Is relying on HUMS acceptable?
CAP 753 (8.3) states that HUMS must detect no less than 70% of failure modes that it is directly monitoring. The oil pump shaft is not directly monitored so presumably the rate of detection may be less than 70%.
At what percentage do you say the risk is mitigated and it's OK to fly?
Can EC or an operator demonstrate 225 HUMS will always give an early warning of impending failure?