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Vibration Causes Components and Structures to Wear and Fail Prematurely, Never Reaching Their Designed Time Before Overhaul (TBO)

Helicopters require significantly more maintenance than fixed-wing because theyʼre significantly more complex. The Apache Program Managerʼs (PM) Office has encountered a number of challenges in dealing with inherent dynamic drivetrain vibration. This vibration causes components and structures to wear and fail prematurely, never reaching their designed Time Before Overhaul (TBO). This creates added burden in areas such as logistics and operational readiness, safety of flight, airworthiness, and numerous other areas inherent in the operation and support of a major aircraft program. The current prolonged surge in aircraft utilization in the Global War on Terror has caused an unplanned acceleration of the airframeʼs useful life resulting in a strenuous burden on the Operations & Support budget and the soldier. The common theme to solving these dynamic component concerns is the requirement to perform new special maintenance inspections and subsequently replace components. Apache users worldwide have borne the cost of dynamic components failing long before their scheduled TBO.

For example: Failures of the main transmission accessory gearbox overrunning clutches have resulted in the transmission being removed every 1,000 hours for clutch replacement followed by a transmission overhaul at the next 1,000 hours of operation; the auxiliary power unit (APU) power take-off (PTO) has been replaced with a completely redesigned/reengineered Commercial Off-the- Shelf Supply Initiative clutch; Another example is the ability for the intermediate gearbox (IGB), tail rotor gearbox and main and tail rotor blades to reach designed TBO have been adversely affected.

Other examples of vibration related concerns;
Structural damage to the aircraft, cracking in the 2L stringers and 530 and 545 bulkheads, existed. The latter issue resulted in an elastomeric vertical stabilizer mounting modification work order (MWO) at a significant cost in terms of dollars, aircraft readiness, and aircrew combat readiness.

Avionics systems such as Target Acquisition/Designation System, Pilot Night Vision System, and associated “black boxes”, are adversely affected by vibration generated from an unbalance drive system. Drivetrain components such as driveshaft, rotor blades, great boxes, etc, are being balanced independently at the manufacturer level and delivered to the assembly line as a “balanced component”. When all the parts are assembled on to the aircraft, the entire aircraft drivetrain is not balanced “as one unit”. Therefore, vibrations are traveling along the entire aircraft drivetrain and fuselage. During flight, torque is forced on drivetrain components and vibration at one area of the aircraft is transmitted to another area.

For example:
It is possible that current vibration monitoring systems will alert a high vibration in the intermediate gearbox, historically a location of high vibration problems. In reality, the root cause of high vibration can be in many places along the drivetrain and the vibration is transmitted to the intermediate great box area from the problem source area. The most common solution to IGB vibrations is the replacement of the gearbox, which requires time and resources. After the component is replaced, in most cases the problem still exists and the aircraft continues to be grounded while maintainers do “guess work” to try and find the cause / source of the problem and possibly replace additional components.