The Joys of Diligent Maintenance
(This is an addendum to the original article which appeared in the March/April 2021 issue of Sport Aerobatics.)
by Tom Myers, IAC 16830
Let’s say a part in your airplane wears out. Where would you prefer to be when you become aware of the fact? At a contest? In the sky? How about on the ground at your home airport? When would you prefer to become aware of the fact? In the middle of a contest? How about at a time when you have the opportunity to properly deal with it? Given the fact that the part is no longer airworthy, the question is really whether you would prefer to find out about it at a time and place of your own choosing, or the part’s choosing?
It is unusual for parts to just up and wear out suddenly with no warning whatsoever. There are often warning signs. I often tell people that a clean airplane is a happy airplane. What I am really saying is that if you really want to discover warning signs, clean your airplane often, and pay attention to what you see while you do it. Warning signs may be subtle and seem inconsequential. The problem is that inconsequential now does not mean inconsequential later. My point is that the time to take care of maintenance problems is when they are inconsequential, not when they are consequential.
This means having the discipline and diligence to chase down warning signs to their root causes when they are subtle and the trail of clues is difficult to follow. Ignoring warning signs because they are difficult and inconvenient to diagnose is a great method of once again proving the accuracy of The Law of Unintended Consequence, aka, Murphy’s Law.
There are plenty of times when the best tools for chasing down such problems are your eyes, your brain, your time, your attention, a little elbow grease, and the desire to keep working until the problems are truly solved. I will illustrate with a recent example.
Over the course of several practice flights this spring, I started to notice a very fine widely scattered spray of tiny oil drops on my canopy while cleaning the plane afterward. I inspected possible engine and propeller sources after every flight, but there was so little oil involved, there wasn’t much evidence to follow. I kept a careful eye out for oil drips or streaks. I monitored the spray pattern, particularly looking for increases in the accumulated amount of oil.
Recently, the amount of accumulated oil spray, though still very small, increased noticeably. Once I got the plane into the hangar, I pulled off the cowling and spinner, and performed a very thorough cleaning and careful inspection of the entire engine and prop. Once again, no source was found.
After the subsequent practice flight, the amount of accumulated oil spray was still attention-getting. Instead of pulling the plane into the hangar, I started my inspection out on the ramp under the bright mid-day summer sun. My hope was that the additional light would make it easier to see even subtle oil streaks. This time, I was successful. I discovered the absolute barest wisp of an oil streak on one of my propeller blades just outboard of the counterweight clamp. There is no oil inside of this part of a propeller blade, so I removed the spinner and went hunting for the source.
The propeller blade roots were all clean and dry. The propeller hub was clean and dry. The propeller dome was clean and dry. There was no leak source of oil anywhere where there could be internal oil. There was just a fine streak of oil in a location with no oil source. I stood there staring at the prop hub thinking about what I could possibly be missing. It was at that moment that I realized that I was staring right at source. Or as it turned out, sources.
Variable pitch prop hubs have fittings for periodically replenishing internal lubrication. When the internal lubrication is grease, Zerk fittings are commonly used. A Zerk fitting contains a ball bearing and a spring. A grease gun is connected to the Zerk fitting, and the grease in the gun is pressurized. The pressure of grease pushes the ball bearing into the Zerk fitting and allows grease to flow into the propeller hub. When the pressure is removed, the spring pushes the ball bearing back into place, thus sealing the fitting and preventing grease from leaking back out. See the accompanying photograph. A small portion of the internal ball bearing can be seen at the very top of the fitting.
Grease is a mixture of solid and liquid constituents. A spinning prop can act like a centrifuge, and separate the liquids from the solids. If a Zerk fitting ball bearing does not form a proper seal, the centrifugal forces can liberate the grease liquids from a prop hub through that grease fitting.
When I pulled the little plastic protective cover off of the Zerk fitting closest to the prop blade with the oil streak, I discovered that it contained grease liquids. When I pulled the plastic protective covers off of the other five Zerk fittings, I discovered that they too contained grease liquids, but just not as much.
In hindsight, I believe that all six Zerk fittings had been seeping for a while, but the amount of liquid lost was too small to be noticed. It was only when the leak rate of one of the Zerk fittings became noticeable that the problem was discovered.
About five years ago during the first annual inspection of the plane, it occurred to me during the process of greasing the prop that I would not want to see the plane grounded for something like an issue with a Zerk fitting. Since Zerk fittings are inexpensive, I bought a full set of six spares, and stashed them away for just-in-case. Low and behold, five years later, I had a set of six brand new Zerk fittings sitting there on the shelf ready to install. Ear-to-ear grin ensued. Onto the prop went the Zerk fittings, and I eagerly looked forward to my upcoming flight with a grease-free canopy.
Except that it wasn’t. There was less grease on the canopy to be sure, but not no grease. Off came the spinner. Off came the Zerk fitting covers. Sure enough, all six covers had grease liquids in them. Yes, at $1 each, Zerk fittings are really inexpensive. As usual, you get what you pay for. Or, in this case, you don’t get what you don’t pay for. Time to find and pay for something that does work.
Internally, my propeller hub is divided into three separate cavities with regard to lubrication. Each cavity has two grease ports. When lubricating the hub, one Zerk fitting is temporarily removed from each cavity to provide pressure relief. Grease is pumped into each cavity’s remaining Zerk fitting. The removed Zerk fitting is then reinstalled.
The section on lubrication in the propeller maintenance manual discusses two options for the grease port fittings. One option is a Zerk fitting. The other option is a solid hole plug. Aha! I could purchase and install six solid hole plugs in the hub for flying. For lubrication, I would remove all six hole plugs and temporarily install three Zerk fittings, one for each internal hub cavity. I would then lubricate the hub, remove the three Zerk fittings, and reinstall the six solid hole plugs.
I am very happy to report that since changing over to the six solid hole plugs, my canopy is back to only requiring the removal of dead bugs after flights. So far, this spans about seven flight hours, and includes one contest.
Oh, the joys of diligent maintenance.
Fly safe.
