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air safety

Apr 13, 2017

REX had no chance of detecting fault in prop that flew off

ATSB finds hitherto unknown dangerous part failure in REX flight that suddenly lost a prop

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REX minus prop at SYD, photo Grahame Hutchinson

Regional airline REX appears was blindsided by a previously unknown flaw in an engine gearbox when a propeller came off one of its SAAB 340 turbo-prop aircraft while it was approaching Sydney Airport with 19 people on board on March 17.

None of the maintenance requirements REX was following in servicing a particular sub-set of its SAAB 340 fleet even required it to look at the component that failed, according to a preliminary report just released by the ATSB.

But that is about to change, with the ATSB warning similarly at-risk-airlines world wide of the fault.

The propeller that came off the aircraft when it was over the Macarthur area not only missed hitting vital control surfaces on the wing or tail of the small commuter aircraft, but then crashed to earth near houses when it came down in a bushland reserve in the Revesby area.

The ATSB says that the cracks and corrosion that were apparent in photos of the aircraft after it landed safely at Sydney Airport started within the so-called mounting flange of the propeller’s gear box, before spreading to a shaft section.

The propeller shaft had then fractured, leading to the separation of the propeller.

It found corrosion and pitting in a dowel pin bore in the engine, and describes the process in considerable detail by which this then led to the ultimate breaking free of the propeller.

The ATSB report says this is the first known critical failure of this type initiating within the propeller hub flange of a GE Aviation CT7-9B engine. It points out that the same propeller gear box is fitted to the widely used CASA CN-235 utility turbo-prop. It warns that any corrosion or cracking within the bore may go undetected until it progresses to the surface of the flange.

The safety investigator has outlined extensive additional work that it will have to conduct before it can issue a final report.

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14 comments

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14 thoughts on “REX had no chance of detecting fault in prop that flew off

  1. comet

    The Saab 340s were all made in the 1980s and 90s.

    It’s amazing that any piece of metal, especially a shaft that transfers the power from the engine, doesn’t need any inspection over a 20 or 30 year period.

  2. Dan Dair

    This is a component of the engine assembly, rather than being airframe,
    so as such, it’s not directly to do with SAAB the aircrafts’ manufacturers.
    It will be pretty amazing if it turns out that neither the aircrafts manufacturer, the engine maker nor the sub-contractor (if it’s not made by GE) who built the gearboxes have ever thought to issue any kind of inspection protocol for this part.?
    You’d think that they’d at least look at it closely on a D-check.?
    This seems to be taking the term; ‘fit & forget it’ to extremes.?

    It was only mentioned earlier today, that the reduction-gearbox on the new P&W GTF engine is ‘sealed-for-life’, but even then, you’d expect that P&W issue instructions as to how long, in Km’s, flying-hours or cycles, that life actually is.?

  3. Tango

    Gentlemen:
    How many of you have inspected the axle of your vehicle?
    Some parts are simply never intended to be intended as they are not ever intended to fail. Housings would be among them.

    The term is life limited parts, solid pieces of metal are not among those and for good reason.

    There was a book, To Engineer is Human.

    There are truly the unexpected failures.

    What we will probably find is that either the factory used a process that failed that was not eer expect to, or someone along the way used an unapproved sealant that propagated into corrosions.

    As this is a once off event, I suspect a once off shop issue.

    I would suspect its a local Australian shop that did not to anything they knew to be wrong but was.

    If you want all risk eliminated then be prepared to pay the price, and it will only cost you more money, not all risk can be eliminated.

    Be grateful it departed gracefully and the got the plane down with passengers safe and we have a chance to correct this.

    Fate is the Hunter, he never rests, but sometimes Fate turns out for the good as well.

    I have spoken, all fall to their knees.

    1. Dan Dair

      Tango,
      I have fallen to my knees,
      in despair.!
      Life limited parts are anything which have a manufacturer useful life limit put upon them.
      The fact that this is a ‘solid’ piece of metal is completely irrelevant.
      It is a section of the power-transfer shaft from the reduction gearbox to the propeller.
      By definition, this is a critical piece of equipment. The fact that there is no inspection program at all for this part is very, very surprising.
      To put this in context, you would inspect a solid leg of a landing strut & they only get used twice per cycle, the propeller shaft assembly is used all the time the engine is running but this part doesn’t even warrant being looked at in a D-check.?

      1. comet

        Well, the ATSB directive has gone out. They’ll all be checked now.

        1. Dan Dair

          I fully accept that I’m being wise after the event,
          but I can scarcely describe my amazement, that these parts weren’t either examined at the D-check or examined as part of a major engine overhaul.?
          How can any part of an aircraft not be subject to some form of inspection when it’s get to be as old as these SAAB’s are.?

