CEN24LA358

The pilot of the helicopter was performing external lift operations when he noticed an “intermittent and unusual vibration.” The pilot performed a precautionary landing, and a postflight inspection of the main rotor system revealed a crack in one main rotor blade, which constituted substantial damage to the helicopter. Postaccident examination of the main rotor blade revealed that a large crack extended from the leading edge spar on the lower surface to the middle of the main rotor blade along the doubler leading edge on the upper surface of the blade. There was an area of chipped paint adjacent to the crack at the outboard tip of the root fitting on the lower side of the blade. After sectioning, the large crack was mechanically exposed and examined stereoscopically. Feathery features consistent with fatigue were observed on the lower side fracture surfaces of the doubler, skin, and spar. Scanning electron microscope examination revealed numerous ratchet marks at the doubler-skin and skin-spar interfaces emanating at the outboard sides of both layers, consistent with multiple initiation fatigue starting from the outboard side. Dark-colored deposits were observed on the spar fracture surface at the tip and on the adjacent skin fracture surface due to the rubbing of mating fracture surfaces. The large size of the crack and the rubbing of the mating fracture surfaces was consistent with fatigue crack growth over an extended period of time. The accident helicopter main rotor blades were subject to an FAA Airworthiness Directive (AD) for regular inspections due to the helicopter’s use for external lift operations. The cracked main rotor blade was installed onto the accident helicopter the month before the accident and had accrued about 67 hours of operation since installation. The operator reported that an inspection in accordance with the AD (AD 2005-21-02) was most recently performed 6 days (12.4 flight hours) before the accident. Maintenance personnel noted no defects during recent inspections. According to the operator, the inboard portion of the main rotor blade was painted gloss white by the operator’s director of maintenance sometime during the month of the accident; however, there were no maintenance records reflecting when or by whom the painting was performed or inspected. It is likely that the fatigue crack occurred sometime between when the main rotor blade was installed onto the helicopter and before the accident flight; however, when the fatigue crack occurred could not be determined. Manufacturer guidance stated that the identification of cracks in main rotor blades requires a careful examination, including the use of supplemental lighting and optical magnification. Whether maintenance personnel performed the inspections with adequate lighting and magnification was not determined. Additionally, it could not be determined whether the fatigue cracking may have been concealed by the recent painting of the main rotor blade.

On September 11, 2024, about 1400 eastern daylight time, a Hughes 369D helicopter, N8969F, sustained substantial damage when it was involved in an accident near Canton, Michigan. The pilot was uninjured. The helicopter was operated as a Title 14 Code of Federal Regulations Part 133 rotorcraft external load flight. The operator used the helicopter for external lift operations. The purpose of the low-level flight was to support powerline maintenance work. The helicopter was operating from an off-airport landing zone delivering cargo to a powerline structure about 300 yards away. While in flight, the pilot, who was sitting in the left seat, noticed an “intermittent and unusual vibration.” The pilot performed a precautionary landing and shut down the helicopter without further incident. The pilot performed a visual inspection of the main rotor system, and he observed a crack in the yellow main rotor blade (main rotor blades are typically marked with various colors to easily identify and reference each individual blade). The helicopter sustained substantial damage to the yellow main rotor blade. There was no further damage to the helicopter. The main rotor blade was manufactured by MD Helicopters (part number 369D21100-523 and serial number 009999-K364) and was subject to a finite life limit of 4,000 total hours. The blade was installed on the helicopter on August 3, 2024, at 3,222.2 total hours. At the time of the accident, the main rotor blade had accumulated 3,289.9 total hours, 67.7 hours since installation, and 6 days (12.4 hours) since the most recent inspection. Postaccident examination of the main rotor blade revealed that a large crack extended from the leading edge spar on the lower surface to the middle of the main rotor blade along the doubler leading edge on the upper surface. There was an area of chipped paint adjacent to the crack at the outboard tip of the root fitting on the lower side of the main rotor blade. After sectioning, the large crack in the main rotor blade was mechanically exposed and