I was asked to post this on another thread but it really belongs to this one - it comes from the HARP report of 1984:
The most complex element of a helicopter, after perhaps the engine, is the transmission gearbox, taking the drive at high speed from one, two or more engines generally in a horizontal plane, and gearing it down by 80 or 100 to 1, or so, to a single vertical shaft carrying the rotor head, with another drive going rearward to a tail rotor (or another rotor head system), the engines having free Wheels, and accessory drives being taken off for generators and oil pumps.
These gearboxes are remarkable examples of the art of the mechanical engineer. Although simplicity is a good starting point in the conceptual stage, complexity is inevitable, and it is not surprising that each designer has found his own solutions. The panel has seen spur gear trains, epicyclics, bevels; it has seen normal straight teeth, 'conformal' tooth profiles; it has seen ball bearings, rollers, tapered rollers, shafts with separate ball races and shafts with rollers running direct on them; we have seen aluminium and magnesium alloy cases. The only common element noted was in the gear material, a traditional low carbon nickel-chrome case hardening steel, either carburised or nitrided, and invariably made of high purity vacuum melted stock.
Notwithstanding the exceptional facilities for gear production we were shown, and the great attention to quality control being paid we are bound to record that in our view it will never be possible to eliminate completely the possibility of failure of some part of so complex a mechanism in service, between specified inspection or overhaul periods.
Although minor surface damage to a gear or bearing is unlikely to be instantly catastrophic, and should be detectable, and although gearboxes are tested to run for a period such as 30 minutes with the oil supply failed, the possibility of a major failure within the box is ever present.
Since with present knowledge a main rotor cannot be duplicated although a tail rotor gearbox might (conceivably a helicopter may survive the failure of the latter) we now come up against the fundamental difference between normal aircraft and rotorcraft -the inability to guard against a possible defect by duplication.
The Panel believes that, while gearbox development and testing should be as stringent as practicable, other means must be introduced to monitor the condition or "health" of the gearbox, preferably in flight. Elementary condition monitoring is already practiced (e.g. oil chip detectors), but much more attention should be paid to this. We make our detail recommendations on this subject later in para. 8.5.
There have been failures of transmission systems taking the drive to the tail rotor, or coupling twin rotor heads. We believe it may be possible to achieve a degree of redundancy here, or at any rate 'damage tolerance'. On the other hand it may be simpler to have some other method of cancelling main rotor torque to use in an emergency (bleed gas jet, rudder surface •••••• )
There is no need to repeat anything in this quote but my underlines emphasize a number of salient points in the text. In original post I also indicated "The text in paragraph 8.5 (referred to above) contains recommendations for 'condition monitoring'; methods include: chip detectors; vibration monitoring; thermal detection or imaging in flight; oil sampling; telemetry (i.e. instantaneous download of HUMS data); and usage monitoring."
The exchange of information between 'The Sultan' and 'flyt3est' was interesting because they both have extensive (design) knowledge of HUMS - and both admit to weaknesses in the system of monitoring of the epicyclic modules. I tend to agree with Sultan, and the message contained in HARP report, that there any number of tools - each of which has its part to play. There is also the issue briefly touched upon in the HARP report that the role of monitoring tools is to deal with unexpected conditions within the inspection and overhaul periods. The setting of such periods, although important, becomes less critical when monitoring is effective – but it must be shown to be effective.
With respect to the S92 accident, no amount of monitoring (using the tools mentioned above) would have prevented that accident - but we have already extensively dealt with that issue in the S92 thread.
Just as we have inspection and overhaul periods in continuing airworthiness, we also need to have confidence that monitoring and early detection of incipient faults will provide sufficient warning to avoid unecessary stress/actions from the pilot in flight. We have also recently reset our tolerence levels to faults extremely low following these two accidents (and hence other threads reporting diversions). We must not put pilots in a position where cockpit indications result in additional hazards; having said that, Sultan's point about fuzz burning and further monitoring is valid.
Unlike the S92, the L2 accident appears to have come out of the blue. It is likely that the precursors to this accident, although subtle, were there; if they were and are pinpointed by the AAIB and EC (as I am sure they will be), the interim policy outlined in the AD provided by EC and EASA will be augmented/replaced by other procedures.
Helicomparator has a valid point that there has been little research into improvement of the HUMS monitoring, and specifically HUMS interpretation, for some time. The fact is that the teams which initially promoted HUMS have exhausted themselves with efforts in ensuring HUMS is implemented and, more importantly, that it is endorsed by the manufacturers (as enthusiastically as it has by Sikorsky). Another factor is that a number of the personalities who were the driving force behind the HUMS initiative have left the scene – some in retirement. HHMAG has also been disbanded since legal competence for airworthiness passed to EASA (and has not been replaced).
In spite of the success of the work of key personalities of the HHMAG and the resultant acceptance of HUMS, it should be noted that although VHM (for larger helicopters) is an existing requirement in the North Sea States and a Recommended practice in ICAO (and work was in progress to include requirement in the JARs by 2012 – which had been accepted by all manufacturers), there is no sign of a requirement in the new EASA OPS Proposal. When these regulations trump the national ones of the UK, Holland, Norway and Ireland, the requirement will be no more.
Why, is that? Is a question we might ask!
Jim