The moments before landing are eerily calm.

Caught on shaky hand-held video, two MV-22B Ospreys appear over a ridge of hills. The first Osprey turns in toward a small landing zone near a chain link fence, its rotors facing skyward for a vertical descent. As it comes within meters of touchdown, a choking cloud of brown dust billows up from the ground, completely obscuring the aircraft from view. The dust cloud grows even larger and more expansive, and the Osprey appears once again, ascending briefly. It hovers for mere seconds above the brownout, and a tongue of flame appears to shoot from its left nacelle.

Then, its rotors still spinning, the aircraft simply drops out of the sky, crumpling on impact as the right rotor tears free and chews the dirt.

The circumstances of this May 17, 2015, crash, which claimed the lives of two Marines and injured the other 20 troops on board at Marine Corps Training Area Bellows, Hawaii, are laid out through the accounts of eyewitnesses in a 2,200-page command investigation obtained by Military.com. The investigation recommends disciplinary or administrative action for the pilots and some aircrew of the aircraft and for Lt. Col. Andreas Lavato, the squadron commander for Marine Medium Tiltrotor Squadron 161, to which the Osprey was attached, and Col. Vance Cryer, commander of the 15th Marine Expeditionary Unit, which housed the squadron.

In findings endorsed by Marine Corps Forces Pacific Commander Lt. Gen. John Toolan, the probe determined that, while the Osprey's engine failed due to "excessive intake of sand and dirt," the pilots should have adjusted their plan based on the brownout conditions of the landing, and the squadron and MEU commanders should have been more involved in the selection and survey of the landing zone and the development of a stronger casualty evacuation plan ahead of the tragedy.

A spokesman for the 15th MEU, Capt. Brian Block, said Lt. Gen. David Berger, commanding general of I Marine Expeditionary Force, now has the authority to act on those recommendations. Officials with I MEF did not immediately respond to inquiries about pending actions. Cryer and Lavato have continued to lead their units in the wake of the tragedy.

The investigation also offers a new glimpse into the chaos that came in the aftermath of the crash, as ambulances took nearly 30 minutes to arrive on scene and Marines resorted to sending casualties to the hospital in pick-up trucks. It also reveals the many acts of heroism -- small and large -- that took place that day. Marines ignored their own painful injuries to tend to their friends, and brave civilians ran to the scene of the burning aircraft without hesitation to offer assistance.

Cryer, the MEU commander, declined to comment on any actions he was facing, but offered his thanks to those who had come to the aid of the wounded and compassion for the families of Lance Cpls. Joshua Barron, 24, and Matthew Determan, 21, who perished in the crash.

"Our heartfelt condolences go out to the families of all those involved in the mishap and we extend our deepest gratitude for the heroic, selfless actions of the Marines aboard the aircraft, along with the quick lifesaving actions of the civilian and military first responders that prevented any further loss of life," he told Military.com in a statement. "We will not forget the Marines who perished. Their memories and legacy of faithful service compel us to live and serve in such a way to bring honor to the sacrifices made."

Crash Day

The last flight of Osprey call sign Mayhem 11 began aboard the amphibious assault ship USS Essex, some 300 miles offshore. The aircraft was one of five birds set to transport 89 Marines to Landing Zone Gull in the Bellows training area, on the southeast coast of Oahu, for a simulated long-range raid. The exercise was scheduled just days into what would be a seven-month deployment to the Pacific and Middle East for the West Coast-based 15th MEU. For many of the Marines, it was their first trip to Hawaii.

The day began without incident. A pre-flight brief noted that LZ Gull, small landing zone of 154 by 506 feet, had moderate reduced-visibility landing, or RVL, conditions: about 1-2 on a scale of 5. The flight to shore was uneventful, but as Mayhem 11 began to approach the landing zone, the two men at the front of the aircraft realized they needed to adjust course slightly.

Two nearly identical landing zones sat on either side of a chain link fence, and another Osprey in the pattern radioed that Mayhem 11 was headed for the wrong one. The copilot of Mayhem 11, who is not identified in the report, opted to wave off the first landing and come around after adjusting course.

According to testimony from the senior crew chief aboard the aircraft, whose name has been redacted, things started to get "squirrelly" at a distance of 85 feet up, when dust began to billow up from the ground. The aircraft began to drift right, and the copilot corrected the drift, but struggled to regain proper positioning. At that point, the aircraft commander, or primary pilot, took over the controls.

Visibility was worse than expected -- RVL conditions of 3 or 4 -- but the senior crew chief said he could see the ground and the bird was clear to land. The pilot continued landing without the aid of a digital hover assist screen designed to facilitate descents in low-visibility conditions.

