BOISE - Military investigators said human error caused an Idaho Army National Guard helicopter to crash during a training mission in November near the Boise airport, leading to the death of two pilots on board.

Right before the crash, the pilots were practicing a routine emergency procedure: flying their Apache attack helicopter to safety on a single working engine, Col. Tim Marsano, a Guard spokesman, said in a statement.

To simulate loss of power, they were supposed to momentarily slip one of the engine power-control levers into the lock-out position, then pull it back, decreasing power to one of the engines.

But investigators concluded one or both of the pilots pushed both control levers into the lock-out position and kept them there for too long, causing the engines to over-speed and shutting down the engines.

When they lost power, the pilots had three seconds to respond before impact, Marsano said, and that's not enough time to restart the engines or otherwise recover the aircraft. The accident occurred about 400 feet above ground level, in the darkness.

Both pilots - chief warrant officers Stien P. Gearhart, 50, and Jon L. Hartway, 43 - were killed instantly upon impact because of blunt-force trauma, the Ada County Coroner's report determined. The two were the only men aboard the aircraft. The crash also resulted in a total loss of the helicopter.

The crash investigation concluded it was not possible to determine which pilot inadvertently placed both engine power control levers into the lock-out because the levers can be operated from either the front or rear seats.

The investigation also determined that all Guard supporting aviation systems were within normal Army standards.

Apache aircrews will be briefed on the investigation, Guard officials said, with the goal of preventing similar accident.

"This routine, hands-on instruction is critical for military helicopter aircrews, since it trains them how to quickly respond to the loss of one engine's power during aircraft operations on both combat and training missions," Marsano said. This emergency procedure is regularly practiced by Apache pilots across the entire U.S. Army, he said.

Both pilots were assigned to the 1-183rd Attack Reconnaissance Battalion headquartered at Gowen Field in Boise. Gearhart lived in Meridian, and Hartway lived in Kuna.

Marsano said the families had requested privacy.