Gear Box Failure @ 43 Hours

Discussion in 'General Auto Conversion Discussion' started by TXFlyGuy, Oct 31, 2019.

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  1. Oct 31, 2019 #1

    TXFlyGuy

    TXFlyGuy

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  2. Oct 31, 2019 #2

    Vigilant1

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    The way I read the report, the gears were not hardened properly by the manufacturer. A similar PSRU by the same folks failed in NZ.
     
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  3. Nov 1, 2019 #3

    TXFlyGuy

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    The NZ failure was not the fault of the PSRU. The owner installed a new, untested and very heavy propeller. This was against the advice of Titan, and Autoflight New Zealand. Naturally, the gearbox failed due to being way overloaded from the prop. The aircraft nosed in from about 40' AGL. The pilot, who walked away from the wreckage, stated that he thought he was dead.
     
  4. Nov 1, 2019 #4

    Vigilant1

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    Did Titan assist with that investigation, or did they fail to return calls from investigators, as the NTSB reported in this more recent crash?
     
  5. Nov 1, 2019 #5

    TXFlyGuy

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    That is a question for the lawyers who represent the parties involved.
     
  6. Nov 1, 2019 #6

    pictsidhe

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    Tex, your fanboy spin-talk is really offputting for anyone who might have been thinking about a Titan. Enough of it is plainly total bullpoop to make anyone wonder about the rest.

    You just posted the link to an accident report from an unbiased and highly professional source, then claimed it was wrong according to rumour. After that, you tell people to get details from lawyers?

    While even the best companies make honest mistakes, nobody likes doing business with eels who won't 'fess up to their errors.
     
  7. Nov 1, 2019 #7

    TXFlyGuy

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    Most find the anti-Titan bias on this forum off-putting...to use your terms.

    There is no legal responsibility to respond to the NTSB. They are not the FAA! What would your lawyer tell you to do in this case? Titan did not build the gearbox, nor the airplane.

    This info was posted in hopes of getting some input, even hypothetical, on why the gears failed prematurely.

    But the usual suspects have reared their collective heads in turning most everything into a Titan bashing thread.

    I pity those of you with nothing better to do.

    Parking brake set.
     
  8. Nov 1, 2019 #8

    wsimpso1

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    The gears may have been soft or had other defects, but that is really tough to tell on these failed parts... As a 23 year experienced transmission engineer, I will give my assessment.

    The dark color of the steel, even on the worn way areas indicates temperatures of 600 F or higher on both the input gear and the mating idler. These levels of temperature will reduce the hardness of heat treated steels.

    Review of the materials lab report shows (in every case examined) that the hardness goes from base hardness to a higher hardness through the case, then drops off in hardness as the surface is approached. This consistent reduction in hardness of the case as the measurements approach the surfaces is surface annealing and is another strong indication that the parts saw high temperatures for some period of time prior to failure.

    Other evidence of high temperatures in the gearbox are the melting and/or combustion and removal of polymer components in the various bearings. Additional evidence of high heat is dark color of the oil, although that could also be due to physical contamination of the oil.

    No mention was made of any measurement taken of the oil. Several types of failure analysis could have been taken. First is to filter the solids from the oil, identify materials and colors of the solids, and perform spectragraphic analysis of the materials. A big clue will be if steel fragments have bright surfaces or are entirely covered in black oxide. If all steel fragments show heat distress prior to being chipped from the gears, this would indicate gross overheat of the gearbox. If there are some bright (not heat distressed) fragments, a lot of information can be obtained from those, including indications of original part heat treat, surface condition, and initial distress of the surfaces. Depending upon fragment sizes and heat exposure, this may allow verification of steel alloy content and hardness as installed and thermal history.

    Polymer solids and the filtered oil could also have been tested (DSC is a great tool for this) and compared to same materials as new (taken from new bearings and oil). The temperature ranges reached can be identified by comparison of the new part and used part results.

    The oil of the gearbox is shared with the hydraulic propellor and we must presume with its governor. I saw no mention of the condition of the prop, the governor, its internal valving and pump. Was the governor contaminated or did it have problems that lead to the failed gears?

    No mention was made of investigation of oil paths within the PSRU for flow blockage or open circuits that are not supposed to be open. Was the gearbox cooled and if so, what in condition was the cooler circuit? No mention was made of any filter and its condition. Was one included and what was its condition?

    I would have difficulty concluding from the evidence that the gears were inadequately heat treated as there is substantial evidence that they may have been run at substantial overheat which will tend to anneal the surfaces and lead to the failures.

    In sum, the NTSB has demonstrated that the gear hardness after failure appear to be somewhat low (HRC 48-54 is hardly butter - many gearsets in our world run just fine at these levels), but got no where close to determining why the gear surfaces were in that condition while lots of evidence does exist indicating substantial over heat. Root cause was not obtained. I am disappointed in the depth of the analysis, and the lack of an overall drive for root cause.

