It was a routine evening in Afghanistan when Marines and contractors with Lockheed Martin launched an unmanned K-Max drone helicopter from a major base in Helmand province to carry food rations to a smaller outpost.  The aircraft had made hundreds of similar trips for the Marines since first being deployed in 2011, and it looked like this would be like any other.

Something went seriously wrong the evening of June 5, 2013, though. As the helicopter closed in on its destination, the Marine lieutenant commanding the mission and Lockheed contractors operating the aircraft remotely expected 15 mph headwinds. Instead, it got a tailwind, shaking up the drone.

The pilots employed a technique known as the “weathervane” effect in an attempt to gain control, allowing the pilots to turn the aircraft into the wind and gain control. But it didn’t work. The oscillation of the 2,000-pound load swinging beneath the helicopter in a cargo net grew increasingly worse, bringing the 52-foot-long helicopter, valued at $11.1 million, down in a heap on the landing zone. No one was injured, but the laptop computer collecting information about the flight was ejected out the left cockpit window, and the tail burst into flames.

The crash report, released to The Washington Post through the Freedom of Information Act, illustrates the dangers that can occur when a drone operator does not realize the peril their aircraft is in.

Military investigators found that the mishap was preventable and occurred because the pilots did not intervene quickly enough when the helicopter experienced unexpected wind, according to documents released by the Marine Corps. Accident investigators also determined that U.S. personnel on the ground near the landing zone should have provided an updated weather report, and also sent a warning back to the pilots at Camp Bastion to let them know it was out of control.

“The aircraft could not recover on its own based on the diverging conditions and its insufficient programming; it required human intervention,” the investigation report said.

The report’s release comes as the Marine Corps brings home the other two K-Max drones it used in Afghanistan. As noted on Checkpoint in July, the U.S. Army will test it in coming months at Fort Benning, Ga., to see how well it operates while carrying another drone vehicle to be used on land and built by Lockheed. It is known as the Squad Mission Support System.

Investigators recommended several changes following the crash, including that the detachment of personnel running the drone have better communication with observers on the ground. Training also should be updated to respond to excessive oscillating or swinging by the load suspended beneath the aircraft, investigators recommended.

“At a minimum, the crew should be trained to point the aircraft into the wind in an auto descent using all available indications and pitch and weight load should be thoroughly monitored through the delivery of cargo and return to the cargo waypoint,” the report said.

A Lockheed spokesman for the K-Max program, Keith Little, referred comment to the military on Thursday. The Marine Corps released a statement saying it has addressed the concerns raised.

“Tactics, techniques and procedures were put in place to address all findings of the report which have helped preclude incidents like this from happening,” said a Marine spokesman, Capt. Dustin Pratico. “The lessons learned have made the K-MAX mission safer and more effective.”

Staff writer Craig Whitlock contributed to this report.