Main Gear Boxes and The Grand Lottery
 

We have had two Main Gear Box failures in the past few weeks that destroyed two aircraft and killed thirty three people causing much sorrow and loss to the families and friends.

Knowing Gann, I have to accept things like this will befall us as we pursue our career in aviation but yet we can question the reasons as to why they happen.

Is it "Fate"....a simple roll of the dice or are we assisted by the system letting us down?

Some questions that came to mind while thinking about this situation....


What does "Extremely remote" mean, and how was it missed by EC? By the DGAC? By Sikorsky? By the FAA?

That gear inside the 332's transmission is a "Primary Structural Element" and as such was certified in the 332L2 and the EC 225 as having a failure probability that was "extremely remote".

That means the probability of failure is less than one in a billion - a fleet of a thousand aircraft flying 1000 hours per year flying for 1000 years should not experience such a failure.

Yet, in spite of Eurocopter's promise and the aircraft certification, the gear broke, suddenly, and with little if any warning.

Clearly they missed it - both Eurocopter and the JAA/Easa, precisely as the oil leakage path on the S92 was missed by Sikorsky and the FAA.

How does one miss such things? Is there a blame? or is this something we have to live with?

If youre looking for 100% mechanical reliablity then youre in the wrong job.
It will just be another AD like the other mechanical issues causing fatalities.
Remember when the blades were flying off the A109s and the mast were coming off Bell mediums, when Arriel 5 modules were blowing up on Astars, and Robbie blades had all the inertia of a balsa model. Tail rotor tip caps came off 500s and all pretty much of the LTS101s early history?
Its that sort of thing.

Jim Beam
 

Jim, maybe the questions SAS is posing is valid for all those failures too. If we just sit back and accept it as part of the wallpaper of life then where, when and how do we progress.

There are stories about things going wrong at the factory that only come out years later. The saga of how the S76 tail rotor problems were suddenly solved overnight with a piece of kit that must have taken months to develop, the spindle saga, Bells combining gearboxes. Are they being straight with us or what?

When the Bell 412 gearbox (epicyclics) seized a couple of years back they managed an autorotation - why not the S92 and the 332L2?

CCD

Mmm ...

To an extent JB is correct ... we are at the mercy of the mechanical engineering .... BUT is that not the reason to examine the result of incidents and accident in our industry and then IMPROVE the design and reliability of of our machinery .... to allow the improvements of computer design tools to make our machinery better ....

SASless as far as transmissions are concerned ... was not the BA Chinook disaster also related to a transmission failure??.

Perhaps ... now the certifying authorities will revisit the criteria for Public Transport Helicopter transmission design and certification in view of recent events!


:eek:

We do not know the precise failure of the 332L2 epicyclic gearing but I have concerns about Sikorsky using titanium studs on a vital part of the MGB oil system, why change from the usual steel studs used for years? Titanium is not very tolerant, the MRH on the Lynx has to be treated with extreme caution, any slip of a spanner causing a ding on the material is a big problem. I suspect that titanium might be similar to stainless steel (SS) in day to day use, we had to be very careful when using SS bolts not to strip the threads and sometimes it couldn't be helped.
With a filter cover that had to opened several times during the tbo of a S92 mgb perhaps the studs were inadvertantly overstressed causing an ultimate failure. We really need to know why the studs failed on the NL S92.

i think to some extent its a "learn from your mistakes" sort of industry.
A bit like, "wow Kapton wiring" and "..all aircraft will be made of composites in the future.."
You dont know about alot of this stuff until its been in service for a while. But then the 332L has been around for a long time, maybe my argument is only valid for the S92 accident...

SAS,

Quote:- "That means the probability of failure is less than one in a billion - a fleet of a thousand aircraft flying 1000 hours per year flying for 1000 years should not experience such a failure."

It does state where in that one billion an incident will occur: it could be during the first 100 hours or equally it could be in the last 100 hours. Who knows? Don't get me wrong; this event must be thoroughly investigated and steps taken to try to make the odds better than one in a billion. As a current EC225 driver, I have a vested interest!

bondu

The BAH Chinook crash was a transmission failure.

JimB,

You failed to catch the point I was making.

