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Accident with back-firing 4-stroke

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Q: Greetings Mr. Lee, I am addressing this to you because I know that a lot of folks read your column, you are most knowledgeable on engines. Late  last October I was starting a new 4 cycle, 1.50 engine for the first time. I had a high quality aluminum spinner attached with the standard adapter that I’ve used many times. The engine backfired and the prop, adapter,spinner etc almost exploded off the shaft. It happened so quickly I could not believe what had occurred.
     However there was blood all over the garage floor instantly and my wife was frantic, as she was holding the a/c. I was in the hospital two days as the local surgeon would not touch the wounds. Several ligaments were cut and one bone fractured. I am still not 100% with the  hand.  To shorten the story I now find that Tru Turn makes a double locking adapter that I am sure would have saved me all this mess..  Most shafts as I have discovered are not long enough to take the main & jam nuts that comes with the engine plus the spinner adapter.  Tru Turn’s product is  a double jam nut set up and the jam nut is internally threaded to receive the 10 -32 screw that retains the spinner. This jam nut has a slit cut perpendicular to the shaft that not only jams the main nut but appears to me to squeeze the shaft as it is tightened.  It may be worth while for your readers that use 4 cycles, to ask for this kit. I have no idea how long it has been on the market. A great product IMHO.  Always enjoy your column. 

A: Sure sorry to hear about your accident. I do find it strange that you suffered so much damage to your hand. I have had any number of 4-strokes back fire during starting loosening the prop, but never had the prop fly off with the force yours must have done. I will be running your letter in an upcoming column. I also intend to contact the Tru-Turn people and get one of the adapters you mention for an included picture.

 Thanks for sharing your experience.

Updated: July 8, 2015 — 5:09 PM
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4 Comments

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  1. I have seen 4-strokes toss the prop despite having the special jam nut.

    I haven’t seen anyone get injured when that happened but with the prop flying more than 10 feet it was more a matter of luck as to which way the prop went vs where everyone was standing. (especially the guy in front of the plane who was starting the engine)

    What happens is the prop gets spinning forward fast enough to be dangerous, the engine backfires loosening the nut(s) the prop keeps turning as the engine shaft stops and the prop spins right off and goes in some random direction

    Fortunately the prop nut rarely comes all the way off, but it does happen.

    One of several good reasons to get behind the aircraft as soon as possible after the engine starts.

  2. I had a YS 120 backfire while going to full throttle.The composite prop and spinner took off so fast it left me stunned for a second as to what happened.I then started checking myself for wounds thinking ,Ok what seems to hurt ! I was hit by something on my neck but there was only a smidgen of blood. Then I saw the 3/16 plywood door that hangs down as a little table off my flight box.The prop had cut through three inches of it on its way buy ! WOW! Glade I wasn’t or anyone else in front of it. I think it was debris from my flight box that hit me in the neck.Glad it wasn’t the prop. The prop was still good after I found it about forty feet away.I’ve always make it a habit to stay away from the front of the plane when I rev it up.Good thing that day !

  3. Contrary to their intended use, I now find it hard to trust the two piece jam nuts. I recently purchased a thunder tiger .91 4stk. After initial break in, I retuned my engine and did some last minute pre-flight checks (before maiden flight). I removed spinner and prop to put on my cowling, and then reassembled spinner and prop. I distinctly remember ensuring the nut/jam nut was tight wheras i hadnt used one like this before. I restarted the motor and just as i gave it throttle the prop and spinner went soaring through the air. Ironically, the engine never even backfired. It only dies after the prop and nut came loose. I have yet to retry these jam nut combinations due to my lack of confidence in them. Any thoughts?

  4. I quickly learned with my YS 4-stroke engines to always start them rich and adjust as required. I also learned not to use plastic spinners. A backfire can crack the back plate and you may not notice until the spinner is removed.

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