HOUSTON -- A fatal medical helicopter crash has lead to questions about the choppers' safety, KPRC Local 2 reported Thursday.

What started as a rescue mission turned into a recovery mission. A PHI air medical chopper flew from Huntsville to Houston early June 8. It crashed in the forest near Huntsville, killing patient David Disman, paramedic Stephanie Waters, flight nurse Jana Bishop and pilot Charles Kirby.

Helicopter Association International statistics show an average of more than 5.5 percent of all crashes happened while the helicopters were acting as air ambulances. Of the 1,908 chopper crashes during the last 10 years, 108 crashed during air medical service.

Austin attorney Mike Slack is a former NASA engineer and a licensed airplane pilot. He said his law firm has been involved in about two dozen cases involving air ambulances. He questioned the benefits of flying ambulances, beyond the crash stats.

"The medical studies point to the fact that fewer than 5 percent of actual transports result in an enhancement or improvement in medical outcome," Slack said.

Slack's writings point to a 1999 Brooke Army Medical Center study of 792 trauma patients in San Antonio. The study looked at the benefits of air versus ground transportation. It said, in part, "No statistically significant variation was seen in mortality for either the ground ambulance or the air ambulance group compared with national trauma outcome standards based on MTOS (Major Trauma Outcome Study) data."

"The ground ambulance is the most dangerous vehicle in America. The death rate on the job for EMS personnel is greater than for fire and police combined," said Dr. Daniel Hankins, an emergency physician who has been a ground EMS and air medical director for the Mayo Clinic for more than 20 years.

Hankins spoke on behalf of the Association of Air Medical Services, a group represents more than 600 air and ground medical providers, including two companies serving Houston.

"Patients in the back of ground ambulances with head injuries actually did worse than they did on helicopters," Hankins said. "The skills for airway were much better on the air crew than on the ground crew side."

Hankins pointed to a 2005 University of California at San Diego study that included more than 3,000 patients transported by aeromedical crews.

"Aeromedical response appears to result in improved outcomes after adjustment for multiple influential factors in patients with moderate to severe traumatic brain injury," the study concluded. "Patients with more severe injuries appeared to derive the greatest benefit from aeromedical transport."

Hankins said the ultimate answer is a mixture between air and ground.

"I think the most important message is you want the right crew for the right patient, in the right vehicle going to the right destination," he said.

The National Transportation Safety Board is investigating the crash near Huntsville. Investigators said it could take up to a year and a half before they announce the cause of the crash.