examined stereoscopically. Feathery features were observed on the lower side fracture surfaces of the doubler, skin, and spar. Scanning electron microscope examination revealed numerous ratchet marks at the doubler-skin and skin-spar interfaces emanating at the outboard sides of both layers. Dark-colored deposits were observed on the spar fracture surface at the tip and on the adjacent skin fracture surface. On February 4, 2004, MD Helicopters released Service Bulletin (SB) 369D-201R2, Main Rotor Blade Assembly Torque Event Inspection, which they classified as mandatory compliance. The SB stated that some operators had experienced cracking of the main rotor blades, and that the cracked blades were associated with a high number of torque events (TE), or external lifts per hour. The SB provided procedures for operators to determine the number of TEs accumulated on each main rotor blade in order to establish inspection intervals. The contents ofthis SB were later adopted into FAA Airworthiness Directive (AD) 2005-21-02 on November 1, 2005, in which the FAA required compliance with the service bulletin and recurring inspections of the main rotor blades at intervals not to exceed 200 TEs or 35 hours time in service, whichever occurs first. MD Helicopters CSP-HMI-2, Handbook of Maintenance Instructions – Servicing and Maintenance, which is applicable to the 369 series, defines TE as “the transition to a hover from forward flight” or “any external lift operation.” The accident main rotor blade part number was subject to this AD. The inspection procedures listed for maintenance personnel in AD 2005-21-02 specifically describe the inspection for chordwise cracks protruding from the underside of the root fitting and the doubler (the location and type of cracking found on the accident main rotor blade). The inspection procedures also specify using a bright light and a 10x magnification glass for the inspection of the lower side of the main rotor blades. MD Helicopters CSP-HMI-2, Handbook of Maintenance Instructions – Servicing and Maintenance, contains additional guidance for maintenance personnel performing a visual inspection. During a TE inspection, the main rotor blade should be lifted off of the droop stop from the outboard end of the blade. The handbook stated that the bottom side of main rotor blades are more susceptible to cracks. The guidance also contains a caution for TE inspections and states in part that, “A crack can be difficult to find if a bright light is not used.” According to the operator, the inboard portion of the main rotor blade was painted gloss white by the operator’s director of maintenance sometime in September 2024; however, there were no maintenance records available showing the paint work that was performed, when the paintwork was exactly performed, who performed the paint work, and who inspected the paint work. Instructions contained in MD Helicopters CSP-HMI-2 stated: It is recommended to only paint the inboard 24 inch (60.96 cm) gloss white to help with crack detection. All blades must be painted the same. The operator’s maintenance records revealed that a TE inspection in compliance with FAA AD 2005-21-02 was most recently performed on September 5, 2024, at a helicopter total time of 18,306.4 hours and TE of 83,119. Since the main rotor blade was installed, four compliance entries with FAA AD 2005-21-02 were found in the operator’s maintenance records, with three different company mechanics performing that work. All four compliance entries contained a note stating, “no defects.” The MD Helicopters MD 500D (Model 369D) Rotorcraft Flight Manual (RFM) discusses preflight inspection procedures and states, in part, for the pilot to check: Main rotor blades for chordwise cracks on the underside of the blade skin and doubler. If existing, the cracks will be at the leading and trailing edges of the root fitting in the area of the two most outboard bolts or outermostend of the lower root fitting. The RFM does not require the use of supplemental lighting, such as a flashlight, during the preflight inspection. The RFM contains an emergency procedure for abnormal vibrations and lists the indications as, “sudden, unusual, or excessive vibrations occurring during flight.” The manual states that the onset of unusual or excessive vibrations in the helicopter may be an indication of problems in the rotor drive or drive train systems, and that the pilot should land as soon as possible to have the cause of vibration identified and corrected before any further flights. On October 2, 2024, MD Helicopters released Service Letter (SL) 369D-144, Inspection of the Main Rotor Blades Assemblies. The SL recommends that owners and operators ensure that inspections per MD Helicopters Service Bulletin 369D-201R2 are completed and FAA AD 2005-21-02 are referenced. The document further states in part: Failure to comply with the bulletin can cause the loss of a main rotor blade during operation and cause significant vibrations and a loss of helicopter control.