Charikot was already a major focal point for foreign aid in Nepal. American personnel had moved in tarps and other supplies in advance of the country’s upcoming monsoon season — and the floods that often come with it.

The Pentagon’s aid force flew MV-22 Osprey tiltrotors — which fly like normal planes, but can land like helicopters — and four-engined KC-130J tankers as well as the smaller Venoms.

Charikot had a functional landing strip, and the larger Ospreys could have landed there, but Marines preferred to use the smaller and less powerful Venoms. The poor weather and the destruction wrought by the quake made the situation dangerous for the bigger aircraft.

A descendant of Vietnam-era Huey choppers, Bell Helicopter’s UH-1Y is the latest member of the family. The newly built helicopters have more powerful engines, advanced electronics and stronger sensors compared to the older types.

Marine intelligence logs noted how the Ospreys had problems operating in Nepal. On May 6, “the downwash from [MV-22] Tiger 11 resulted in severe brownout and unsafe conditions for nearby structures,” one official log said.

A day after Vengeance 01 went missing, the Marines sent Tiger 11 back to one of Charikot’s established landing zones. When the Osprey arrived, the crew said the site still couldn’t handle their unique plane and diverted to center of town.

“Tiger 11 reported a nearby town which was heavily damaged by a new landslide, and then landed in Charikot Proper HLZ,” the summary said. The acronym “HLZ” refers to an identified helicopter landing zone.

Major aftershocks in the area had triggered the landslides, adding to the existing damage and causing new injuries and fatalities. The MV-22 picked up injured civilians, flew a brief search for the downed chopper and then flew back to Tribhuvan International Airport in Kathmandu.

At that time, the Marines rated severe weather as the highest risk to their operations. Identifying “damaged infrastructure or obstructions hindering or posing possible threats” was also a “priority intelligence requirement,” the summaries explained.

“Add in winds, cloud ceilings and a number of other weather-related factors, and flying becomes even more difficult,” public affairs officials with the Pentagon’s task force told War Is Boring in an email. “However … the performance limits of both the aircraft and crew have not been exceeded.”

But even without the weather and other hazards brought on by the recent disaster, American pilots would have had to use all their training in the skies north of Kathmandu.

“Altitude alone makes operating in Nepal difficult,” the public affairs personnel added. “The higher a helicopter flies, the thinner the air density, and thus less air for the rotors to push.”

“This can cause performance issues relating to maneuverability,” the officials pointed out. No one knows exactly why Vengeance 01 went down, but environmental factors likely played a part.

After the search parties found the wreckage, Wissler called off the mission because the situation was still dangerous.

“Due to the extremely difficult terrain at the site of the mishap, below freezing temperatures and violent winds and thunderstorms, I made the decision to cease the recovery efforts,” Wissler explained to reporters. “We cannot afford to put U.S. or Nepalese service members at any further risk.”

Four days after the helicopter went down, rescue personnel recovered the bodies of all the fallen Marines and Nepali soldiers. As would be the case in any fatal crash, the Marine Corps is conducting a thorough investigation into the accident.