    Billski
     
    Last edited: Nov 1, 2019
  9. Nov 1, 2019 #9

    TXFlyGuy

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    Billski - Thank you very much for your insight, and professional analysis of this accident / PSRU.
     
  10. Nov 1, 2019 #10

    pfarber

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    Is that site quoting the NTSB report?

    I read this :

    .. lost thrust to the propeller ..

    You don't supply the propeller with thrust, or is this a new thing I've never heard of??
     
  11. Nov 1, 2019 #11

    TFF

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    Lawyers and the NTSB. The NTSB are not the FAA. They are federal marshals, they are essentially judicial for transportation. FAA is data gathering. NTSB puts the data into legal order. The interesting thing is NTSB has no enforcement to the industry only recommendation. Enforcement is for the FAA. When it comes to an individual involved in a case it’s different. Say you will not answer without a lawyer, they bring 40. Will not testify, they can put you in jail with no due process. It’s like not obeying a judge in their court. Will they go that far for one dinky homebuilt? No. If no one crashes an airliner and a bunch of are standing around the water cooler, they will add that talent on to your case.
    They probably spent $100,000 on a little tab that broke on my old company’s aircraft and it would have been worse if we were not transparent and willing to help. No one was hurt in the accident. What really closed it was someone getting killed in a similar type unrelated to us and problem, and they moved all the force to that and said wrap ours up.
     
  12. Nov 2, 2019 #12

    Aerowerx

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    Has anyone gone to the NTSB website and read the actual report? It is best to get it straight from the original source.

    Since it has only been about 2 weeks since the accident, I would hazard a guess that the quoted report is a preliminary report.

    By the way, you can learn a lot about how to, or not how to, fly a small plane from browsing the NTSB reports. I highly recommend it.
     
  13. Nov 2, 2019 #13

    wsimpso1

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    Yes. There were picks to it on the posted page. I would not hold me breath on more analysis -this is a homebuilt crash and nobody died.
     
  14. Nov 2, 2019 #14

    pfarber

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    Its amazing how many people crash/die because of fuel mismanagement.
     
  15. Nov 2, 2019 #15

    Wanttaja

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    Depends on your "amazing" threshold. It's actually a pretty small percentage. From my 1998-2017 homebuilt accident database, 1.4% were related to managing the fuel onboard (e.g., there WAS fuel, but the pilot didn't get it to the engine). That puts it about about 12th in my list of 25 potential categories. Fatal accidents is about the same.

    About 3.4% of all the homebuilt accidents were due to fuel exhaustion (no fuel left). That puts it at about eighth of my 25 categories.

    Combining the two moves them up to the fourth position. However, eight times as many homebuilt accidents were due to pilot stick-and-rudder errors vs. any combination of fuel management issues.

    Ron Wanttaja
     
  16. Nov 2, 2019 #16

    TXFlyGuy

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    Another big cause, at least in airplanes that have gear reduction, is PSRU failure.

    One of the more notable ones was Terry Kronk, in Australia. Custom built Chevy, around 600hp. Terry had built an 80% scale P-51, maybe one of the most beautiful planes ever. Even nicer than a Stewart.
    He was on his way to an airshow, took off and suffered an immediate gearbox failure. His friends who witnessed the crash could not get to Terry in time due to the heat and flames that engulfed the cockpit.



     
    Last edited: Nov 2, 2019
  17. Nov 2, 2019 #17

    Wanttaja

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    Depends, again, on what your definition of "big" is. ("that's what SHE said....")

    I've got 544 auto-engined accidents in my database, of which 327 are NOT Corvairs, VWs, Aerovees, or any other traditionally-direct-drive conversions. Of those 327, there are 15 accidents involving the PSRU, or about 4.5% of the accidents. 37% of the accidents involving these airplanes were due to the pilot's stick-and-rudder skills.

    I'm all for work to increase reliability of these systems, but let's not lose the fact that majority of accidents are due to pilot mistakes, not the hardware.

    Ron Wanttaja
     
  18. Nov 2, 2019 #18

    Aerowerx

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    I was just going to say that the majority of the accidents could be classified in one category: "Stupid"
     
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  19. Nov 2, 2019 #19

    BBerson

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    There aren't very many PSRU aircraft. The question is the percent rate of accidents of PSRU aircraft compared with direct drive.
     
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  20. Nov 3, 2019 #20

    Winginitt

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    I think I would be asking what kind of inspection/certification (not to be confused with airplane certification) the company producing.....or at least the heat treat vendor provides. I would also request that a "loaner" set of gears be provided (preferably from the same batch as yours) so you can have an independent company verify the mechanical properties of the (or your) gears. Then I would think about installing a temperature and an oil level sensor in your reduction drive along with an "idiot light" in your cockpit. Without researching anything, it does seem that many gear drive reduction units have the same failure indicators.....oil level and temperature.

    Edit: maybe even a "sight glass" like air compressors have so you can see the oil level during preflight.
     
    Last edited: Nov 3, 2019

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