Operator error likely caused a medical helicopter to break apart mid-flight and crash during a training exercise last year, killing the pilot and flight instructor, National Transportation Safety Board investigators said in their final report.

The Mercy Flight helicopter was on its second training run of the day when it went down in a field in the western New York town of Elba in April 2022. No patients were on board.

In a report issued Dec. 8, investigators cited the pilots' "inappropriate flight control inputs" while practicing for an unstable condition known as vortex ring state. The errors caused the main rotor blade to make contact with the helicopter's tail, leading to the mid-flight break-up. The flight instructor's "inadequate monitoring of the flight" also contributed, the NTSB said.

NEW YORK MEDICAL HELICOPTER CRASHES, KILLING TWO PEOPLE: POLICE

Killed were pilot James Sauer of Churchville, a retired New York state police pilot, and Bell Helicopter instructor Stewart Dietrick, of Prosper, Texas.

According to the report, the pilot from the day's earlier training flight described being surprised that the instructor did not intervene when, after instructing the pilot to perform a vortex ring state recovery maneuver, the helicopter developed a very high descent rate.

"While in VRS, the pilot stated that he didn’t know why they were going so deep into VRS and that the instructor was just sitting there, ‘hands on his lap,’" the report said. "So, the pilot, feeling uncomfortable at that point, had to exit this very high descent rate on his own rather than waiting for further guidance from the instructor pilot."

A spokesperson for Bell, based in Fort Worth, Texas, did not immediately respond to a request for comment.

Buffalo-based Mercy Flight transports patients to area hospitals in a Bell 429 helicopter, according to the not-for-profit company’s website. The twin-engine aircraft was housed at the Genesee County Airport, which is a little less than 3 miles from the crash site.