A civilian contract employee who was killed when she walked into the moving propeller of an unmanned aircraft at a Southern California airfield in September lost "situational awareness" before the fatal incident, Air Force officials said Friday. A detailed investigation report revealed the frantic moments just before and after 32-year-old Stephanie Cosme was fatally struck as she carried out testing at Gray Butte Airfield.

Cosme, of Palmdale, California, was working as a test engineer on the evening of Sept. 7, 2023, when she accidentally walked into the rotating propeller of a parked MQ-9A Reaper drone while performing a ground test on the aircraft, according to a final accident investigation report. The 40-page report concluded there were two underlying reasons for the accident — first, that Cosme was incorrectly trained on how to take telemetry readings when approaching the aircraft and, second, "she lost situational awareness while walking around" the drone as she took readings with a hand-held device.

Brig. Gen. Lance R. French said two other factors also "substantially contributed to the mishap," including a lack of communication between the contractor test team and ground support workers and, because of previous delays, "the tests conducted on Sept. 7 were rushed."

Cosme, who was employed by Sumaria Systems, LLC, was providing test support services when she was fatally struck.

According to the report, after taking measurements with a power meter, Cosme walked down the right side of the fuselage toward the rear of the aircraft, "looking down at the testing device the whole time, and appeared to be pressing buttons, possibly to keep the backlight on." As she came closer to the propeller, colleagues began "shouting and waving" in an attempt to get her attention, the report said.

"Without looking up to determine her position relative to the aircraft, (Cosme) proceeded to walk directly into the propeller of the (aircraft) sustaining fatal injuries," the report said.

Just before 7:58 p.m., the engine RPM dropped from 100% to 99% and engine torque jumped from 16% to 31% — apparently the moment of impact. Seconds later, the report says, the crew chief yelled over the radio to the aircrew, "Kill, kill, kill, kill!" Moments later, a crew member can be seen on the aircraft's front camera running from right to left, presumably running to the drone's kill switch.

Cosme, who was struck in the head and the left arm, was unresponsive when team members arrived. Resuscitative efforts were unsuccessful and she was pronounced dead at 9:06p.m.

In its report, the Air Force said officials later attempted to replicate the fatal sequence of events in the same spot with the drone's engine running at the same power setting with similar twilight conditions and airfield lighting that were present on the night Cosme was killed. In that simulation, the propeller was not visible when looking to the rear of the aircraft, from where Cosme approached.

An obituary posted on Legacy.com noted that Cosme was "known for her passion for crafting, baking, aviation, and all things strange and unusual." She was survived by her parents, two siblings, fiance and stepdaugher.

"Though her time on this Earth was far too brief, Stephanie left a lasting legacy of love and kindness that will continue to shine brightly," the obituary reads.

The MQ-9A Reaper is a U.S. Marine Corps drone that performs intelligence, surveillance and reconnaissance missions over land or sea, according to the Air Force. The $56.5 million aircraft is 36 feet long and has a 66-foot wingspan. A Reaper drone can reach an altitude of up to 50,000 feet and carry up to eight laser-guided Hellfire missiles, according to the Air Force.

Reaper drones have been used to carry out U.S. military attacks, including the 2020 strike that killed top Iranian military commander Qassem Soleimani in Iraq, and in 2021 against the Taliban as the militants were regaining control of Afghanistan amid the pullout of U.S. forces.

Alex Sundby contributed to this report.

In:
• Death
• U.S. Air Force
• California
• Drone

Stephen Smith

Stephen Smith is a senior editor for CBSNews.com.