Speculation continues as to what caused a helicopter crash that sent five people into Pearl Harbor on Thursday, but a forensic expert specializing in aviation says that while video of the accident is valuable — it could take years before investigators reach an answer.

So far theories include main rotor failure, but have discounted engine failure due to lack of noise on the video. In an interview with Inverse’s Jack Crosbie, aviation expert Matthew Robinson of Robson Forensics agreed that footage of the crash was some of the most valuable information available.

“I would take the video and put it up on a big screen, analyze the pitch of the main rotor blades and tail rotor blades; I would watch it many dozens if not hundreds of times in real time and slowed down,” Robinson says.

A 16-year-old boy remains in critical condition after the crash. A 45-year-old woman and a 50-year-old man both remain hospitalized in stable condition. The two remaining passengers walked away with only minor injuries.

Robinson emphasized that the investigators would apply the scientific method to their investigation of the crash.

“As an aircraft investigator we don’t deal in possibilities, we deal in probabilities,” he says, adding: “Right now it’s [the investigation] in data collection mode — there are a lot of possibilities and we need to narrow it down to the probabilities.”

Robinson believes there’s still a possibility the crash could have been caused by Vortex Ring State, which can be caused by even relatively mild winds from the right direction and velocity, although it would be uncommon and proving it from video evidence would be difficult.

“I saw no evidence in the video, no way to determine the direction and velocity of these winds. It [VRS] doesn’t really occur in mild conditions but it can.”

As redditors have already noted, the lack of engine noise could be an important clue.

“There doesn’t appear to be any significant change in engine noise, I didn’t hear the engine spool down and quit, that would leave me to believe power was being applied to the drive system,” he says. “If all that comes out as good then VRS is now starting to become a likely possibility.”

Another possibility was transitional lift, a term for improved rotor efficiency caused by incoming wind entering the system during directional flight.

“If the power required begins exceeding power available, that’s when you receive transitional lift, which is essentially a high hover,” he says. “When that happens it’s a gradual loss of power and it’s more of a slower descent rate.”

Robinson said helicopters rarely have flight data recorders, known as black boxes, but that “there may be other sources of non volatile memory within the helicopter’s instrumentations.” He said it was good that the helicopter crashed in water, as instrument data can be rescued from immersion, but often is destroyed in post-crash fires when an aircraft goes down hard.

The next step for investigators, Robinson said, would be to conduct extensive interviews on all the passengers and the pilot to gain information on what they were seeing, hearing, and smelling. NTSB investigators would also simultaneously look at all of the physical evidence and components of the helicopter, doing “non-destructive examinations” on them.

If you can establish that the helicopter had both “power and controllability,” Robinson said, “Then you can start narrowing it down pretty fast. They’re going to go through all of these components and narrow them down.”

They can use X-rays, bench tests, and more to examine components.

“Aircraft accident investigators do this for one reason and one reason only, and that’s to prevent future events,” he says. “We’ll go through and figure out what happened, but the real question is the why.”

He said the investigation would have to examine not just the aircraft, but the pilots, the organization they were flying for, and their policies and procedures, so it can make recommendations for safer practices going forward. FTSB investigations are public, but can take a year and a half to two years to completely finish, so it may be a while before there’s a definite answer as to what caused the crash.

“In the first 24-48 hours there’s so much speculation,” he says. “You know, to quote Sherlock Holmes: ‘It’s unwise to speculate in advance of the facts.’ Invariably, it biases the judgment. It’s good to let the facts speak for themselves.”

With reporting by Jack Crosbie.

Photos via YouTube