The Army's Kiowa Warrior helicopters are being upgraded with improved engines and computer control systems as part of a series of safety and performance modifications the Army is doing to keep the armed reconnaissance helicopters flying.

According to Bell Helicopter, maker of the OH-58D Kiowa Warrior — the type that crashed yesterday at Wheeler Army Airfield, killing its two pilots — the Army's fleet of Kiowas has accumulated more than 1.7 million flight hours and 460,000 combat flight hours.

Officials yesterday said an investigation into the cause of the crash is under way.

The Army, meanwhile, has suffered some devastating losses both in Hawai'i and in combat in helicopter crashes.

On Feb. 12, 2001, six Schofield Barracks soldiers died in the crash of two Black Hawks in Kahuku during training.

The six who perished were in a chopper carrying a Humvee in a sling beneath the helicopter. The last aircraft in a group of four, it lagged behind, then overtook and collided with the lead helicopter as the choppers approached the landing zone at 7:40 p.m.

On Oct. 16, 2004, two Schofield pilots in an OH-58D died when their helicopter collided with another in Baghdad. Killed were Capt. Christopher B. Johnson, 29, of Excelsior Springs, Mo.; and Chief Warrant Officer William I. Brennan, 36, of Bethlehem, Conn. Both were assigned to Company C, 1st Battalion, 25th Aviation Regiment.

On May 28, 2007, 1st Lt. Keith Heidtman, 24, and Chief Warrant Officer Theodore Church, 32, were killed when their OH-58D was shot down in Diyala province, Iraq.

On Aug. 22, 2007, 10 Schofield soldiers who were scouts with the 2nd Battalion, 35th Infantry were killed in the crash of a Black Hawk helicopter in northern Iraq.

The four-man crew, also killed, was from Fort Lewis, Wash. It was the single worst loss of life for the 25th Infantry Division at Schofield since the Vietnam War.

Seconds into flight, when the 18,000-pound chopper was about 150 feet in the air, the helicopter fell back to earth.

An "unknown foreign object" in the tail rotor housing had traveled aft and became lodged between the housing and the tail rotor drive shaft, causing the tail rotor to fail, an Army investigation determined.