"It's an unfortunate consequence of the high tempo of operations," said retired Marine Col. J.F. Joseph, an aviation safety consultant. "They're out there working on the edge trying to exploit the maximum capabilities of the aircraft and their tactics. Just by the virtue of that, in becoming combat ready, these unfortunately are not uncommon occurrences."
The Marine Corps and Navy, nonetheless, stand out in their efforts to mitigate that risk and make training as safe as possible, he said.
Officials said it could weeks to determine what caused two helicopters, an AH-1W Cobra and a UH-1 Huey, to crash in midair during a routine exercise Wednesday night, killing all aboard the aircraft. Skies were clear and the weather was mild.
The accident occurred near the Chocolate Mountains along the California-Arizona border - a sprawling desert range favored by the U.S. military because its craggy mountains and hot, dusty conditions are similar to Afghanistan's harsh environment.
With 17,500 Marines and sailors, including personnel stationed at Camp Pendleton and Marine Corps Air Station Yuma in Arizona, the 3rd Marine Aircraft Wing conducts hundreds of aviation training exercises a year so troops can get as much experience as possible before they go to war.
It was the fifth aviation accident since March involving the 3rd Marine Aircraft Wing headquartered at Miramar Marine Corps Air Station in San Diego. Throughout the Navy and Marine Corps, there have only been two other aviation training accidents in the past five years involving seven or more deaths, according to the military's Naval Safety Center.
Chaplains and counselors were called in to talk to troops. Six of the Marines killed were from Pendleton - the West Coast's largest base - and one was from the base in Yuma.
Their identities will not be released until their families have all been notified.
Gov. Jerry Brown issued a statement on behalf of himself and his wife: "Anne and I offer our condolences to the families and friends of the Marines who died last night. We honor their bravery and sacrifice."
Two of the Marines were aboard the AH-1W Cobra and the rest were in the UH-1 Huey utility helicopter. They were flying in a remote section of the 1.2-million-acre Yuma Training Range Complex as part of a two-week standard training called "Scorpion Fire" that involved a squadron of about 450 troops from the 3rd Marine Aircraft Wing.
The helicopters collided near dunes at the edge of the Yuma range about an hour before the range was to shut down for the evening. Ground troops were in the area, but they were not affected, said Gunnery Sgt. Dustin Dunk, a spokesman at the Yuma base, which is a 90-minute drive from the accident site.
Part of the exercise involved having helicopters low on fuel descend to ground troops that have set up a refueling outpost, Dunk said.
He did not know if that's what the pilots were doing at the time of the crash.
"Our training is always evolving, safety is paramount, and being prepared is paramount," he said. "It was a very standard exercise for what we do. Our thoughts and prayers go out to the family members. ... Our investigation will look to see what went wrong and how to correct it."
The AH-1W carries a pilot and gunner and is considered the Marine Corps' main attack helicopter. The UH-1Y, which is replacing the aging version of the Huey utility helicopter first used during the Vietnam War, carries one or two pilots, a crew chief and other crew members, depending on the mission.
Hueys often are used to pick up and drop off ground crews, while Cobras hover by ready to fire if the Huey comes under attack.
In other crashes in the past year, a twin-engine, two-seat AH-1W Cobra helicopter went down in September during training in a remote area of Camp Pendleton, killing two Marine pilots and igniting a brush fire that burned about 120 acres at the base north of San Diego.
In August, two Marines were ejected from their F/A-18 Hornet fighter jet as it plunged toward the Pacific Ocean. The two Marines spent four hours in the dark, chilly ocean before they were rescued. Both suffered broken bones but survived.
In July, a decorated Marine from western New York was killed during a training exercise when his UH-1Y helicopter went down in a remote section of Camp Pendleton.
Another Hornet sustained at least $1 million damage when its engine caught fire on March 30 aboard the USS John C. Stennis during an exercise about 100 miles off the San Diego coast. Eight sailors, a Marine and two civilians were injured.
In one of the worst accidents in the past five years, an AH1-W flying in formation with three other Marine helicopters on a nighttime training mission from Camp Pendleton to San Clemente Island collided with a Coast Guard C-130 airplane in October 2009, killing two aboard the Marine helicopters and seven aboard the C-130.