The trouble started within two seconds of the helicopter’s engine drive being disengaged from the rotors — the equivalent of putting a car transmission in neutral — in a test of the chopper’s ability to "autorotate."

Pilots are trained to use the spinning momentum of the rotor and upward airflow to keep flying.

Instead, the Army OH-58D Kiowa Warrior quickly lost airspeed and plummeted from an altitude of 250 feet, according to an Army investigation. The two-seat chopper crashed at Wheeler Army Airfield, killing the pilots, Stanley Blane Hepfner, 29, and Jonathan Bryce Millward, 28, both chief warrant officers.

The accident investigation, obtained by the Star-Advertiser through the Freedom of Information Act, cites pilot error as the cause. The report says a drop in airspeed, a failure to abort the autorotation and a belated attempt to power back up sent the aircraft into a descent greater than 300 feet per minute into its own downwash.

"The pilots did not correct airspeed to stay within acceptable safety parameters," the report said.

The ill-fated flight on May 27, 2009, included a required test of the Kiowa’s ability to autorotate, an emergency procedure in case the engine conks out in flight.

Civilian helicopter flight instructors say autorotation practice can be challenging and is a leading cause of hard landings.

Following the investigation, the 25th Combat Aviation Brigade modified standard operating procedures for Kiowa maintenance test flights, including changing the minimum altitude for recovery to 1,000 feet from 500 feet above ground level, officials said.

Jynnae Smedley just knows that she lost her brother, Millward, in the crash.

"I think we’ve accepted" that it was pilot error, the Idaho resident said. "We don’t know which pilot made a mistake. We know that they were both excellent pilots, and we also are aware there’s an awful lot to flying one of those helicopters. Just any slight mistake, obviously, can cause critical injury."

Although it was impossible to determine who was at the controls at the time of the crash, Hepfner was the "pilot in command," rated as a maintenance test pilot, and "was ultimately responsible for every aspect of operating the aircraft," the investigation said.

Hepfner had 1,108 hours of rotary-wing flight time. Millward had 293 hours and didn’t have the requisite experience to be a pilot in command or maintenance test pilot.

Also instituted following the crash was the requirement that both pilots be at the controls of a Kiowa during flight maintenance tasks including autorotation checks.

"These changes ensure that … there will be more time for the pilots to react if there is a problem," the 25th Infantry Division said in an email. "They also ensure that the chain of command is considering the increased risk when they have less experienced test pilots conducting autorotations so they can mitigate the risk by pairing them with more experienced pilots."

Hepfner had completed the maintenance test pilot course six months before and was on his first "utilization tour" as such with the 2nd Squadron, 6th Cavalry Regiment, according to the investigation.

The OH-58D is one of the oldest helicopters in the Army inventory. The 34-foot, two-seat chopper, which can be outfitted with missiles, rockets and a .50-caliber machine gun, is designed to fly at relatively low altitude and is not very fast, but is maneuverable and responsive.

The six-page investigation executive summary provided by the Army said Hepfner and Millward started the autorotation check at a "suitable altitude," but the report doesn’t state what altitude.

According to the investigation, the standard airspeed for the maneuver is 55 knots (63 mph), plus or minus 5 knots. Cockpit indicators were acceptable initially, but airspeed decayed within two seconds to an "unacceptably low level" likely due to simultaneous rotor adjustment and winds shifting to the tail of the chopper.

The maneuver should have been aborted once airspeed dropped below 50 knots, but the crew continued on, the report states.

Forward airspeed reached 0, drag on the rotor system dropped rotor speed, and a power recovery was started.

There was a lack of crew coordination and failure to reference airspeed and rotor blade pitch with the pilots apparently "fixated" on the autorotation, investigators found.

A duty log first notes the crash on the airfield at 3:30 p.m. A handwritten notation states, "Aircraft conducting non-standard approach appeared to never regain engine power. Hard landing on the runway, rolled over and caught on fire."

Blunt force trauma killed both pilots. The monetary loss was valued at $4.1 million for the helicopter and $124,829 for equipment.

Ben Fouts, a 14-year helicopter pilot and flight instructor with Mauna Loa Helicopters, said civilian helicopter pilots are tested on autorotation during proficiency and license tests.

"Anybody that’s (flying) single-engine helicopters, you need to get really proficient at it," he said.

Mauna Loa usually does them at 700 feet, and a minimum of 500 feet, above ground level.

"Things happen pretty quick," Fouts said. "From initiation of the maneuver to where you are on the ground with a power recovery or when you’ve completed it is maybe 20 seconds."

Autorotation training is probably the most common reason for accidents such as bent skids and hard landings, but "it’s rare that it’s fatal," he said.

Smedley, the pilot’s sister, said by phone that he loved flying — and wasn’t scheduled to be on the maintenance test flight that day.

"This was a test flight that he was not assigned to, but from what his friends told us, he was always running down to the test pilots wanting more flight time, (saying), ‘Do you need a co-pilot? Can I go up with you?’"

The pilot from Chubbuck, Idaho, was outgoing, a musician who played multiple instruments and sang, and loved to mug for photos, she said.

"Nobody will ever forget Bryce," Smedley said. "There’s a Facebook page for him that his friends set up, and they just keep adding to it. Lots of memories of Bryce. Everybody loved Bryce."

The accident occurred as the brigade, its 2,600 Hawaii-based soldiers and nearly 100 helicopters were set to deploy to Iraq. Now the brigade is preparing to deploy to Afghanistan in January with about 26 Kiowas, 14 CH-47 Chinooks and about 55 UH-60 Black Hawks based here.