Reno 2010: Relentless Loses Prop in Flight
Is there something we can learn?
By Patrick Panzera, EAA 555743, firstname.lastname@example.org
Art by Steve Edmundson
You might already have read EAA’s Reno National Championship Air Races report by Tim Kern, but if you didn’t, you might want to read it before continuing.
During a qualifying session on Tuesday, September 14, the propeller departed Relentless (Nemesis NXT, designed and developed by legendary air racer Jon Sharp) with its builder, Kevin Eldredge, at the stick. A 400 mph pull-up exchanged airspeed for altitude, and an uneventful, safe landing was executed. Safe, if you consider a small fire from oil being spewed on a hot turbo as being uneventful. The fire was quickly extinguished by the ground crew at Reno.
According to Eldredge’s Reno 2010 blog, an aluminum oil-return fitting failed. The fitting was necessary to allow the connection of the oil return hose that runs from the oil cooler to the accessory case. Eldredge speculates that the fitting broke off just before the prop ran away. As soon as the prop governor lost oil pressure, rpm could no longer be controlled and the engine “spun to some ridiculous rpm,” according to the blog. Of course Eldredge was flogging his custom-built Lycoming TI0-540 at the time, hoping to beat Reno Air Races National Champion Jon Sharp’s record of 412 mph. The runaway rpm is the probable cause for the rear counterweight breaking off the crank; the sudden and catastrophic stoppage of the engine (the result of the counterweight failure) snapped the prop flange off the crank due to its rotational inertia.
Not many of us will ever race at Reno or push our planes beyond 400 mph, but is there something we can learn from this? Yes, probably.
Are Your AN Fittings Really AN?
Check your oil and fuel lines. Are you running with AN fittings or inexpensive fittings that look like AN hardware? Some might suppose that there isn’t a lot of difference between the fittings sold by hot-rod shops and true AN fittings except maybe the cost, but that’s not entirely true. Rather than go into the details here, I recommend the following website: www.MechanicSupport.com/ArticleStronger.html. The long and short of it is that the less expensive fittings that look like AN may be as much as 40 percent weaker than the real deal, due mostly to the way the threads are cut.
Are Your Hoses Supported and Not Causing Strain on the Fittings?
“Weaker” is an ambiguous term at best. How strong does a fitting really need to be? Thousands of us are using fittings from speed shops and not aircraft parts suppliers, and we aren’t seeing a rash of failures. I’m not trying to imply that a cheap fitting was the cause of the Relentless failure, but rather I’m taking this opportunity to point out two very important things we all need to know about the fittings we use. The first is to know what you have. (Is it AN or a replica?) And the second is to see that the hoses connected to them are supported properly.
Threads can act as stress risers, and if one end of the fitting is securely affixed and the other end is free to vibrate, it’s just a matter of time before the aluminum begins to fatigue and crack. How much time? I don’t know, but I would assume that with all else equal, the “weaker” fittings will fail before true AN fittings, especially if they’re damaged or otherwise weakened by overtightening.
The illustration below is very similar to what a lot of us have in our planes – an AN (type) fitting threaded into a stationary part, be it oil filter, oil cooler, firewall bung, carburetor, or one of any number of locations on the engine.
Onto this fitting we have a hose connected. The hose has weight (mass) and is capable of moving and imposing live loads on the fitting if the hose isn’t properly secured. Additionally, it’s entirely possible that one such hose could reach its resonant frequency and the forces will be multiplied. These loads can cause fatigue on the part, which can eventually lead to failure. How many hours will it take for an unsupported hose fitting to fail? Maybe more than the life of the engine/airframe combination, maybe only a few. I personally don’t want to find out the hard way, and I don’t want anyone else to find it out that way either.
So let’s err on the side of caution and make sure our hoses are secured, ideally as close to the fitting as practical.