnel kept the helicopter ready to go and performed daily preflight/airworthiness checks, but never checked the inlet to the first-stage of the axial compressor to make sure it was free of ice.

According to the NTSB, the company that operated the helicopter, Air Methods Corporation, as well as the engine builder and the FAA have all issued recommendations about operating helicopters in icing and/or snowy conditions based on this incident.