The Army National Guard has ordered an aviation safety stand-down of all its helicopter units following two accidents in February, including a crash that killed two Mississippi Army National Guard pilots.

The length of the stand-down, which went into effect Feb. 26, is “undetermined,” said Capt. Jamie Mason, an Army National Guard spokesman.

“Units and crews will be allowed to resume flying once they’ve completed the directed review of safety policies and procedures,” Mason said. He added that because of the distributed nature of Army National Guard aviation units, “we are providing leaders with enough flexibility” to effectively conduct the review.

Lt. Gen. Jon Jensen, director of the Army National Guard, said in a news release that he ordered the safety stand-down “to ensure all of our crews are prepared as well as possible for whatever they’re asked to do.”

“We are a combat force with helicopters training or on mission worldwide every day,” Jensen said in the release. “Safety is always at the top of our minds.”

Each of the crashes in February involved AH-64D Apache helicopters. The incidents are under investigation by the Army Combat Readiness Center, and no cause has been determined, spokesman Jimmie Cummings said.

On Feb. 12, an Apache helicopter flown by pilots with the Utah Army National Guard crashed in West Jordan, Utah, during training. The two pilots were injured.

A Feb. 23 crash during training in a wooded area of Prentiss County, Mississippi, killed Chief Warrant Officer 4 Bryan Andrew Zemek and CW4 Derek Joshua Abbott.

Zemek, 36, was an AH-64D Apache standardization instructor pilot with 1st Battalion, 149th Aviation Regiment. Abbott, 42, was a maintenance test pilot with 2nd Battalion, 151st Lakota Medical Evacuation Unit.

They served at an Army aviation support facility in Tupelo, Mississippi.

“Today is a devastating day for the Mississippi National Guard as we mourn the loss of two of our brave brothers in arms,” Maj. Gen. Janson Boyles, the state adjutant general, said in a Feb. 25 statement.

The Army National Guard’s 5,500 rated pilots, of which some 4,500 are assigned to flying positions, will be permitted to resume flying once they’ve completed the review process, Mason said. The stand-down does not apply to Army National Guard fixed-wing aircraft units.