In the early hours of Dec. 13, 2017, the airmen aboard Pyro 76 -- an Air Force MV-22 Osprey -- were jolted awake in the skies over Arizona by loud noises and aircraft movement. They were falling out of the sky.
Based on testimony in the official Air Force investigation and conversations with two airmen aboard the aircraft on that flight, Pyro 76 illustrates the danger that hard clutch engagements, or HCEs -- a problem that recently grounded all of the service's Ospreys -- pose to aircraft and crew.
Testimony in the incident report that was provided to Military.com says the crew immediately took action and prepared for an emergency landing after the aircraft dropped in altitude and they "heard what sounded like the [right-hand] engine spooling down."
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Jack, one of the airmen aboard the Osprey that night, told Military.com in an interview that the incident started with a "flameout" in the aircraft's left engine. A flameout is a somewhat generic term that refers to an aircraft engine powering down as a result of the fire in the combustion chamber going out.
"When the left engine flamed out, the right tried to take over for it ... and [the clutch] just failed to engage," Jack explained.
"By the time it did, that engine was running at like 110% power," he added.
Military.com has changed Jack's name because he fears retaliation for speaking about an incident the Air Force has chosen not to publicize.
The Air Force grounded its Osprey fleet Aug. 16 over concerns related to the problem. That prompted the Marine Corps, which flies the Osprey more than any other branch, to reveal that there have been 15 mishaps involving this issue since 2010, including 10 on Marine aircraft.
Maj. Jim Stenger, a Marine Corps spokesman, said that the military has a long-term goal "to find a material solution to the root cause of this problem." Another, unnamed, Marine Corps official described the problem as "common knowledge with the fleet" while stressing that in every case there were no injuries and that most incidents happened "within seconds of takeoff."
The Air Force lifted its Osprey flight ban after two weeks without that long-term fix.
The incident with Pyro 76 is the exact scenario the Osprey's complex system of clutches and linkages is designed to address. If the aircraft loses power in one engine, the second engine can still turn both propellers and keep the aircraft flying. But in a hard clutch engagement, this doesn't happen.
"Anytime you lose one engine, this clutch needs to engage," Jack said, adding that "it's a sole backup to be able to power the other side of the aircraft.
"Without that, you're lost. There's nothing else you can do."
In Pyro 76's case, not only did the system not engage, the incident led to the complete shredding of every gearbox on the aircraft mid-flight. Jack said that the clutch issues meant their one good engine "was running at like 110% power."
"That immediate surge of trying to take over when it couldn't is what blew everything apart," he said.
A second airman on the flight told Military.com that, after the hard engagement of the clutch, one of the proprotor gearboxes ruptured and dumped oil into the engine. They then followed emergency procedures for flying on one engine and a gearbox issue and landed the aircraft immediately at nearby Flagstaff airport. According to crew testimony in the report, the aircraft was on the ground within minutes.
The crew ran out the back of the aircraft and, when they turned around, they "saw all of the … oil being spilled out of the [right-hand] nacelle and it covered the whole [right-hand] side of the aircraft," the report said.
The incident caused more than $5 million in damage to the aircraft, according to the Air Force report. Both engines and five gearboxes needed to be replaced, as well as nearly a dozen other components. It took a team of six, working 12-hour days, 45 days to repair the aircraft, according to the report.
However, more important than the cost, the incident illustrates that the hard clutch issue -- one that the military says has been known about since the early days of the aircraft -- has the potential to end in catastrophe.
Jack said that, in the case of Pyro 76, "thankfully, our other engine started back up." He also stressed that the crew was fortunate to be close to an airport where the Osprey ended up landing.
"If this would have happened overseas ... this could have spelled … death for at least the crew with whatever operators or medical personnel would have been in the back," he added.
In fact, the Air Force is currently wrestling with how to get another of its Ospreys that experienced a hard clutch engagement off a remote island in Norway.
Lt. Gen. James C. Slife, head of Air Force Special Operations Command (AFSOC), said Wednesday during an interview with the Air Force Association that one of the service's Ospreys had to make an emergency landing in one of Norway's protected nature preserves, complicating the retrieval of the aircraft.
"We had one of these hard clutch engagement events; the aircrew put it down," Slife said. "And so we're in the midst of a recovery process to get that airplane to a place where we can swap out the engines and the gearboxes and all the things that need to be replaced."
The Norway incident is one of the more recent hard clutch engagement problems Air Force Special Operations Command has had with its Ospreys since 2017.
Last month, after two hard clutch engagements happened within a matter of weeks, Slife ordered a stand-down of AFSOC's Osprey fleet to conduct inspections, collect data and provide guidance to pilots.
The stand-down ended last week -- but with no immediate fix to the clutch issue. AFSOC's hope "remains identifying the root cause of HCE and finding and implementing a material solution," according to a command spokeswoman.
Mackenzie Eaglen, a researcher at the Washington-based American Enterprise Institute think tank who focuses on Air Force defense budgets and military readiness, said any mechanical issues with the Osprey that cause it to be out of commission will severely harm training and mission readiness.
"The Air Force, AFSOC in particular, is feeling acute pain by having any grounding of the fleet or any Osprey out for longer than they had been planned," Eaglen told Military.com. "Many of them are currently deployed to Europe and Asia; it's really hard to lose this capability on a moment's notice."
Eaglen also said that, because the immediate focus has been on training pilots how to address hard clutch engagements, it likely has delayed a push from the manufacturers and services to get an immediate mechanical fix.
Slife said during his Air Force Association interview that he credits his pilots' skill with surviving those emergency landings and working around the hard clutch issue.
But he knows the ultimate solution is fixing the part that's causing the problem.
"In AFSOC, we haven't had a catastrophic mishap," Slife said. "Each one of them results in a kind of a Christmas tree of lights, caution lights in the cockpit. ... I'm really, really proud of our crews and the way they've been able to safely land these airplanes, but I'd rather they not have to demonstrate their superior skill because we put superior controls in place to prevent them from having to do that."
Although there have been two fatal Osprey crashes this year that have left nine Marines dead, none has been tied to the hard clutch engagement issue so far.
In June, five Marines aboard an MV-22B Osprey were killed when the aircraft crashed near Glamis, California. The cause is still under investigation.
In March, an Osprey crashed near Bodo, Norway, while participating in a military exercise, killing four Marines. Investigators concluded that the pilots of the aircraft that crashed in Norway turned too sharply, causing the craft to lose altitude and speed and, ultimately, crash.
Those crashes were the first deadly incidents with the aircraft since 2017. Between 1991 and 2006, while the Osprey was undergoing testing, there were four crashes resulting in 30 deaths.
While military leaders search for a fix, service members will continue to fly in an aircraft with a known issue that could -- on rare occasions -- lead to the destruction of a system meant to keep them alive and flying.
"Everything is supposed to have a redundancy, but the redundancy is failing," Jack said.
"The fact that no one has been injured with it is obviously a great thing, but it's just a matter of time."
Editor's note: This article has been edited to include new details from a second airman that more accurately portray the narrative of the incident involving flight Pyro 76.
-- Konstantin Toropin can be reached at konstantin.toropin@military.com. Follow him on Twitter @ktoropin.
-- Thomas Novelly can be reached at thomas.novelly@military.com. Follow him on Twitter @TomNovelly.
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