"I got the response from the [aircraft commander] that he was just trimming it out and to start bringing it down at that point, which we fell out of the sky at that point," the crew chief told investigators.

Aboard the aircraft, many of the Marine passengers sensed the sudden drop. Some heard a loud "pop" sound and felt the bird dip to the left. They started screaming "power, power, power," as the aircraft plummeted. The Osprey was in a hover for 45 seconds during the landing attempt and in free fall for only about three seconds.

That was plenty of time to alert most of the 22 Marines on board that something was seriously wrong.

"I had no warning other than the gut feeling of: we're dropping," an automatic rifleman with Lima Company, 3rd Battalion, 1st Marines who was aboard the aircraft told investigators. "So I clenched up my body as a reaction to the falling feeling."

Another Marine with Lima company said he instantly thought of his family in what he believed were his final moments.

"Before we hit the ground I closed my eyes and thought of my wife and child and how I was going to die before my child could understand who I was," he said in a written statement. "At that point, I didn't want to accept death but felt like it was inevitable."

As the aircraft made impact, the wings fractured, the tail cracked, and the roof split wide open, letting daylight in. The senior crew chief, anchored to the aircraft by a gunner's belt so he could perform his duties, felt instantly that his back was broken. Barron, the other crew chief and also in a gunner's belt, was under some broken fuselage and apparently in cardiac arrest. Determan, a rifleman assigned to Lima Company, was still buckled in his seat with a massive head injury. No one emerged completely unscathed.

The Aftermath

In the moments after impact, chaos reigned, but Marines went to heroic lengths to care for each other. Knowing that the Osprey could blow up, the Marines who could walk worked to get their more injured team members unbuckled and free from the bird.

Some seven Marines -- one with a shattered ankle -- were unable to walk and were deposited at an improvised casualty collection point by a speed limit sign at what was deemed to be a safe distance from the Osprey. Despite a number of back and head injuries in the group, these Marines would be moved twice more to get further clear of the aircraft as no one knew for sure what its blast radius might be.

Lima's company commander ordered a Marine to find a poled litter to transport the worst of the injured and moved around the scene, taking names and making sure all 22 Marines were accounted for.

As the troops were triaging each other, others began to converge on the scene as well. A civilian registered nurse who was camping with a group of cub scouts rushed over when she saw the crash and offered her help. Several Marines threw a flak jacket over the barbed wire portion of an eight-foot chain link fence separating the 51-year-old nurse from the crash site, and in a burst of adrenaline she scrambled over it and went to work.

Lifeguards from the nearby beach also arrived on scene and began to administer first aid. Later, a doctor who happened to be camping nearby would arrive and prioritize the injured for transport to three local medical and trauma centers.

One man was checking in at Brother's Paintball Field at Bellows, half a mile from LZ Gull, when he heard that an Osprey had gone down. He jumped into his pick-up truck and floored it, headed for the crash scene. Once there, he jumped into action, pulling a barely conscious Determan out of the Osprey and carrying him away from the smoking aircraft. Eventually an off-duty Air Force officer who also had a truck was able to load the Marine into his vehicle and drive him to Castle Medical Center, about 20 minutes away.

The first ambulances would not reach the Marines until 25 minutes after the Osprey's crash-landing around 11:30 a.m. Emergency medical services was notified instantly when the aircraft touched down, but were hindered by several physical obstacles that cost precious minutes. Several ambulances pulled up about 17 minutes after the crash, according to the investigation, but the drivers were confused about the location of the aircraft and could not get through a locked fence that stood between the vehicles and the Osprey. More ambulances tried to pull up on the west side of the aircraft, but were held at bay by the intense flames shooting out of the bird.

Finally, fire department personnel cut the fence, allowing the emergency vehicles to get to the injured.

The decision to transfer some of the Marines to the hospital via pick-up truck speaks to the desperation and frustration those trying to manage the scene felt as they waited for ambulances to arrive.

Speaking to investigators following the incident, the civilian nurse said she remained troubled by the episode.

"I did not think getting in the back of a pickup truck and rattling down the road with a head injury was very smart," she said. "You know, the goal was just to get away from the scene. Get away and get to the hospitals. And that's why I feel people did it. But it wasn't -- I believe it was not correct to do."

Ultimately, though, she said she was certain the move had not been one with deadly consequences. Multiple witnesses testified Barron was dead at the scene of the crash, and Determan would die surrounded by family at Castle Medical Center two days later from impact injuries the nurse believed were not survivable.

Troubling Choices

The Hawaii casualty evacuation plan showed transit routes to three different medical centers: Castle, The Queen's Medical Center, about 26 minutes away, and Tripler Army Medical Center, the furthest away at about 35 minutes from LZ Gull.

