Avnx EO said,
"Advocates argue that the HUMS vibration monitoring allows the prediction of impending failures..... but remember how this works: HUMs detects failures which follow a predictable pattern of failure. By definition, if its predictable, then it should have been factored into determining the certified life of the component, and with the current margins, you are sure to replace the component long before you get into the pattern the HUMS was designed for. "
What about the human factor errors, like installation errors and misalignment? While these are not factors in certification, mistakes do happen when driveshaft clamps or shims are not properly put back in place. These type of failures can be effectively tracked.
A customer recently rejected an engine due to high turbine vibration. At teardown there were deposits on the wheel. Turns out that aircraft had been flying to a platform that was spewing effluents (sp?) bad enough that the pilots had to clean their winsheild prior to taking off. They thought nothing of it, but vibration analysis removed the engine prior to any catostrophic damage.
You also downplay the ability to know the vibration levels with respect to ride quality. Many pilots have a different perspective on how smooth their aircraft flies. I have seen a new crew come on hitch and immediately ground the aircraft due to high vibrations, yet the pilots that were flying the aircraft said it felt fine. Knowing these levels ahead of time and not waiting for someone to down the aircraft because the aircraft does not feel comfortable does offer quite a bit of payback as it allows you to schedule the maintenance. Quite a few aircraft have recurring vibe checks of different components. Reviewing the data from HUMS allows them to sign the check off without the need to install portable equipment and the associated ground run. This also saved a start cycle too.
On non bussed non FADEC aircraft, exceedance monitoring will offer one of the greatest paybacks. I've seen write ups where the pilot stated torque went to 105% and the copilot said it went to 95%. What would you do on a non HUMS aircraft? Most would perform an overtorque inspection, a costly endeavor. Yet when reviewing the data the aircraft only went to 99%. That one instance made a considerable payback on the acquistion cost of HUMS.
Aircraft parameter recording as used in Exceedance monitoring can also be an effective troubleshooting tool. Take the writeup that the engine starts hot... First question asked of the pilot is what was the battery voltage, he shrugs his shoulders and say, I am not sure, you look at the parameter data and see the battery bus voltage dropped to 15 volts during start. You now replace the battery instead of adjusting fuel controls. How about a write up that said, #1 engine failed power assurance due to high N1. A review of the parameter data shows the N1 lower than indicated on the gage. At next run up, real time viewing of the parameter data indicates that in fact the N1 gage is reading higher than actual. You change the N1 gage not the engine...
Safety does play an important factor, but HUMS also plays a large part in planning and logistics.
I too agree that in the beginning that HUMS was billed as the answer to all problems with Usage credits as the biggest selling point. However, Health monitoring plays an effective role too. Useage without credits also has paybacks, as in flight time tracking can save a considerable amount in actual usage, just from rounding errors alone.
HUMS without credits does offer payback, it just does not offer the magic bullet that usage credits offer in a distinct number that you can assign a cost per hour payback.
chuckolamofola