Gearbox Issue’s Root Cause Elusive As USAF Investigates V-22 Crashes

For most of the Bell-Boeing V-22 Osprey’s service life, operators have tried to work around a phenomenon inside the tiltrotor’s gearbox: The sprag clutch slips, causing a subsequent hard engagement that can damage the engine and affect flight safety.

In mid-August, U.S. Air Force Special Operations Command (AFSOC) grounded its CV-22s after an Osprey was forced to land in a remote area of Norway. The incident prompted AFSOC Commander Lt. Gen. Jim Slife to ask: If the sprag clutch phenomenon forced this Osprey into a hard landing, could it be responsible for other mishaps? And as the problem becomes better understood, are there interim steps that can be taken to improve safety, even if the root cause of the slippage remains elusive?

• Service explores installing new gearbox assemblies
• Marine Corps, Navy and Japan are not yet following U.S. Air Force’s lead

“When we did our stand-down in August, at the time I highlighted that we did not know of any catastrophic mishaps that had been caused by the hard clutch engagement,” Slife tells Aviation Week. “But it is a serious phenomenon, and AFSOC has had a number of these events that were kept from being catastrophic by pretty impressive feats of airmanship by the crews flying the airplanes.”

The problem centers on the sprag clutch component of the proprotor gearbox, a part that connects the tilting Rolls-Royce AE 1107C engines to the proprotors. AFSOC recently conducted a review of 62 severe mishaps over the entirety of the CV-22’s service life, looking at both classified investigations and publicly released reports. A group of subject matter experts and legal advisors combed through the available data to discern whether what is known about the hard clutch engagement could have been a contributing factor to the incidents. The V-22 Program Office, along with the Navy and Marine Corps, followed suit, looking at their own history of mishaps to see if the issue could answer questions about the crashes.

The results, Slife and the other services say, is that none of the findings would be “materially” changed based on what is now known. That does not mean a hard clutch engagement was not involved in some way in the mishaps. It just means that the existing investigations would not be substantially changed by what could be gleaned from new reviews, so there is no merit in the extremely rare step of reconvening an investigation, Slife says.

AFSOC, the only command to ground its V-22 fleet because of the hard clutch engagement, is also the only one looking to go forward with a potential change to its aircraft to mitigate the issue. The command said in a statement that it is exploring with Bell-Boeing potentially installing rebuilt input quill assemblies on all CV-22s—a remanufacture of a key component of the proprotor gearbox that is sheared during a hard clutch engagement—with new inner and outer races and sprag clutches. Slife says the command is finalizing its decision, determining overall impacts to the fleet if the replacements go forward. The V-22 Program Office said in a statement that it supports AFSOC’s plan. The Marine Corps and Navy are still determining whether to follow suit.

Controversial Crash

One of the worst mishaps in AFSOC’s recent history was a driving factor in the decision to conduct the review—an April 2010 crash in Afghanistan that killed four and injured 16 on board. The crash became well known because of some controversy surrounding the investigation, with the command at the time publicly rebuking the top investigator.

The Air Force’s official account of the crash, the publicly released Accident Investigation Board (AIB) report, did not identify a main cause because key pieces of evidence, including one entire engine and the flight data recorder, were missing or destroyed. The report stated that engine power loss was a potential contributing factor, with ground markings and video from a nearby A-10 showing engine degradation and an “abnormal engine response.” An analysis showed that proprotor speed was low when the aircraft attempted a rolling landing on a rough surface, then-AIB President and Brig. Gen. Donald Harvel wrote.

However, AFSOC at the time disagreed with Harvel’s findings, issuing a statement that engine power loss could not have been a major factor in the crash. It instead blamed the pilots.

Harvel retired shortly after the report was released, but he later wrote a book about the crash, the AIB process and the command’s response. The book, Rotors in the Sand, includes a chapter titled “My Unofficial Opinion of What Really Happened,” which asserts that gearbox problems were a cause. He writes that when the Osprey was descending, the left engine experienced a compressor stall, and the flight engineer dumped fuel to reduce weight. Crewmembers were overwhelmed by a sudden loss of engine power, and the pilot increased thrust to full power to try to reverse the rapid descent. The stall led to a surge in torque to the right engine, breaking the proprotor gearbox clutch and destroying the engine.

“The crew had no idea of what caused the power loss situation,” Harvel writes. “There were no procedures, and no information in their flight manuals about clutch failure. However, they would get other ‘cautions’ posted on their engine instruments relating to driveshaft and gearbox components attached to the right engine.”

A Rolls-Royce analysis of an engine recovered after the Osprey was destroyed found that it was running, not shut down, at the time of the crash. Harvel agreed: “The engine was running, but the proprotor gearbox clutch was broken. In fact, I think both engines were running. They were operating at an abnormally low power.” The incident was referenced in later reports about Osprey crashes, but the investigation was not revisited. Harvel died in 2020.