The FAA has a definition of "Extremely Remote" as does the DGAC and CAA and JAA and EASA....just to mention a few of the certifying organizations.

My question is how they apply that definition to critical parts and how they determine when to apply the definition to those parts as compared to requiring documented testing of the items.

Look at the Sikorsky situation with the 92 gearbox.....it runs for hours with almost any amount of oil left in the sump....but only minutes with no fluid. It might have run longer if the aircraft had been flown at a reduced power setting than was used. No one anticipated the oil filter failing and causing such a failure as has occurred twice now.

The EC gearboxes have shown themselves to be subject to an unexpected cracking of gears based upon one known event and probably this latest tragedy. Despite HUMS and top notch maintenance engineering we see this failure occur and we now have to question the risk assessment that was applied to the potential failure of the gear box and its critical components.

I am not suggesting we can achieve perfection although that should be the goal otherwise we should not be involved in aviation to begin with.

What we need to do is assess the thinking behind some of the decisons and promises made by the builder and certifying agencies.

In light of this MGB failure at Bond.....the "Extremely Remote" concept just flew out the window!

Bondu,

You posted as I was editing my post.

I feel you prove my point about the definition....if it can happen in the first hour or the last hour....then what does that say about statistical probability.

If we had two failures on the same type aircraft, on the same operation, on the same day.....would that definition still hold? All we would have to do is to have the fleet fly the rest of its service life without a another failure and everything would be okay I guess.

The "Problem" with aviation is that it is constantly changing.

Equipment, Materials, Personnel, Processes, Repairs Schemes, Regulations, Research, Development, Standards, Companies, Manufacturers. All changing, all the time.

And nearly all the time it is to save money.

I cant remember which astronaut it was (John Bell?) who I paraphrase as saying:

"All the time I was up there, I couldn't help thinking all the capsule's parts were made by the cheapest bidders!"

The same goes for all parts made for any aircraft. If you think a broken Main Rotor is a critical item - try flying a plane with a broken Spar!

Yes, I am being simplistic! But I am trying to say that it can be a matter of Luck. As it can be lucky just to get to work every day, and then get home safely too.

Every care is taken with getting these critical products right - Manufacturers do worry about getting it wrong!

Perhaps it's time to change the "business equations" of safety levels - but I think that would be sooo costly as to prevent future development of any aircraft.

The main point is that we all want to make money as easily as possible, with the most risk permitted. We all accept inflated amounts of money to take part in those risk mitigations too.

We don't accept a high level of risk in anything we do - because we partake in daily risk assessment and mitigation.

Sometimes, someone gets something wrong - but not often. Often, it has been found, the person who 'does it wrong' is not aware of what has happened or why it happened until years after it happened.

Aviation, like medicine, is not a science. It is a "best guess" with some maths thrown in as a check.

We must learn from it - and then move on. As we always do.

We are all as good as the last accident/incident proved.


My respects to those that died, and to their families.

Rigga

I take your point SASless.
All these "one in a billion" statements are dervived mathmatically though.
Two things spring to mind, bigger and heavier.
Aviation parts, and particularly helicopter components, are built as light and as weak as possible.
Thats probably the reason Sikorsky used titanium studs, lighter than steel.
Build in more overload, more safety features, larger saftey margin.

I re read your first post about the death toll, 33 in two weeks, id imagine the oil companies (who pretty much drove up the aviation standards single handedly) will have quite a bit to say on this matter and things may change from their side rather than the regulators.
It will be interesting to see how this might affect the EC175s production testing.

Step One:

Put chip burning chip detectors to adequately monitor the gearbox and link it to pilot caution warning system. You do not need a $500K HUMS to do this. Let the crew either command a burn (Bell 222 technology from the 70's) or automate it with a CAS message alerting the crew. Multiple successful burns should be indication of bad things and prompt immediate crew actions. Unsuccessful burns should have the same result.

Step Two:

Implement step one as part of the regs (i.e. do step 1)

From the L2 design chips may have been accumulating on a HUMS read mag plug (not annunciated) which if annunciated could have given this crew actionable information.

The Sultan

Sultan,

Your post assumes the defective item is making metal.. It is possible, and reasonable that a crack in a gear will make an alteration to vibration signals, before it makes significant amounts of metal.. Thus $ 1/2 million HUMS systems.

