Originally Posted by
TenTon
Anyone who thinks HUMS data could be interpreted in real time has never seen the Data before. Please stop thinking HUMs is a magic tool to determine maintenance requirements. HUMs has always been and still is a maintenance TOOL. Not a determining factor nor reliable enough to make a decision such as ditching a Helicopter as some seem to think or those in suits want everyone to believe. Some HUMS indications are so vague it can take a LONG time to determine if the HUMs system or aircraft is faulty. Much longer than your typical out and back trip offshore helicopters take making real time impossible. Whatever failed on this Helicopter happened incredibly fast. Faster than even the Pilots onboard to realize something was wrong. HUMs=USELESS in that situation. If the data is still intact on the card then it may help investigators determine what failed easier.
A newbie on this site, but interested in rotorcraft safety for sometime.
I agree in principle that HUMS indications are generally vague, need to be analyzed in the right statistical context, and, from the operational perspective, can result in an unacceptably high false positive rate. As a result, they are generally not expected to be used on-line on-board at their current level of maturity.
However, I am reading the Aircraft Accident Report for G-REDW and G-HCN from 2/2014
https://www.gov.uk/aaib-reports/aar-...cn-10-may-2012
As a reminder, two ditchings in 2012 were caused by fatigue cracks in bevel gear vertical shaft. One of the findings of that report was that original finite element analysis (FEA) used for certification of EC225 underpredicted the stresses in the bevel gear vertical shaft by a factor of 3.8 (page 98). As a result, the estimated safety margin was reduced from 5.4 to 2.1 (page 152), which basically invalidated the original certification (for the safety margin under 3, FEA alone would not sufficient and fatigue test would be required).
At the same the relevant HUMS indicators, and MOD45 in particular seemed to provide an early warning, but only about two-to-four hours in advance (this is my interpretation rather than how it was stated in the report, but take a look at Figures 22 and 23 of the report).
So, one of the recommendations of that report has led to ASB No EC225-45A010 ‘Central Maintenance System – HUMS – M’ARMS MOD45 on-board monitoring system’, dated 8 July 2013. I quote: "The purpose of this ASB was to upgrade the MFDAU (Miscellaneous Flight Data Acquisition Unit) software to:
● Calculate the MOD-45 indicator in real time.
● Increase the acquisition rate.
● Display the MOD-45 indicator status on the HUMS Control Panel or (Man-Machine Interface)"
I tracked ASB No EC225-45A010 document to the most current AD No.: 2014-0078R1 from July 08, 2014. The best I could tell from this last document is that the recommendation was indeed implemented for some rotorcraft, but there was another option:
"after EASA AD 2014-0078 was issued, Airbus Helicopters redesigned the MGB bevel gear vertical shaft through modification MOD 0752525, which provides new nitrided shaft P/N 332A32-5109-00/01/05/06. This new shaft design eliminates a possibility of mechanical failure of the shaft. However incorporation of the new shaft requires to keep previous installation of a new MGB oil jet (MOD 0753021), and is incompatible with M’ARMS MOD45 monitoring function (MODs 0726994 and MOD 0728083) as initially required for EC 225 helicopters equipped with a VHM"
In summary, while it might be unwise to rely on HUMS for safety warnings on board, the Aircraft Accident Report has recommended to do just that. Two questions:
1. Is it possible from the evidence we have so far to rule out the possibility that this current accident was caused by bevel gear vertical shaft failure?
2. Was this specific rotorcraft equipped with M’ARMS MOD45 monitoring function or had a redesigned MGB bevel gear vertical shaft that "eliminates a possibility of mechanical failure of the shaft" (or neither)?