There was some confusion, though, about what air evacuation assets would be available to the Marines in case of an emergency.

During an April 24 confirmation brief ahead of the Hawaii-based training, the 3/1 battalion surgeon told commanders that Tripler could send a CH-47 helicopter to pick up casualties, significantly trimming emergency response time. That was not the case. Cryer, the MEU commander, learned of the error ahead of the May 17 flights, the investigation found, but decided a ground emergency response would be adequate.

"So I had to break out, 'Well, let's take a look … You know, what routes are we going to take? How long is that going to take?'" Cryer recalled to investigators. "You know, he estimated 25 minutes' response time. And so that was sufficient in my mind in accordance with your typical MEU mission."

Investigators ultimately faulted Cryer for the "lack of rigor" in the design of the mission's medical evacuation plan, saying the failure to plan in detail resulted in delayed evacuation, unforeseen obstacles including locked gates, civilian vehicle transit to local hospitals, and the lack of a single predetermined casualty collection point for wounded Marines.

However, the investigation found, none of these circumstances had resulted in a more deadly outcome for the Marines due to the nature of the injuries sustained in the crash.

Another problem described in the report indicates the Marines narrowly avoided what could have been an even greater loss of life. Investigators found a number of the Marine passengers aboard the aircraft had unclipped their shoulder restraints and lap belts seconds ahead of landing in keeping with an understanding that doing so was permitted in order to exit the aircraft quickly.

"Marines avoided injury despite, and not because of, their failure to properly utilize the restraint system," the investigation found. "Had this aircraft landed with more of a rolling moment or actually rolled over, the injuries and fatalities sustained would have likely been much worse."

The crash itself, investigators found, was caused by an engine compressor stall after the engine sucked in a significant amount of sand and dirt in LZ Gull's brownout conditions. This intake resulted in "turbine blade glassification," as the report puts it -- essentially, the melting of reactive sand at high temperatures in the back of the engine. Such a reaction changes the airflow and makes the engine less efficient, a former MV-22 test pilot told Military.com.

Interviews with the co-pilot of the aircraft indicate everything seemed normal in the cockpit until the instant rapid descent began.

"In retrospect, that's kind of what to me is really shocking, sir, is that everything was very normal," he testified. "There was no caution advisories, there was nothing … I just remember the settling.

I didn't see any temperature indications, and then I did see a right engine hot indication."

When the pilot was finally receiving treatment for injuries at Tripler later that day, he described a conversation with an unnamed individual who queried him about the crash. The exchange suggests the pilot believed the cause of the crash to be mechanical.

"'Tell me straight up, was this a mechanical fault or an aircrew fault? A lot of people are relying on the Osprey to come to Hawaii and Japan,'" the pilot recalled. "And I told him that we lost our engine in a 75-foot hover, and [he] said, 'Okay.'"

What Went Wrong

The command investigation found the Osprey had hovered twice in brownout conditions: for 35 seconds during the first landing attempt, then again for 45 seconds during the landing that would result in a crash. While the Naval Air Training and Operating Procedures Standardization manual used by Navy and Marine Corps aviators warned that hovering in brownout conditions for more than 60 seconds could lead to engine shutdown, that hover time was not considered cumulative; the pilots of Mayhem 11 were operating well within guidelines.

A Sept. 9, 2015, Naval Air Command report on "V-22 … engine surge in RVL environments," obtained by Military.com recommends that Osprey pilots limit total RVL exposure to 60 seconds per mission, and NATOPS now recommends hovers of no more than 35 seconds in brownout conditions. That report was generated partially in response to the Bellows tragedy.

The unique construction of the Osprey, with its tiltrotors that allow it to hover like a helicopter and then fly horizontally like a plane, also make it uniquely susceptible to engine failure in brownout conditions, the former Osprey test pilot said. The pilot, who spoke under condition of anonymity because of his standing in the aviation community, said the Osprey has higher disk loading than other helicopters because of its smaller rotors, meaning it must pump a larger volume of air at a higher velocity to stay airborne.

"The velocity coming out of that Osprey rotor is considerable higher than anything that's out there," the pilot said. "You hover over that sand and you make one hell of a mess."

He added that since the engine inlets on the Osprey are vertical in descent instead of horizontal as they are in CH-53s and other helicopters, the Osprey can suck in significantly more sand than comparable aircraft.

While the pilots hadn't violated any rules or guidelines, the investigation found it showed poor judgment to execute a second hover and landing attempt in the severe brownout conditions at LZ Gull. They also found that the pilots should have used their "hover page" digital hover assist in light of the landing zone's limited visibility.

