I'm experiencing an odd oil pressure issue with my Lycoming GO435-powered Twin Bonanza. Oil pressure specs are 65-85 psi for normal operating range. Left engine oil pressure is fine on start (mid-90s on cold start, quickly comes down to mid-80s), good on runup (high 70s to low 80s depending on temps). The issue starts when I push the power up for takeoff.
My engines go to 3400 RPM on take-off thanks to the gearbox on the front. Oil pressure comes up steadily as power increases, to around 80psi, then after a few seconds at full power the oil pressure on the left engine starts to drop (the right holds steady). Depending on temps, at 3400 RPM the oil pressure on the left engine will steadily drop to the low 60s and then stabilize. If the oil is hot, it will drop to the mid-50s. It always stabilizes, but it's below the green arc. When I reduce the props from 3400rpm to 3100 rpm (max continuous), the oil pressure RISES a couple of PSI. Then when I reduce the props again to 2750 for cruise, the pressure rises again a couple of PSI. If I reduce throttle further to 2500 rpm, it'll tick up another PSI or two.
Once it's all settled, in cruise the left oil pressure at 2750 is high 50s to low/mid 60s, depending on oil temps. It'll hold steady there for hours, with only the normal slight variations as oil temp changes. That engine tends to run 150-160* oil temp in cruise (about 10-15* cooler than my right engine). I run Phillips XC oil.
It gets even more interesting when I start to descend.
When I reduce throttle (not RPM) and start down, the oil pressure will start to steadily rise a little, even if I leave the RPM alone. Reduction from say 22" MP to 17" MP, with no change in RPM, will result in a 1-2psi increase in oil pressure. As I continue to descend, leaving RPM alone (but reducing throttle and increasing airspeed), the oil pressure will keep slowly ticking up a couple of PSI, even with no change in temperature. By the time I'm on final, the oil pressure will be in the low 70s. Landing and taxi in, it's in the mid to high 70s.
Based on all of the above, it appears that the oil pressure is changing based on prop pitch (not necessarily RPM only).
Now note that my props are a little odd. I've got original Beech 214 props, which utilize a rear mounted governor with oil lines feeding a prop transmission mounted to my gearbox. That transmission uses external gears on the transmission and prop to adjust prop pitch (and has an electric feathering motor). It does not have any oil in the prop hub. And oil is only fed to the transmission by the governor when a pitch change is needed; one oil line feeds oil to increase prop pitch; another oil line feeds oil to decrease prop pitch; the oil is then returned to the case via a separate return line.
The governor on the left engine is old; don't know the last overhaul date, but it's been awhile. It's an old Woodward 210055 governor. But, the governor generally holds RPM well and it doesn't hunt much more than the right side (which has a NOS governor on it with maybe 100hrs on it).
No evidence of oil leakage at the governor.
No "new" or increasing oil leaks on that engine (it's got a persistent minor oil leak somewhere that we've been unsuccessful at locating, but it's very minimal).
So, what could be causing this issue? Given the apparent correlation to prop pitch, internal leakage in the prop governor seems like an obvious possibility, but I don't know the internal workings of prop governors well enough to know if that's a real possibility.
Most recent engine monitor data is below:
https://apps.savvyaviation.com/beta/flights/6675825
Thoughts appreciated.
My engines go to 3400 RPM on take-off thanks to the gearbox on the front. Oil pressure comes up steadily as power increases, to around 80psi, then after a few seconds at full power the oil pressure on the left engine starts to drop (the right holds steady). Depending on temps, at 3400 RPM the oil pressure on the left engine will steadily drop to the low 60s and then stabilize. If the oil is hot, it will drop to the mid-50s. It always stabilizes, but it's below the green arc. When I reduce the props from 3400rpm to 3100 rpm (max continuous), the oil pressure RISES a couple of PSI. Then when I reduce the props again to 2750 for cruise, the pressure rises again a couple of PSI. If I reduce throttle further to 2500 rpm, it'll tick up another PSI or two.
Once it's all settled, in cruise the left oil pressure at 2750 is high 50s to low/mid 60s, depending on oil temps. It'll hold steady there for hours, with only the normal slight variations as oil temp changes. That engine tends to run 150-160* oil temp in cruise (about 10-15* cooler than my right engine). I run Phillips XC oil.
It gets even more interesting when I start to descend.
When I reduce throttle (not RPM) and start down, the oil pressure will start to steadily rise a little, even if I leave the RPM alone. Reduction from say 22" MP to 17" MP, with no change in RPM, will result in a 1-2psi increase in oil pressure. As I continue to descend, leaving RPM alone (but reducing throttle and increasing airspeed), the oil pressure will keep slowly ticking up a couple of PSI, even with no change in temperature. By the time I'm on final, the oil pressure will be in the low 70s. Landing and taxi in, it's in the mid to high 70s.
Based on all of the above, it appears that the oil pressure is changing based on prop pitch (not necessarily RPM only).
Now note that my props are a little odd. I've got original Beech 214 props, which utilize a rear mounted governor with oil lines feeding a prop transmission mounted to my gearbox. That transmission uses external gears on the transmission and prop to adjust prop pitch (and has an electric feathering motor). It does not have any oil in the prop hub. And oil is only fed to the transmission by the governor when a pitch change is needed; one oil line feeds oil to increase prop pitch; another oil line feeds oil to decrease prop pitch; the oil is then returned to the case via a separate return line.
The governor on the left engine is old; don't know the last overhaul date, but it's been awhile. It's an old Woodward 210055 governor. But, the governor generally holds RPM well and it doesn't hunt much more than the right side (which has a NOS governor on it with maybe 100hrs on it).
No evidence of oil leakage at the governor.
No "new" or increasing oil leaks on that engine (it's got a persistent minor oil leak somewhere that we've been unsuccessful at locating, but it's very minimal).
So, what could be causing this issue? Given the apparent correlation to prop pitch, internal leakage in the prop governor seems like an obvious possibility, but I don't know the internal workings of prop governors well enough to know if that's a real possibility.
Most recent engine monitor data is below:
https://apps.savvyaviation.com/beta/flights/6675825
Thoughts appreciated.
