The Air Force Special Operations Command has released its report on the cause of the crash of a CV-22 Osprey off the coast of Japan last November, which killed eight crew. The accident investigation attributes the mishap to a catastrophic failure of one of the tiltrotor’s gearboxes and also points to decisions made by the pilot as a significant contributing factor. The crash had already led to all V-22s across the U.S. Marine Corps, Air Force, and Navy being grounded for more than three months, an order that was lifted in March.
The accident, on November 29, 2023, involved CV-22B serial number 10-0054, callsign Gundam 22, assigned to the 353d Special Operations Wing at Kadena Air Base, Japan. The tiltrotor was taking part in a joint interoperability exercise when the mission had to be aborted, following multiple advisories and a caution message displayed in the cockpit, instructing them to “land as soon as possible.”
The crew then diverted toward Yakushima Airport, around 60 miles away. While on final approach to the runway, and around 800 feet above ground level, “a sudden materiel failure” put the aircraft into an immediate left roll. The tiltrotor rolled twice and then impacted the water, around half a mile off the coast of Yakushima Island, Japan, at approximately 2:40 p.m. local time.
All eight crew on board the Osprey were killed on impact, with seven bodies subsequently being recovered in the search-and-rescue operation.
The following section from the Accident Investigation Board (AIB) report specifies the factors determined to be behind the accident:
“The Accident Investigation Board (AIB) President (BP) found, by a preponderance of the evidence, the mishap was caused by a catastrophic failure of the left-hand Proprotor Gearbox that created a rapidly cascading failure of the MA’s [mishap aircraft] drive system, resulting in an instantaneous asymmetric lift condition that was unrecoverable by the MC [mishap crew]. The BP further found, by the preponderance of evidence, Mishap Pilot’s decisions were causal, as they prolonged the mishap sequence and removed any consideration of an earlier landing at a different divert location.”
Other contributing factors were identified by the AIB, namely inadequate risk management and ineffective crew resource management.
Taken together, all these issues resulted in what the AIB described as an “insufficient sense of urgency throughout the entire mishap sequence, beginning with the first PRGB [Proprotor Gearbox] cockpit advisory approximately 49 minutes prior to aircraft impact.”
The CV-22 has a pair of PRGBs, one in each engine nacelle. Their job is to transmit power and provide speed reduction from the engine to the Osprey’s trademark proprotors. The PRGB is a part of the Osprey’s design that has proven to be problematic in various ways over the years.
After an analysis of the parts of the left-hand PRGB that could be recovered, it was determined that the catastrophic failure in this instance was “most likely initiated by a crack in one of the high-speed pinion gears and fatigue cracking of the associated pinion gear’s bearing cage. Additionally, at least one piece of the failed high-speed planet pinion wedged in the high-speed carrier assembly, grinding against the high-speed sun gear’s teeth until finally removed.”
Looking in more detail at the AIB’s conclusion that the Mishap Pilot’s decisions were a causal factor in the accident, the board president states that the pilot’s decisions “caused a prolonged mishap sequence of events that removed any consideration of an earlier landing at a different landing location.”
Specifically, those individual decisions started with the Mishap Pilot electing to continue with the mission after a third chip burn advisory posted in the cockpit. A chip burn advisory gives a visual indication when the V-22’s chip detection system detects debris in the PRGB’s gear oil. At this point, the situation became Land as Soon as Practical, with the pilot then deciding to land at Yakushima Airport, although closer locations were available.
According to the AIB, the pilot and crew did not “plan for, deliberate, or even discuss” closer suitable landing options after they received the chip burn warning about the left-hand PRGB.
In response to the board’s findings that the pilot’s actions had a causal role in the mishap, Tim Loranger, an aviation attorney and senior partner at Los Angeles law firm Wisner Baum, representing the families of two of the dead crew, provided the following statement:
“One fact remains clear: Gundam 22 suffered a catastrophic failure initiated by a crack in one of the high-speed pinion gears, which led to pieces of the high-speed planet pinion grinding against the sun gear. This mechanical breakdown resulted in a cascading failure of the drive system and an instantaneous asymmetric lift condition that was impossible to recover from.”
Loranger adds that, based on the report’s findings, “no amount of skill or experience could have fully overcome” this kind of catastrophic mechanical failure. Loranger adds that “it is crucial to understand that these decisions were made under extreme pressure and what became a rapidly deteriorating situation.”
“The loss of these brave individuals is a profound tragedy, and any suggestion that places blame on those who are no longer here to defend their actions only adds to the deep pain their families are already enduring.”
Asked by The War Zone about possible plans for a lawsuit, Tim Loranger said: “There has not been a lawsuit and no decision has yet to be made. The report was just released.”
Meanwhile, we will also have to wait and see what, if any recommendations are made to V-22 operators to reflect lessons learned in the investigation.
In a statement provided to The War Zone, the U.S. Marine Corps — by far the largest operator of the Osprey, with some 348 MV-22Bs spread across 17 squadrons — deferred any comment about the impact of the investigations on the V-22 to the Joint Program Office.
The statement continued: “The Marine Corps remains unwavering in our commitment to the airworthiness of our aircraft and ensuring the safety of our Marines who fly and fight in support of the Marine Air-Ground Task Force, Naval, and Joint forces.”
Another Osprey subvariant, the Navy’s CMV-22B carrier onboard delivery (COD) is currently also unable to perform fully in its main mission set, being limited to flights and missions that stay within 30 minutes of a suitable divert airfield. Most critically, this prohibits the use of the CMV-22 for carrier onboard support of deployed aircraft carriers once they have ventured far from shore. As we have discussed in the past, the result is that the CMV-22 is essentially operating within the range of what a laden heavy-lift helicopter could achieve. In retrospect, this imposed limitation mirrors the findings from this report, considering the gearbox warning came 49 minutes prior to catastrophic failure. Theoretically, if the exact same circumstances were to occur that struck the doomed CV-22B, thirty minutes would put the CMV-22B within range of a divert location with significant margin to spare.
Overall, the Bell-Boeing tiltrotor has had a checkered history, punctuated by numerous fatal crashes since its first flight in 1989, though U.S. military officials regularly point to a broadly improving track record in the past few decades. For now, we will have to wait and see to what degree this accident affects the broader V-22 community.
Contact the author: thomas@thewarzone.com