FLAGSTAFF, Ariz. – A fiery collision that killed six people aboard two medical helicopters has underscored the dangers of emergency flights and renewed questions about whether they are worth the risks.

Experts agree air ambulances can save lives when the victim is in grave condition and the hospital is a long way off or hard to reach by road. But they say there are other cases in which an ordinary ground ambulance is just as good, and perhaps safer.

The collision involved two helicopters that were arriving with patients Sunday at Flagstaff Medical Center. It was the ninth accident this year involving emergency medical aircraft, bringing the number of deaths to 16, National Transportation Safety Board officials said Monday.

“This has been a serious issue,” NTSB chairman Mark Rosenker said. “We’re going to work very, very hard to make sure we understand exactly what happened here, determine the probable cause and make recommendations to prevent it from happening again.”

Dr. Bryan Bledsoe, an emergency medicine physician who teaches at the University of Nevada and has researched accident rates of medical helicopters, said the flights benefit only a small subset of patients, such as those needing a cardiac stent or balloon within a 90-minute window.

While helicopters are faster than ground transportation, traditional ambulances can often get patients there quicker, given the difficulty of finding places to land helicopters and the decision sometimes to turn off the engines when the aircraft arrives and then power them up again when it is time to take off, Bledsoe said.

But Gary Sizemore, president of the National EMS Pilots Association in Alexandria, Va., and a pilot for Air Methods of Englewood, Colo., operator of one of the choppers that went down in Flagstaff, said that air ambulances are especially useful in rural areas, where narrow roads and rugged terrain can make ground rescues difficult.

“It may take them an hour, an hour and a half to get definitive care by ground, and we may be able to make that transport in 20 minutes,” Sizemore said. “So then that 40 minutes we shave off of that timeframe usually means a better outcome for the patient.”

Medical helicopters are sometimes forced to land in rough terrain and navigate through bad weather. The difficulties include extreme winds, poor visibility from darkness or fog, and tight spots in which to land, such as accident scenes with power lines nearby, said Bill Waldock, a safety science professor at Embry-Riddle Aeronautical University in Prescott, Ariz.

“It’s the rigorous environment that’s causing these accidents,” Waldock said. “You are putting yourself and your aircraft into a situation that’s already not good, and you get in trouble.”

The patient on the helicopter from the Grand Canyon was a firefighter who had suffered anaphylactic shock – a life-threatening allergic reaction – from a treatment he received for a bug bite, authorities said.

Three people died on each of the two Bell 407 helicopters: the pilot, the patient and a flight nurse on one chopper, the pilot, the patient and a paramedic on the other. A flight nurse on one of the helicopters was hospitalized in critical condition.

Will Summers, 74, said he was working in his yard nearby when the helicopters smashed into each other. He said one helicopter’s tail appeared to clip the other aircraft. One chopper exploded.

“There were rotor blades everywhere,” Summers said.