Fault was also laid at the feet of Cryer, the MEU commander, and Lavato, the squadron leader, for not ordering a more thorough site survey ahead of the landing. Members of VMM-161 had landed at LZ Gull during the Rim of the Pacific exercise in 2014, and investigators found reports of the site's conditions were based largely on prior experience from then. Selection of the nearby LZ Tigershark, they found, would have resulted in much better landing conditions.

Lavato, like Cryer, declined to comment on the report's findings.

Ultimately, investigators made a slew of recommendations to improve flight safety in low-visibility conditions:

A better filtration system to prevent sand and contaminants from being sucked into V-22 engine turbines;

Engine performance monitoring showing real-time engine performance and stalls;

Advanced brownout technology using sensors and autonomous landing systems;

A warning advisory to alert the pilot when engine power declines to 95 percent;

Better preparation for V-22 pilots to use the "hover page" landing aid; and

Analysis of sand that is found to have a greater potential for turbine blade "glassification."

Some, though, expressed dismay that the Marine Corps would lay some of the blame with the pilots for an apparently mechanical issue that policy did not adequately address.

"It's always pilot error," said the former Osprey test pilot. "It's damn easy and it sucks that they would blame the pilot for this … 99 percent of the time when they find pilot error, it means the design of the aircraft and the circumstances piles so much onto the pilot's plate that he got overloaded."

Mike Determan, father of Lance Cpl. Matthew Determan, one of the fallen, also said he was disappointed the pilots were faulted.

"By not doing the pre-work and knowing that this sand is reactive, they're just putting these guys at huge risk," he said.

One action that was not recommended was a grounding of the Osprey fleet for further analysis -- something Determan said he would have liked to see.

"By its very nature, there will always be inherent risk in combat aviation. This is due to the expeditionary nature of U.S. Marine Corps operations and the varied types of missions we fly," Marine Corps spokeswoman Capt. Sarah Burns said. "When mishaps occur, we diligently investigate them, and we are transparent with regards to the findings of each investigation. In this investigation there were no indications that there is an issue beyond that of the aircraft involved and consequently did not lead to a determination that a grounding of the fleet would be warranted."

The Heroes

Within the oral testimonies and written statements on which the Bellows command investigation is based are numerous stories of selflessness and courage of Marines and civilian responders, their names redacted, who unhesitatingly sacrificed safety and comfort to care for the wounded.

Among many accounts of heroism and humanity, a few stand out.

An automatic rifleman with Lima Company recalled, in moving detail, tending to a badly injured Marine in the immediate aftermath of the crash.

"Looking at him and talking to him, I could see in his eyes that he was still conscious, that he was kind of speaking to me through his eyes -- kind of that feeling of: 'I am hurt. I can't. I can't talk. Please help me,'" the automatic rifleman told investigators. "So I took off his Kevlar, and I kissed him on the forehead and told him it was going to be okay. I grabbed him underneath the arms and as gently as I could laid him in the center aisle [of the aircraft] on top of the packs."

Later, the Marine testified, he kept watch with Determan's family, crying and spending hours in shifts at his friend's bedside until he died May 19.

A platoon radio operator with Lima Company may have bought one of the wounded more time through preparedness and foresight. Prior to the crash, the radio operator had asked his roommate, a corpsman, to give him a nasopharyngeal airway, or NPA -- a rubber tube inserted through the nose to assist breathing. They didn't come standard in the Marines' infantry first aid kits, but the radio operator believed he needed one, just in case.

After Determan was removed from the Osprey, he struggled to breathe. The corpsman who had been aboard the Osprey could not recover his combat lifesaving bag from among the Osprey wreckage. The only one who had an NPA was the radio operator. He handed it to a civilian medical responder, who inserted it immediately.

"If you could, could you ask them to put NPAs in the [infantry first aid kit]?" the radio operator said when he was interviewed. "Because I feel like that's what kept [Determan] alive long enough for his family to at least see him."

"That's a guarantee from us," the investigative team assured him.

The request would become the 11th of 14 recommendations in the investigation report.

The civilian nurse who jumped into the fray assumed a key role in initiating CPR procedures, cataloguing injuries and providing triage care. After the most serious cases had been attended to, she said, she transitioned to the role of a comforter, holding the hands of the injured and reassuring them as they waited for transport. Altogether, she said, she spent some 45 minutes working the crash site.

During her interview with investigators, she marveled at the strength she'd been able to summon to scramble over the tall fence separating her from the Osprey wreckage.

"Well, that's the big joke at work now," she said. "I work with a couple of old Army respiratory therapists. And they are still cracking up the fact that I got up and over that fence. Because if they asked me to do it today, I couldn't do it."

-- Hope Hodge Seck can be reached at hope.seck@military.com. Follow her on Twitter at @HopeSeck.