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Vacuum pump failure modes

JKevin

Well Known Member
While flying VFR in seriously clear conditions last week I experienced an abrupt vacuum pump failure (Rapco dry pump). The flight was purely visual, I was not using the DG to navigate nor was I using the AI. Just noticed the low vacuum light in my scan, and then saw that the vacuum gauge was out of the green.

I had read that vac pump failures were often not noticed at first as it took time for the gyros to spin down. I can verify that this is correct. It was probably 5 minutes before the DG became sluggish, and shortly thereafter the AI slowly started indicating a slow descending left turn. I can see how this could occur so slowly as to not be noticed if you were in the soup!

Thanks to the generosity of a VAF member, I have a replacement pump installed and everything is working again.

I just had to take the old pump apart to see what went wrong with it. I was expecting to see broken vanes (pump has about 700 hours on it) but they looked to be in good shape and not excessively worn. What had failed was the coupling (plastic / nylon)--just sheared in two parts.

Is this a normal failure mode? My filter was replaced last condition inspection and the gyros are relatively new so I don't think the load on the pump was excessive.

This is really an intellectual exercise as much as anything. I am taking the plane (RV-6, 2001) down this winter for upgrade to glass panel upgrade and the vacuum pump will go at that time.
 
I had a Rapco vac pump fail in exactly the same way (coupling failed) at exactly the same number of hours (700 hrs). I also had replaced the intake filter a couple of times while the vacuum system was in use.

My fix was similar to what you are planning. Dumped the vacuum system and the gyros. Installed a uAvionix AV-30E and use it in attitude indicator mode. I'm not ready to jump into full glass panel mode.

So, yea, I'd say your failure mode was normal.

BTW, the cheapest "fix" it to replace the coupling: https://www.aircraftspruce.com/catalog/inpages/coupling07-03568.php?clickkey=30845
But then you still have a vacuum system.............
 
Normal failure mode. The coupling will typically show it is nearing failure when visually inspected during the condition inspections.
 
You can buy the coupling on its own for less than 20$. I have one on hand, as I had a couple (different engines) fail that way. Since being prepared - no more failures. Ops, I might regret saying that!
 
It is technically a Shear Coupling. It is designed of plastic or composites to allow it to shear in half and save your engine gears if the pump ever seized up. Unfortunately heat and time are enemies of plastic and they eventually become brittle and fail.

Replacements are inexpensive.
 
Based on conversations I have had in the past with the overhaul techs at Aero Accessories, the most common cause of coupler failure is an increase in internal friction.
The increase in friction is caused by the surface of the concentric bore in the pump body becoming wash boarded / rippled from wear induced by the carbon vanes.
When vacuum pumps get overhauled, the bore gets lightly machined to make it smooth again.
If a failed coupling is replaced without doing anything else to the pump, it usually fails in a pretty short amount of time.
 
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