Your dismissal of the system is in itself symptomatic of the kind of attitudes that prevent system improvement. If half a million bucks worth of HUMS isn't quite good enough, then why not spend another 1/4 of a mil making it better?? At least then when we start making the oily bits out of non-magnetic materials, we will have system maturity on the fault detection.

I'm not Rubbishing your statement, just highlighting limitations. I guess though, nothing that involves a human interpretation or judgement will ever be fool proof.


PS to whoever mentioned "spacecraft made of parts supplied by the lowest bidder"

One rather well known American manufacturer openly tells its supplier base that it will be insisting on a 2% year on year price reduction for the next 5 years.. How do you think that is going to be achieved?? A bit of lean manufacturing? Six sigma? Improved investment in new technologies? That must be it.. For sure.. Phew! No need to worry!

Flyt3est

I am making no assumptions. I am very knowlegdable in HUMS and I am not dismissing it as not beneficial, as history has shown it is one piece of the safety puzzle. It is an extremely useful tool, and if it is the primary tool for chip detection it is critical.

The issue is that those who claim vibration monitoring is superior to, or can eliminate, debris monitoring border on delusional. It is only a great addition to the tool kit. For vibration to be generated you have to have an actionable level of damage. Problem with cracks on high stress gears are that they may propogate to a critical level before any loss of stiffness is noticed in a daily check of HUMS. It is basically good, good and bang. Relative to root cause of many failures debris generates before enough surface damage occurs to provide an actionable vibration signature. If the chips are magnetic and the chip detectors are well placed, chip lights will be the first indication. Obviously, if the result of the Bond accident is causing a more frequent check of the mag plugs, the root cause is made from magnetic material.

As stated above all detection methods have a place. The industry and regs need to pick the low hanging fruit which has been missed.

The Sultan

Low Hanging Fruit
 

One of the major oil companies has gone on record asking a major US manufacturer (and the message was, I believe, passed to their French, German and Italian brethren) ......"Here is a flight bag with 10 kgs of metal - please put that back in your helicopter where it will do the most good, we will gladly accept 10kgs less payload as a price worth paying".

That after having to live with fragile tail-rotor systems and components that are scrapped if they are even scratched.

G

Main Gear Box failure from the Past
 

A sad accident from the past, on Google type " Puma Helicopter accident 16th December 1980 near Kuala Belait Brunei ".

About 30 years ago.

Tmb

Am I right in thinking some aircraft including some Eurocopter models have a weak link in the Main Rotor Shaft, that will break and allow autorotation in the event of a sudden gearbox failure ??

If this is the case, why is it not a feature of all helicopters. As a pilot, I've never missed it before, but as of February, I am beginning to consider it a necessity (if viable and available)

Sultan,

Ta for clarifying, slightly misunderstood your first post.

Concur with your thoughts on the 2nd one.

FT

EC gearboxes
 

I think we are missing the point here. The original cause of these failures is often an overtorque that has caused damage to a load bearing surface. This eventually results in stress fractures or similair failures. Correct me if I am wrong but I seem to remember that after an event the IHUMS is redatumed and virtually continues from this new baseline.
Regarding the 330J Puma failure I was there and involved both in the recovery and investigation. There is more to the story than revealed.

I know this thread has concentrated on the two most recent accidents but can any 76 drivers please comment on what the 76 RFM requires us to do ie we can conceivably have all three MGB caution lights illuminated + gauge presssure zero + gauge temperature high and it is a LAND AS SOON AS POSSIBLE ! When we get Grinding , Abnormal Vibration or Torque increase then we go to a Land Immediately or Ditch. I've always thought that by then , even if we are at zot feet and reduced speed , we are stretching our luck.....comments ?

Answer to a previous query. The only machine I am familiar with that had , inside the gearbox) , a waisted section on the main mast was the AS350. Supposedly the gear box could fail/seize and the mast would shear below the main upper gear box bearing thus allowing an autorotation. I wonder how they tested that ???

Peter

Gearboxes again
 

Ref wasted portion in gearbox, the Alouette series had that feature in 1963 when I did the course so they have had plenty of time to test but have never heard of it been used in earnest.

