Bell 525 Relentless - Using Tail Rotor Torque Measurements for Maintenance Credit

In the course of operation, rotorcraft structural components will encounter loading cycles that are often associated with excursions in the main rotor and tail rotor torque. Rotorcraft designs anticipate these load excursions via a design usage spectrum that factors a certain number of such load conditions per flight hour. In general, the usage spectrum assumes worst-case usage regardless of how the helicopter is actually flown. Thus, maintenance intervals are the same regardless of how the helicopter operator uses the aircraft. If a means exists to detect the actual occurrence of these load cycles (per flight hour) and the measured occurrence is less frequent than the assumption in the design usage spectrum, a usage credit could apply. Such a usage credit would reduce maintenance cost by allowing longer intervals between inspections and longer intervals between replacement lives for certain life-limited components. Such a methodology is being sought for the Bell 525, which has a tachometer-based tail rotor torque measurement system. Main rotor torque is also available and is calculated from known engine torque, tail rotor torque, and losses. The torque measurement system has almost two thousand hours of flight test results that show it to be highly accurate. A methodology to use the tail rotor torque measurement and main rotor torque calculation to safely substantiate future usage credits and achieve reduced maintenance cost is put forward. Finally, an application for this sensor technology to detect drive system faults is introduced.