          I don’t know if I’m criticising SAAB directly, or the collective engine/gearbox/propeller manufacturers.?
          But it strikes me as the blindingly obvious that by the time an aircraft gets towards the end of its ‘accepted-life’, extra scrutiny should be given to everything and even more so when the manufacturer agrees to extend the ‘useful-life’ of the airframe.

          This isn’t about some roughneck-Australian outfit with a ‘she’ll be right’ culture.
          This is about a worldwide failure to provide a proper inspection program to ‘legitimate’ operators of this aircraft type & others which utilise the same failed part.

          I genuinely think it’s a worry.
          What else on any aircraft isn’t being inspected because “it’s never failed before, so we never thought to initiate an inspection protocol”.?

  4. Tango

    Dan D: You don’t seem to get that they don’t totally disassembly every nut and bolt in the airframe and engine and assemble it again during the checks.

    If they did so, you would never be able to afford to fly as the ticket prices would reflect that and it would be even higher than it is.

    Some item are inspected, some replaced, some replaced on condition. Those that have been assessed as never going to fail are not.

    Your mechanic is not going to inspect your axle unless there has been a bulletin out that the axle has issues.

    Gearbox (where this occurred) teardown is usually on indicator of wear. If there are known wear out parts they will have an hours to be replaced but often there are not.

    Do you tear down your cars gearbox every 100K?

    And while checks and inspections seem to comfort tyou, th4e worst air4crat are the ones that just came back out of D checks.

    Why? Because a great deal has been disturbed and much of the fine tuning is gone wrong now and you have to start over (ask any aircraft mechanic)

    Tear down and assembly has a risk of its own that its not done righty.

    My dad dies as a direct result of a failed repair. He and his friend also put themselves in a high risk situation where nothing could go wrong or they would die.

    An DC10 or L1011 almost went down as a result of a mechanic doing an inspection and putting oil plugs back in wrong. The good news was it was the two ground access engines and not the tail engine and they were able to dump fuel and make it back on one engine.

    I am willing to bet you think nothing of getting in your car and driving someplace.

    But then the this happens and the balance shifts, how could they?

    Which is safer? Driving or Air travel?

    What it does amount to is (for the most part) mfg use their best experience and judgment on this and they issue the inspection, maint or watch lists accordingly .

    Tires and hot engine sections wear out at a known intervals. Gearboxes can be life rated (short of warning indicators)

    Sometimes it can slip through.

    If it was indeed a locally induced situation, you would have them inspecting every engine like that in the world on every major inspection. And that’s a huge added cost for a one off event.

    An old adage, if you try to watch everything you wind up watching nothing.

    There is a balance and a cost benefit or its vastly too costly period.

    I understand the risk of driving. When we got our last car, it was one that had more airbags than required at the time. That and defensive driving are what I can do to mitigate the risk.

    I drove motorcycles for many years, I only did it out of town. I also wore full leathers, helmet, gloves and heavy boots. Again that was risk mitigation, but it did not mean it was even close to risk free.

    1. Dan Dair

      Tango,
      Clearly air travel is safer than car per passenger kilometer.
      However, I’d be much more interested to know what percentage of car accidents result in death & the same figure for air accidents. I’d be surprised if the accident to death ration is not noticeably higher for aircraft in Australia & the US.?

      I do understand your point about what can be economically checked;
      but these unchecked parts have done hundreds of thousands, possibly millions of kilometers, thousands of cycles over twenty-plus years & have never, ever been properly looked at since the day they were fitted. Surely we can justify checking them at least once in their operational-life.?
      These are not static parts, but working under high twisting and compression / expansion loading everytime the engine is in operation.

      I do check my car before I take a long trip.
      It’s also regularly tested for the integrity of it’s parts at it’s annual service at the garage.
      If they find defective components, they’re replaced.
      They don’t routinely dismantle everything, but they would do a deeper check if they discover a problem…..
      & of course, if I have car trouble as I’m travelling, I can just pull over & stop;
      self-evidently, that’s not the case for an airliner.!

      A program of checking these otherwise-ignored parts seems to me to be an obviously sensible plan.
      Identify what doesn’t get looked at & then set a timetable for checking them. Some at the D-check, when the aircraft is closest to a kit-of-parts
      & other parts when the ‘normal’ program of inspection & maintenance check makes them most accessible of most convenient. (or perhaps least inconvenient.?)

      I honestly can’t accept that a propeller literally, just falling-off an airliner in-flight can be considered an acceptable-risk.
      If the prop had come-off towards the fuselage instead of away from it, this would have very likely resulted in the loss of the aircraft & probably all lives on-board.
      Surely that can’t be considered as ‘just one of those things’.?

