AMNA NAWAZ: For months, the U.S. military has been flying the Osprey, despite questions about whether it should be.

The tiltrotor aircraft was grounded after a crash in Japan last November that killed all eight airmen on board.

That crash was the fourth by an Osprey in the last two years.

The military allowed the fleet to fly and said it mitigated the problem that caused these accidents, but the families of those killed in the crashes aren't convinced the Osprey is safe and want it grounded again.

Nick Schifrin and producer Dan Sagalyn report from North Carolina.

NICK SCHIFRIN: At Marine Corps Air Station New River, the Marines in the Fighting Griffins approached the aircraft they call their workhorse.

The MV-22 Osprey had been grounded for three months before we visited this summer.

So, Captain Dave Schroeder is eager to fly and make sure it's flight-ready.

He's an instructor who trains junior pilots.

Do you get rusty?

Do other pilots get rusty?

CAPT.

DAVE SCHROEDER, Osprey Instructor Pilot: Absolutely.

Flying is a skill.

NICK SCHIFRIN: He's been flying military aircraft for more than a decade.

He calls the Osprey one of a kind and gives it his full faith.

CAPT.

DAVE SCHROEDER: I'm fully confident in the way that this aircraft performs and I trust my life in it.

The flexibility it provides and the speed of range that we can operate is really unmatched.

NICK SCHIFRIN: The Osprey takes off like a helicopter and flies like a plane at twice the speed of other helicopters.

The Marines call it essential to their mission and on average deployed 20 to ships every day last year.

But it also has a history of high-profile crashes.

JIM LEHRER, Co-Founder and Former Anchor, "PBS NewsHour": An experimental Marine Corps aircraft crashed today.

MAN: In Afghanistan, a U.S. Air Force Osprey went down.

AMNA NAWAZ: A U.S. military Osprey aircraft crashed into the sea off Japan today.

NICK SCHIFRIN: Since 1992, 11 fatal Osprey crashes have killed 66.

Four have crashed just in the last two years in California, Australia, Japan and Norway, killing 20 people.

CAPT.

DAVE SCHROEDER: It was such a heartbreaking and traumatic experience for so many of us.

We were friends and colleagues with a lot of the families and crews, so, initially, we're just dealing with the loss.

NICK SCHIFRIN: Did some of the guys in Japan or California?

CAPT.

DAVE SCHROEDER: Yes.

One of my first initial thoughts was that could have been any one of us.

I have got three young children at home, hoping to have another one soon.

I have got a gorgeous wife.

And they are one of my top priorities.

So considering that that could have been me, it was very emotional.

NICK SCHIFRIN: The 2022 crash in California killed five people, including 19-year-old Lance Corporal Evan Strickland, Brett and Michelle's son.

BRETT STRICKLAND, Father of Fallen U.S.

Soldier: I don't think many people got to see the serious side, the caring side.

He was an animal lover, musician.

He loved jazz.

MICHELLE STRICKLAND, Mother of Fallen U.S.

Soldier: We found out after his passing that he was probably going to elope, a crazy kid.

That sweet little face.

BRETT STRICKLAND: One of the things that always hits me is when I'm looking at pictures, because there are no more new pictures.

NICK SCHIFRIN: When he was 14, Strickland attended an air show and sat in an Osprey cockpit.

Five years later, he was killed in one.

He was the crew chief his very first day on the job when his Osprey crashed during a training mission.

A military investigation found the crash was unpreventable, caused by a catastrophic and unanticipated mechanical failure.

The Osprey is one of the world's most complex aircraft that has two engines mounted at the end of each wing.

Each engine is attached to a gearbox and clutch that power the rotors.

If one engine fails, the other engine can provide power to the other rotor through a driveshaft.

And in California, the Osprey crashed because of what the military calls hard clutch engagements.

The clutches slipped on both sides, then reengaged, causing a power surge, damaging the driveshaft and creating a catastrophic loss of thrust.

The military acknowledges Ospreys have suffered a hard clutch engagement at least 19 times over the life of the program, but they don't know the root cause.

The Stricklands and other families sued the Osprey's manufacturers for failing to address what they say are previously known mechanical failures.

BRETT STRICKLAND: My job going forward for my son, for those that have fallen with him is just to seek answers and accountability for something that should have been taken care of over a decade ago.

REP. STEPHEN LYNCH (D-MA): This repeated drumbeat of fatalities is totally unacceptable.

NICK SCHIFRIN: In June, the House Oversight Committee questioned the Osprey's safety.

Naval Air Systems Commander Vice Admiral Carl Chebi blamed hard clutch engagement on aging aircraft.

VICE ADM. CARL CHEBI, Commander, U.S.

Naval Air Systems Command: Over time, the clutch wears out and has a higher susceptibility to slipping.

We grounded the fleet and mandated that all aircraft will remove clutches that have over 800 hours.

That has been completed and we have flown numerous hours since then without a hard clutch event.

I want to make this point clear, though.Hat has not eliminated the risk.

