Life usage indicator

Apparatus for in-flight determination and display of the fatigue life usage of a plurality of helicopter transmission gears, the gears having gear teeth which are subject to differing stresses at different times as a result of differing torques being transmitted thereby, the rate of fatigue life usage of each gear at any instant being dependent on the level of torque being transmitted thereby, said apparatus including:

• a plurality of torque measuring monitors, one or more of the monitors being associated with each one of said gears, the said one or more of the monitors associated with each one of said gears providing a torque monitor signal representing the level of torque being transmitted by the associated gear;
• conditioning means for receiving said monitor signal for each gear and includig a multiplexer for sequentially switching between said monitor signals for said gears, said conditioning means being operative to condition the particular monitor signal being switched by the multiplexer at each time instant for further processing;
• a life usage memory having stored therein as look-up tables the expected life usage for each of the transmission gears for each of a plurality of mutually exclusive torque ranges;
• a life usage determining means connected to receive the conditioned monitor signal being switched by the multiplexer and being connected to said life usage memory, said life usage determining means being operative to carry out a range determination, said range determination comprising determining in which one of the torque ranges the level of torque represented by said conditioned monitor signal falls at that time instant and said life usage determining means being further operative to carry out a life usage determination, said life usage determination comprising reading from the appropriate life usage in said memory a first measure of expected life usage for the respective gear which is having its monitor signal being switched to the life usage determining means by the multiplexer at that time instant, said determining means being further operative after a known time interval to repeat for the same gear the operations of range determination and life usage determination so as to provide a further measure of expected gear life usage being a measure of expected life usage for the said same year within the same time interval;
• and display means connected to said life usage determining means and operative to provide a cumulative display of determined fatigue life usage for each of said transmission gears.