Abstract:
Helicopter dynamic components are substantiated against fatigue using the
safe life methodology. The advance of usage monitoring systems suggest
the extension of the safe life according to measured usage, leading to
maintenance credits. The question is whether the extended life is
associated with a different probability of failure due to elevation of safety
factors embedded within the safe life methodology.
The fatigue substantiation process was studied and its conservatisms were
identified. Both S-N curve behaviour and service loading have been
investigated. Three components were studied. One was a location on the
lift frame, the others were a main rotor blade linkage and a rotor pitch
change link. The variation in manoeuvre loads and usage during normal
operation of a UK military helicopter was statistically estimated. Service
loads were measured using strain gauged components and a data recording
system. Usage was monitored by manual identification and recording of
manoeuvres throughout the helicopter flight.
It was found that usage variability is very great, with coefficients of
variation in excess o f 100% for the majority of manoeuvres. It was found
that usage measured in service was significantly more benign for all types
of mission, than that assumed in design. Mechanical test results showed
significant errors in damage accumulation and mean stress models for life
prediction. Monte Carlo simulations demonstrate that calculated
probability of failure is dominated by material and modelling errors; usage
and manoeuvre load variability playing a minor role.