We will not eliminate that risk until we have a redesigned clutch that is in testing right now.

NICK SCHIFRIN: The new clutch won't be ready until at least mid-2025, not fast enough for Massachusetts Democrat Stephen Lynch.

REP. STEPHEN LYNCH: If another Osprey goes down, we're done.

This program is done.

Ground them now.

We will bite the bullet for the next year or so until we get this clutch figured out.

But we have already had too much carnage.

We have already lost too many good men and women.

MICHELLE STRICKLAND: You're almost asking those still flying to, like, roll the dice.

It feels like almost to play Russian roulette.

BRETT STRICKLAND: Are they doing what's necessary to fix the problem and to find the actual cause?

Not just, how can we... MICHELLE STRICKLAND: A patch.

BRETT STRICKLAND: Yes, exactly.

NICK SCHIFRIN: The Marines told "News Hour" the Osprey has flown more than 600,000 flight hours and that its 10-year average mishap rate is lower than some of the other aircraft they fly, such as the Harrier, Hornets and Super Stallion.

Do you believe the Osprey's safe?

LT. COL. JEFFREY PULLINGER, Commander, Marine Medium Tiltrotor Squadron 266: I do believe the Osprey's safe.

I fly it right along with every other pilot that I have in this squadron, and I don't have a single doubt.

NICK SCHIFRIN: Back at New River, Lieutenant Colonel Jeffrey Pullinger is the Fighting Griffins' commanding officer.

CAPT.

DAVE SCHROEDER: If you hear, see, smell anything that requires the aircraft to abort while we're in a hover, call abort, abort, abort.

NICK SCHIFRIN: Schroeder, who, since we filmed this, left the military, trains on mitigating emergencies even if he doesn't always know the cause.

If you don't know the root cause, if you don't officially know the root cause, then how do you know it's safe?

CAPT.

DAVE SCHROEDER: When I look at the numbers that were provided to us, I feel very confident that there's such a low chance of it happening again that I wanted to get back in the aircraft.

REX RIVOLO, Former Department of Defense Osprey Analyst: They keep discussing that the root cause is not known.

To me, logically, the root cause is, the clutch is too small.

NICK SCHIFRIN: Rex Rivolo is a retired Air Force pilot who flew combat missions over Vietnam and was, for 16 years the Defense Department's chief Osprey analyst.

He says the military's mitigation doesn't address fundamental flaws.

REX RIVOLO: The drivetrain clearly is inadequate to handle the loads.

The history of the clutch engagements, and now this complete failure of the gearbox clearly indicates that there is a design problem.

NICK SCHIFRIN: Failure inside the gearbox that's behind the rotors in part caused last year's crash in Japan.

The military investigation found that the aircraft's left-hand gearbox started chipping, causing the gear's teeth to grind down, leading to -- quote -- "rapid cascading failures of the drive system.'

The report also said the crew could have landed earlier, when the chips were initially detected.

But gearbox failure is another risk the military has known about for years.

An unreleased safety investigation board review obtained by Military.com found a gearbox failed in 2013 due to a manufacturing defect in the way the gears were made, and documents obtained by the Associated Press showed seven previous cases of cracking.

The Japan crash report, unusually, blamed senior Pentagon leadership -- quote -- "Safety assessments and their findings were given insufficient treatment at the program level and have been inadequately communicated to the military services."

REX RIVOLO: There is a strong desire to keep these aircraft flying, and basically you compromise good judgment in order to do that, and that is a leadership problem.

NICK SCHIFRIN: Rivolo urges the military to install an additional warning system similar to the kind that exists on this type of British military helicopter that would alert the pilot to unusual vibrations.

REX RIVOLO: The V-22 does not do it, even though they have the onboard systems to make the detections.

In today's environment, that's unconscionable.

NICK SCHIFRIN: Until those changes to design that you're advocating can be made or there can be a better warning system, should the aircraft be grounded?

REX RIVOLO: Yes, the aircraft should be grounded.

GABRIELA LAVOY, Mother of Fallen U.S.

Soldier: We all think that they should be grounded, that no mother should go through this.

NICK SCHIFRIN: Gabriela Lavoy is the mother of 33-year-old Technical Sergeant Zachary Lavoy, who died in the crash in Japan.

He'd been in the Air Force for 10 years.

He'd planned to get out, get married, and start a family.

GABRIELA LAVOY: It's been hard.

It's been really hard.

You don't expect to outlive your children, especially somebody that we're not in times of war.

It was just a training mission.

NICK SCHIFRIN: How hard is it to talk about it now?

GABRIELA LAVOY: I had talked to him maybe two days before, and we were making plans for him to come and visit for Christmas.

It was a very happy time.

We had a good talk, and then suddenly he's gone.

NICK SCHIFRIN: It is a plane that airmen and the Marines love, and the military calls vital.

But all these families want is to make sure it's safe.

For the PBS "News Hour," I'm Nick Schifrin in Jacksonville, North Carolina.