What happened
On 20 November 2024, a CV-2_2B, tail number 11-0060, was conducting takeoff and landing practice approaches at the Wilderness Helicopter Landing Zone within the Melrose Air Force Range, New Mexico. The aircraft, assigned to the 20th Special Operations Squadron of the 27th Special Operations Wing, was performing day qualification and tactical training as part of a return-to-fly syllabus.
While executing a climbing right-hand turn at approximately 315 feet Above Ground Level, the aircraft experienced a rapid loss of left-hand proprotor gearbox (PRGB) oil pressure. This was immediately followed by an engine overspeed condition and a subsequent shutdown of the left-hand engine commanded by the digital engine control system. The crew responded to the cascading warnings by initiating an emergency landing profile. The aircraft successfully reached the ground approximately one nautical mile north of the range, resulting in no fatalities and no injuries, though the incident caused estimated damage to the aircraft of $2,795,884.
The investigation
The Accident Investigation Board examined the mechanical failure of the left-hand proprotor gearbox components. Investigators analyzed the sequence of cockpit warnings, noting that the aircraft's voice warning system suppressed certain specific announcements due to the simultaneous nature of the failures, instead using an aural "WHOOP" tone to indicate multiple concurrent conditions. The board also reviewed the automated power transfer from the right-hand engine to the left proprotor via the interconnect drive system, which allowed the crew to maintain controlled flight during the descent.
Findings
- The primary cause of the mishap was a catastrophic failure of the LH PRGB Lower Input Idler Helical Gear (P/N 901-044-105-101) caused by a non-metallic material inclusion in the gear's rim-to-web radius interface.
- This failure resulted in an eight-tooth section of the gear puncturing the gearbox case, creating a hole measuring 12 inches by 6 inches.
- The puncture led to the immediate loss of gearbox oil and subsequent engine overspeed.
- A contributing factor was identified as the poor design of the -105 gear within the CV-22 proprotor gearbox.