Spinwing, the BAH Chinook crash was caused by a transmission failure.

Briefly a big ring-gear (about 2 feet across) is fitted to the vertical mainshaft with a lot of bolts to take power from the horizontal high-speed shaft. Some, that is some, of these bolts were found to be losing torque slightly over the x000hr inspection period. A hugely complex and costly fitting of oversized bolts with higher torque capability was instigated.

Primarily due to a change in the washers used under these new bigger bolts (a lubrication channel was deleted) salt particles got into the thread area and started a corrosion pit which propagated a circumfrential stress fracture in the ring gear which finally failed. The 'box was fitted with a comprehensive chip detector system. Chip detectors cannot detect a stress fracture until metal starts spalling off, by which time it's probably far, far too late.



Post modified in deference to the remarks in the post below, though very little of the original was apparently incorrect.

Addendum...

At the time it was widely & popularly believed within the BAH community that the modifications were unnecessary, that the BA and Boeing engineers had developed a perfectly acceptable solution to the torque loss that did not involve oversizing bolts (and thus the incorporation of the non-scalloped shim. This, iirc, did involve Loctite) and that this mod had been forced against their better judgement by the CAA despite strident objections. Further, it was stated categorically within my hearing by people who should have known that one feature of the scalloped (grooved) shim was that it allowed lube oil to flush salt contaminants out of the joint, thus removing a potential cause of corrosion. The new-mod solid shims could not do this, and the view was that this lack of flushing almost certainly promoted, if not caused, tha corrosion pit that started the process.

Cetainly not a forseeable error chain, but a good example of the complexity of such events.

Agaricus bisporus
AAIB tell a different story: Air Accidents Investigation: 2/1988 G-BWFC

New technolody - old mentality
 

With the new helicopters like S92 EC225 AW139 we fly the latest technology, full glass cockpit, fully computerised, fully monitered (CVR,FDR, HUMS), and basic magnetic plugs ... :ok:

In flight we only have access to basic informations like temp and pressure and chip warning. :confused:

What would be the cost to bring all those datas centralised and available in flight with a proper monitoring.

As an example in the 139 I can know my hydraulic level at any time in flight, the level is monitered, but any significant change in a short time (leak) won't be noticed until the level is low. We always now too late :ouch:

If something goes wrong in the MGB IGB or TGB any significant change in vibration level and increasing in a short time should be displayed on the screen and not only "recorded".:ugh:

Don't tell me it's difficult to do, what computers can't do that ? :eek:

When you find yourself in one of those extremely remote probabilities, the few minutes or even seconds advance warning may change everything.


Manufacturers have to think differently when they design new aircrafts.
3rd millenium technology with last centenary thinking and design :yuk:

You engineers out there correct me if I am wrong.....you certainly have in the past!

Do not chip detectors tend to find "bearing" bits rather than "gear" bits?

Gear wear is more likely to produce fewer but larger chunks as bits of gear are broken off thus fewer bits are produced and are of larger sizes than bearing bits.

Chip detectors don't do squat for finding cracked gears.

Gearbox again
 

Hi SASless you are quite correct. although the chip detectors/mag plugs collect any ferrous material floating about in the oil. You may recall that a couple of years ago I found an entire gear tooth from a combining gear box in Port Harcourt that showed up only as an intermittent flash on the chip detector warning light.

I was asked to post this on another thread but it really belongs to this one - it comes from the HARP report of 1984:

Quote:

---

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.

---

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

With most of the chip detectors I come by, they worked only on ferrous materials, and of size approximately 100µm and more. As even the electric ones work on principle of collecting chips with magnetic field, and then shorting an electric circuit inside with that said chip. If the chip is not ferrous (coatings, light alloys, polymers etc.) or too small - it will not indicate.

Better chip monitoring systems include spectrography and lubricant flow monitoring... known in aviation industry by common name: "expensive".

Unfortunately in a scenario of protective coating failure it can lead to relatively fast destruction of the gear teeth, although producing small debris - around 20-40µm - for such case only constant gear monitoring system that includes magnetic field monitoring could pick such failure in time... although this require Hall sensors sets on each and every gear, making it very expensive.