Slife says this crash and another two years later—at Hurlburt Field, Florida, that resulted in the aircraft being destroyed but no fatalities—remain mysteries to him. He was personally close to both incidents. A longtime friend, Senior Master Sgt. James Lackey, died in the Afghanistan crash, and Slife attended his funeral and gave the eulogy. At Hurlburt, he was the wing commander at the time of the crash.

“Essentially the question that I asked and impaneled our team to go answer was: Would what we now know about the hard clutch engagement phenomenon—when it happens, how it manifests itself and so forth—would that knowledge materially change any of the findings or conclusions of any prior board, where perhaps we just didn’t understand the phenomenon of these sprag clutch slippage events and the subsequent hard clutch engagement? That was the scope of what we did,” Slife says.

“And the answer that came back [was] that there are no findings or conclusions from any of the prior investigations that would have been changed by what we now know about hard clutch engagements,” he continues. “That doesn’t mean that hard clutch engagements were not present in any of those previous mishaps that we looked at; it just means that the findings and the conclusions of the investigations themselves would not have been materially changed by our current understanding of that phenomenon.”

For the Afghanistan crash, the ability to reinvestigate the incident was limited. There was very little evidence. The aircraft was destroyed and the crash site ransacked after the responding ground force did not retrieve the data recorder. The AFSOC team looked at testimony given at the time, though the co-pilot said during the original investigation that he could not recall the crash. Slife says Harvel’s formal report at the time concluded that an unexplained mechanical malfunction was the cause. What is now known about the hard clutch engagement “neither supports nor undermines that conclusion,” Slife says.

“Could that have been part of the sequence of events that led to that crash?” he asks. “It could have, we just don’t have any evidence that would suggest that’s the case. And so . . . it didn’t warrant a reopening of the investigation because it didn’t tell us anything about that specific mishap that we didn’t already know.”

Looking for the Root Cause

None of the V-22 operators know the root cause of the hard clutch engagement issue, although the Marine Corps, for example, says it has been a known issue since at least 2010. When the Air Force grounded its fleet in August, the Marines and Navy kept flying and said their aircrews know how to work around it. It is one of 13 Category 1 deficiencies that could affect flight safety of the V-22 fleet. The program office declined to identify the other deficiencies.

Slife says AFSOC crews have changed how they fly, including procedural changes to adjust how power is applied for tactical takeoff-and-departure operations as part of risk controls in place.

A data collection effort is underway to continue investigating the issue. When an Osprey experiences a hard clutch engagement, the clutch is sent to Bell-Boeing to examine the material and help determine a long-term fix.

“We know we have to identify the root cause of why the clutch is slipping in the first place and fix whatever the root cause is,” Slife says. “So I don’t know that the clutch is the root cause. But we do know that the clutch is slipping, and so we’ve got to work back and figure out what the root cause is.”

In the short term, AFSOC is looking at replacing the input quill assemblies to get ahead of a potential hard clutch engagement incident. “In the meantime, can we replace the components that are subject to slipping? [These are] some of the details inside the clutch and the sprags themselves, the inner and outer races of the clutch,” Slife says. “Can we replace components on a time-change schedule to make sure that we’re avoiding clutches with enough time on them that they might be subject to slipping?”

AFSOC is working with Bell-Boeing on the effort. It would not be a quick fix, as AFSOC and the program office are trying to understand industry’s timeline for building the new components and plan for installations across the Osprey fleet. The command estimates installations would take about six months to incorporate across the fleet, once new components are delivered. Bell-Boeing has not outlined any potential manufacturing timeline, referring questions to the program office.

As AFSOC progresses, the Marine Corps, Navy and the Japan Ground Self-Defense Force are determining whether to follow suit, the Marine Corps and the program office said in statements.

Slife says his connections to the 2010 and 2012 incidents, along with experiences early in his career, shaped his decision to ground the CV-22 fleet in August and how he is proceeding with AFSOC’s Osprey plans.

“Early in my career, the battle of Mogadishu, [Somalia,] in 1993 was the first time that people I personally knew were killed in combat,” he recalls. “It left a mark on me. I never, as a leader, want to be in a position where I would look back at something that I asked somebody to do and wish that I would have changed something when I had the opportunity to. So this is kind of the Mogadishu test for me.”

If the fleet kept flying after the incident in Norway, and then another incident happened during which airmen were killed and a subsequent investigation found a hard clutch engagement had contributed to it, Slife says, he would look back at his decisions and wonder if he had done everything he could have to get to the root cause of the problem.

“I’ve had to bury my friends, and I don’t want to do any more of that than I have to,” he says. “That was kind of what led us down this path in the first place. I think we’re in a better place, even if we don’t fully understand what’s going on inside the input quills themselves.”