When you find your oil pressure much lower than it should be and are stuck far, far  away from home, there is NO better place to have it happen than at Shady Bend Airpark.

  

 

This was when I found that the key on my oil pump sprocket had not been inserted properly when I overhauled the 91 Turbo block and was found lying on the bottom of the oil pan – after 10 hours of flight time!!!!

 

Fortunately, unintentionally leaving my throttle wide open the evening  when we  examined  my new throttle body mount innards - combined with low temperatures the next morning and the 20-50 W oil in the engine, caused the engine to zoom to 5000 rpm almost instantaneously when the engine was started the next morning.  The sudden  load of the cold, heavy-weight oil apparently was sufficient to overcome the clamping pressure of the nut on the oil shaft and the chain sprocket proceeded to slowly began to rotate on the oil pump shaft (rather than rotating the shaft {:>)) 

 

We were preparing to take off (Tracy, Finn and I) from Shady Bend, there was only a light wind that morning, but being the wimp  I am about such things - I insisted on taxing down the length of the runway in order to take off into the wind (Tracy and Finn and already taken off down wind).  However, on the 3000 ft taxi to the take off end, I notice my oil pressure which is normally up around 50 psi at those rpm was only reading around 20-25psi.  Sensor! I thought, possibly the gauge – no problem! -  I mean you flew down here from NC with no problem.  But, could not bring myself to take off with the oil reading what it was.

 

So taxied back to Tracy’s hangar where  trouble shooting cleared the sensor and gauge leaving no choice but to tear down the engine – while walking over to the parts washer with the oil pan , I heard a metallic clanking in the pan and as soon as I had the piece of metal in my hand knew immediately what my oil pressure problem was.  The key was not in the oil pump shaft keyway.    Tracy donated a used oil pump enabled me to fix it there and make it home.

 

  Inspection showed that after the sudden engine RPM increase, the nut apparently lost its clamping grip and the sprocket gear had started to turn on the oil shaft – sufficient that the shoulder of the oil shaft near the pump housing had begun to carve a grove into the sprocket gear.  This resulted in lower oil pressure and would only been a matter of a minute of two at higher rpm and the oil pressure would have gone to zero – like maybe at 500 ft {:>(

 

It appears that when I pushed the sprocket gear on the shaft, I had shoved the key out of the keyway.  Normally, I would have hear it hit the oil pan or floor if it fell out, but an indention in the

Aluminum oil pump housing showed that apparently when I pushed the gear on the shaft and pushed the key out – I trapped it between the sprocket and pump housing.  When I torqued the nut down, it put sufficient pressure on the trapped key that it indented the aluminum oil pump housing and was trapped there.  Therefore no “clink” of a key fall on the floor.

 

Undoubtedly shortly after I cranked or rotated the engine it must have fallen – but, unable to hear it at that point.

 

 

So a Shady Bend Break Down is not a bad place to have it happen – but best not to have it happened.  Later discussion with Bruce Turrentine indicated this is not exactly a rare event, so check and double check that key is in there if/when you rebuild your engine.

 

Ed

 

Ed Anderson

Rv-6A N494BW Rotary Powered

Matthews, NC

eanderson@carolina.rr.com

http://www.andersonee.com

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http://members.cox.net/rogersda/rotary/configs.htm#N494BW

http://www.rotaryaviation.com/Rotorhead%20Truth.htm