It is possible to design an "intelligent" gearbox that will sense any type of failure, and warn crew about it, although I'm not sure anyone would like to pay 3 or more times more for a helicopter than one with "dumb" design.

The thread kind of drifted a bit. SASless originally asked:It's probably fate.

In reading the accident report of the BV-234 G-BWFC in 1986, it became clear that the manufacturer had been having issues with that forward transmission since the introduction of the civilian model 234 in 1980. (From that we can surmise that Boeing-Vertol was having the same issues with the CH-47, but were less evident to the general public.) They were working on it; they thought they had a solution.

Nevertheless, the failure of G-BWFC's forward transmission happened so quickly and unexpectedly that the crew had no time to analyze and do something about it. A similar fate seems to have befallen the crew of the Puma that just crashed in the North Sea.

No amount of testing and computer-generated data can predict every failure. A manufacturer can run a transmission in a test rig for hours and hours, but can it accurately simulate all of the inflight and harmonic vibrations and loads that the airframe either generates or is subject to? Hardly.

Helicopters keep finding new and/or different ways of killing us. It occurs to me that no matter how "safe" the manufacturers tell us these products are, those of us who fly these wacky machines on a routine basis are still very much test-pilots, much more so than our fixed-wing counterparts.

That is a sobering thought.

FH1100 Pilot,

The purpose of collecting data on flights is not to simulate but to measure; and with that measurement comes the ability to spot emerging anomalies (clusters and trends) and predict when failure is about to occur.

The great thing about neural nets is that they support machine learning and do not need the setting of thresholds; eventually, after establishing what normality is, and without human intervention, they can spot abnormality and therefore the potential anomaly.

Perhaps what some of us are bemoaning, is that we saw a demonstration of at least one of these programs (the GE software running over the Bristow data-store) a couple of years ago. In fact we also saw the same software running over the HOMP data. What this can spot is something like a pilot who constantly approaches faster than the rest of the pilots - not fast enough so that it triggers an alert (because thresholds have to be set high enough to avoid nuisance alerts) but a group of data points that sit in a cluster and so become an abnormal pattern.

There will always be faulty parts (bad material, poor machining, incorrect assembly etc) and they will conspire to break outside the normal inspection pattern or before overhaul. The breaking of such parts will always take the occurrence outside the 'extremely remote'. What collection of data and monitoring does is to allow us to eliminate premature failure by spotting the precursors.

In the same way, HOMP (FDM) permits us to identify behaviour patterns that, when isolated are not themselves dangerous but, when put in the mix with other elements might be the final link in the causal chain of the accident.

What most of us really want, is to take the human out of the heavy process that is post flight data analysis; much better that software systems do that for us and then alert when the abnormal pattern is observed - they are much better at this than we are. Leave the humans to the standard intervention (nuts, bolts, mag-plugs, inspection) - they are really good at that.

The problem I have with the arithmetic of a probability of 'extremely remote' is that it has to encompass the knowledge of continuing airworthiness (when do I target my inspection, when do I do my overhaul); hence the target figure is preserved because the faulty element, when found, is removed from the calculation. For a practical example look to the introduction of the EC155 to Nigeria; there never was going to be an engine failure because no engine sat in the aircraft for more than 200 hrs. It is setting these intervention intervals that is the real skill and the one which permits a very small figure like 'extremely remote' to exist.

In an extremely complex system, we can mitigate the errors made in the establishment of such intervals only by monitoring. It was this very point that was the basis of the quote from the HARP report.

Jim

Titanium alloys may have low strength - lower then Aluminium ones, although only when the % of titanium is higher than 98% - those almost pure Titanium alloys are used only in medicine, as are extremely resistant to corrosion. Not useful for aviation industry though - here are usually alloys with less than 90% Titanium, with the rest being Aluminium, Vanadium, Chromium etc. Those alloys are very tough, light, and both corrosion and heat resistant. The only reason why aircrafts are not made entirely of Titanium alloys (Black Bird and X-15 excluded) is because of high cost of the TiO ore, and manufacturing - as it is not very efficient - milling is very expensive because of high material strength, welding was not possible on the wider scale until 1990s, and casting is problematic because of other materials usage, and time.

