By Suzanne Roig
HONOLULU (Reuters) - Sand and dusty particles got into an engine of a military helicopter during a landing attempt in Hawaii earlier this year and contributed to a crash that killed two U.S. Marines and injured 20 others, officials said on Monday.
The crash raised questions about the safety of the aircraft which is built by Boeing Co and Textron Inc's Bell Helicopter and is designed to take off like a helicopter and then rotate its propellers to fly like a plane.
The results released on Monday of a military investigation into the May 17 crash of the MV-22 Osprey found sand and dust was kicked up when the aircraft hovered at low altitudes during attempts to land at Marine Corps Training Area Bellows on the island of Oahu.
Particles got into an engine on the aircraft, and the buildup of matter on turbine blades and vanes led to a stall which caused the helicopter to come down hard, the U.S. Marine Corps Forces, Pacific, said in a statement.
"The pilots did not violate any regulations or flight standards; however, pilot decision-making failed to take into account the contributory events that led to the mishap," the statement said.
Specifically, after the pilots encountered a high level of sand and dust on their first attempt to land, they should have taken an alternate approach such as opting for a different flight path or landing site, the statement said.
It added that the investigation resulted in a recommendation to improve the engine air filtration systems for the MV-22.
Also, due to the crash, the military on Nov. 17 issued a directive decreasing the amount of time pilots who are landing can spend in brownout conditions, according to the statement from the Marines.
The investigation resulted in a number of technical recommendations to aid pilots who might face a similar situation.
Those included displaying engine performance and stall margins on a display in the cockpit and advisories to alert pilots when engine power falls below 95 percent.
Representatives for Boeing and Bell Helicopter could not immediately be reached for comment.
Development of the MV-22 was nearly canceled after the deaths of 23 Marines during flight testing in 2000, but its speed and range have made it very popular in recent years.
The aircraft is used by the U.S. Navy to deliver people and cargo to aircraft carriers and for humanitarian missions and disaster relief.
(Editing by Alex Dobuzinskis and Kavita Chandran)