NATIONAL HARBOR, Md.–The U.S. Air Force Special Operations Command’s (AFSOC) fleet of 51 Bell [TXT]-Boeing [BA] CV-22 tiltrotors may be flying without restrictions by the end of this year or early next year, AFSOC Commander Lt. Gen. Michael Conley told reporters on Wednesday.
“About 60 percent of our fleet is fully operational,” he said. “We’re adding back three or so every 10 days.”
The figures would mark a return to flight acceleration, as AFSOC said that it had cleared
14 CV-22s for flight in late July (Defense Daily, Aug. 1).
Eight airmen died last Nov. 29 when metallic chips caused their CV-22’s left-hand proprotor gear box (PRGB) to fail, and the Pentagon grounded the Navy/Marine Corps V-22s and the AFSOC CV-22s.
A PGRB, mounted in each of the CV-22’s nacelles, receives same side engine power through the respective input quill, which houses the clutch assembly. The PRGB also provides speed reduction to the proprotors.
“At least one piece of the failed high-speed planetary pinion wedged in the high-speed carrier assembly, grinding against the high-speed sun gear’s teeth until they were completely removed,” the AIB said. “The removal of the gear teeth prevented torque being applied to the left-hand mast…Once the left-hand PRGB failed, an onset of rapidly cascading malfunctions occurred, to include low/lost left-hand PRGB oil pressure, ICDS [interconnecting drive system] failure, and right-hand PRGB over-torque–all occurring less than six seconds after failure.”
Naval Air Systems Command (NAVAIR) PMA-275, the V-22 Joint Program Office (JPO), said on Aug. 1 that “all gearboxes produce metallic chips” and that “chips were a consideration when designing the V-22, and the aircraft has a robust and reliable debris detection system.”
While NAVAIR approved a return to flight with safety controls in place–such as flights within 30 minutes of an airport–for the U.S. Marine Corps MV-22 and U.S. Navy CMV-22 on March 8, AFSOC said that it is taking a phased approach to resumption of flight. NAVAIR has projected the middle of next year for a resumption to full flight of the Navy and Marine Corps tiltrotors.
“The V-22 debris detection system consists of active and passive monitoring,” the V-22 Joint Program Office (JPO) has said. “Active detection provides aircrew in flight alerts and the passive detection system allows for precise monitoring of any gearbox metal debris. Debris is collected and analyzed allowing maintenance and engineering support teams to make informed decisions with the gearbox. The JPO PMA-275 continues to explore new solutions to prevent chips from occurring in the gearbox.”
G22’s left Rolls-Royce Liberty AE1107C engine was built in December 2016, installed on the aircraft in October 2019 and had 1,253 flight hours, while the right engine was built on Feb. 1, 2010, installed on G22 in August 2017, and had 1,364 flight hours. Both engines passed the minimum 95 percent power assurance check last Nov. 16–the left engine at 106 percent and the right engine at 100 percent, the report said.
“Chips are a normal occurrence in rotary and prop aircraft,” AFSOC said on Aug. 1. “There is no indication that this was due to older engines and engineering analysis is ongoing by the Joint Program Office to better determine what causes the fatigue cracking in the gear. This is the first time a failure has occurred like this in nearly 800,000 flight hours across the V-22 enterprise.”
The CV-22 cockpit received the first of five “chip burn” notifications about 49 minutes before the crash–a notice indicating that a safety system electrical current was burning off metal fragments in the engine. After the third notification, the aircraft is to “land as soon as practical,” but the pilot has discretion to continue and did so, as he assumed that the notifications were false reads by a chip detector, the report said. The G22 at the time of the third notification was still close to mainland Japan and “several divert airfields,” AIB said
Conley said on Wednesday that such PRGB failures look to be extremely rare and that the V-22 JPO is exploring materiel improvements with Bell in addition to the institution of new flight procedures. Previously, the guidance was to land as soon as possible after a maximum of three “chip burn” lights, and now the guidance is after one such light, Conley said.