I played with few things made of titanium alloys, and those were almost indestructible. Also take a look at the wreckage pictures of F-14s - main central wing spar that is also a main fuel tank and the whole aircraft hang from it - is always intact. I can't imagine that Titanium parts designed with aviation industry safety factors could be broken under normal operation loads, although as we all know it happened few times already, but I don't believe the material selection had anything to do with it.

Sometimes I think manufacturers are stuck in very old ways of solving problems. Gearboxes being one of them.

It would obviously be fully feasible to design a rotor system without a metal-against-metal-soaked-in-oil grinding gearbox - either with belts or by having a bigger diameter turbine stage and direct drive the rotor.

Maybe manufacturers of big helicopters should take a look at how far the RC heli's rotor systems have improved over the years.

Get your point and, except for the poor example,very well put.....

Nigeria is not the only place the EC155 was introduced and it has been more successful (with much longer engine 'sitting' times) in those places. So there was still the probability of an engine failure with that aircraft/ engine type, just not in Nigeria.

What is the technical reason that prevents putting a fluid level sensor in a gearbox sump and connecting it to at least a warning light or contents gauge and warning light?

I have never heard of one on a helicopter transmission ever thus there must be a good reason why would not work.

I think the reason theres no xmsn fluid indicator is the oil is really getting thrown around in there and it may give false readings. If youve ever watched a sight glass in a gearbox with the rotors turning youll know what i mean.
Thats the problem with using a sump as a reservoir
Theres also the problem of blocked jets and oil starved bearings which would not be picked up by a fluid level.

Sobering, yes ... but also somewhat to be expecetd given that the acft our fixed wing counterparts are flying have several more decades of practical cumulative development experience - about 100 years for FW vs about 60 years for RW (give or take a few). And, a similar but even more exaggerated experience gap in terms of cumulative units produced. The next 40 years will likely see considerable advances in most aspects of helicopter safety and reliability.

I personally feel that given the types of people who frequent our industry, with their engineering know- how, HUMS knowledge, pilots and those with experience and influence in regulatory spheres, the biggest crime here would be to fail to seize the current situation and use it as a catalyst to kick new life into flight safety initiatives.


My biggest fear is that this will not happen, and in 12 months, this whole tragic episode will be forgotten, and we will not have learned and improved to the best of our collective abilities. That would be a failure.

JimL, your post #27 has that piece of information which has caused some measure of debate. I've got the scars to show for it!!!! :) I know context is everything, but its little wonder people may have erroneous beliefs.

Brian,

Perhaps some clarification is necessary here; at the time that this report was produced - i.e November 1984, the members of the HARP (a committee set up by the Airworthiness Requirements Board (ARB)) would have been aware of the contents of the proposed amendment to FAR 29 but would have had no direct interest because Certification in the UK was to an alternative set of requirements - i.e. BCARS. (Without checking, I would make the assumption that this requirement was contained in BCARS. It was also not clear at that time that the UK would eventually be forced to abandon BCARS in favour of FARs (in the form of harmonised JARs).)

Perhaps a more interesting fact is that in the proposed NPRM, the text was slightly different.By the time the requirement was finalised as amendment 29-26 in 1988, the rule contained the conditioning text we now see. Normally one can examine the explanatory text of the final rule and establish why the text has been subsequently amended. In the case of the text of 29.927(c), the explanatory text is as follows:The 'law of unintended consequences' in action?

By putting in this clarification, there is no intent to reopen the discussion on the 30 minute run-dry capability (and the reason why it was not underlined in my previous post) only to clarify the extract from the HARP report.

Jim

Sultan wrote:

To detect a crack the only concept I can see as practicable would be a delta resistance detection system of some kind. (thermal or magnetic resonance imaging being cited as too costly above). Strain and stress guages have been around for a long time. In simple terms I envisage a system that runs a current through the gearbox from entry to exit and gives a resistance level readout which would have a known behavioural pattern for load/torque/temp settings. A crack or significant fatigue (chip loss)would hopefully give a warning fractionally in advance of chip /temp/pressure warning systems.

Does anyone know of such a system?

The added benefit of such a system is that it may be applied in non magnetic material systems.. as long as they conduct a current. (Ceramics can be made conductive or not as required.. I believe.