      1. Tango

        Dan D: There is a difference between acceptable and realistic.

        Again, that part has been assessed and its been asses as NON LIFE LIMITED. In other words, the universe will end before it breaks. It is not realistic to do an inspection of that part.

        In order to do so, you have to break the casing apart. That in turn leads to the issue of putting it back together for a non issue item.

        It is of course casually inspected while a mechanic is in doing maint, various other checks, but its not going to be an item of intense inspection or teardown as that has been covered.

        https://www.flightglobal.com/news/articles/air-canada-a330-takes-off-after-losing-wheel-during-436310/

        So, do we start inspecting all A330s in the world or do we find out what the root cause (someone left the nut off?) ?

        In both cases we put out an alert due to a totally unforeseen incident AND we follow up with the investigation that caused those parts to come off.

        From history we can clearly say the gear box cover does not corrode spreading to the shaft, nor does a wheel fall off a A330 (which could also be a broken shaft though I doubt it)

        In the US we average something like 35,000 traffic deaths a year.

        We usually have NO airlines deaths and a handful of them form general aviation (call it a hundred).

        Obviously something is wrong with automobiles and how they are operated.

        That is where the real return is.

        Again you miss the concept. The engine casing has been assessed. Its been quality made process, it may have been magnafluxed as part of the process.

        Something totally abnormal happened.

        It is most likely something imposed on it by unknown factors.

        Rather than inspect the wheels of each A330 each time it is going to take off you have a quality control program in place that not only deals with that it deal with all the others aspects of things you take apart on an aircraft to maintain or repair it.

        1. Dan Dair

          Tango,
          I think it’s you that’s missing the point…..?

          The bit that broke on the REx SAAB wasn’t a casing, but a flange.
          The flange is part of the drive-train between the gearbox & the propeller.
          I understand your point that the universe will end before the part breaks (although clearly that isn’t so), but equally, that assessment is based upon the original lifespan of the airframe.
          Also, it isn’t yet clear to me whether this is in fact an engine component (as opposed to airframe) & consequently, it may be that the engine manufacturer & not SAAB are the ones who have been lax in promoting inspections of components beyond the original lifetime of the airframe.?

          The fact is, that A330 wheels, landing struts & axles are all thoroughly visually examined as part of every D-check. They are no-doubt routinely examined at all other major checks and will be given a cursory examination before each take-off.
          Despite all that, a wheel still fell-off.!
          OK, that was probably human error.?
          You can’t fix that, stuff happens because people aren’t perfect.

          My point is that the stuff you can fix, you should be fixing.
          If SAAB are saying to the ATSB; “we had an extended-life examination protocol for this, but it was low on the schedule-list, because we didn’t believe it would fail”, I would find that acceptable.
          If SAAB or the engine makers are saying; “The universe will end before the part breaks, so we don’t need to inspect it, Even after it’s been flying around unexamined for 20+ years”, under the circumstances, they’re probably going to have a bit of a problem convincing anybody of their competence.?

          1. Dan Dair

            Just to add,
            If the prop had come-off at full-speed, rather than whilst being feathered &/or come -off in a different direction & then caused the aircraft to crash, presumably with the loss of all onboard,
            would you still be as relaxed about the incident as you appear.?

            IMO the consequence of such a crash would by now, have resulted in the worldwide grounding of all SAAB’s & anything else which use the same propeller flange setup.?
            Since it’s all REx fly, they’d be in a bit of a sticky spot for the time-being.

    1. Tango

      Dan D:
      You mix up the term relaxed to realistic.

      We could follow your approach and air travel would be cost prohibitive. Each item and action costs. Those add up.

      I do find it puzzling on a laser focus on this sort of thing when there have to be at least 1000 car deaths a year in Australia and how many deaths caused by hospitals? There truly small difference would have huge returns.

      1. Dan Dair

        Tango,
        You’re comparing apples & oranges.?

        If you get in your car & have an accident, it’s probably your fault unless it’s the other guys.?
        When you’re a passenger on a plane, if you have a crash, it’s extremely unlikely to be anything to do with you.?
        Consequently, everyone is looking around for who to blame.

        If you crash your bus or train it crashes. It doesn’t fall thousands of feet first, before it crashes.

        If you check your aircraft over properly, the preventable won’t happen. If the thing that fails has been attached to your aircraft since the day it left the factory & no-one has ever, ever looked at it, you can only point an accusing finger at the manufacturer & operator.
        Sure as eggs-is-eggs, it’s not the fault of the passenger.
        & there’s no point in saying ‘shit happens’, because that fall into the category of the UNpreventable. Something that COULD have been examined at any time during it’s 20+ years of service, but wasn’t, can hardly be described as unpreventable.

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