Rear engine power loss during flight
 

Hello Everyone, I live in the west coast area of Canada and own a 1967 337B model that just went through extensive rear engine work over the past year from a well respected (and expensive) shop, basically a "top end" done. Plane was stored outside when the shop had to wait for parts and work on other planes. Finally in February plane was ready for pick up. Pre-flight checks show fuel level okay and drains checked for water. Run up checks went okay. Went to do a test flight around the circuit a few times at high power setting to break in new cylinders and for 10 minutes everything was fine. Just about to leave circuit when I noticed rear engine rpm's dropping off slowly 2500 to 2000, no reaction to prop control, switching tanks, throttle adjustment. Return to airport and landed safely. When taxiing off runway and stopped noticed oil pressure was below green arc but not in the red and engine temperature higher than normal approximately 400 degrees, oil temp was okay just a little above front engine reading.
Inspection done no oil leaks seen and oil level just below 8 quarts(normal). Attempts to restart rear engine unsuccesful with front engine running and starting normally. Prop would move but not even crank over. Shut down completed after fuel starting to drip and possibly engine being flooded. Any ideas would be appreciated.

The situation you described while taxiing off the runway is normal. It illustrates the typical rear engine cooling while not in flight.

After landing the engine is hot. Even with the rear cowl flaps open there is not enough airflow over the cylinders to reduce their temperatures. Cylinder head temperatures normally increase. Slightly low oil pressure is the result of hot thin oil and low oil flow rate while taxiing. Normal oil temperature is the result of a the oil cooler oil pump continuing to remove the excess heat accumulating in the cylinders.

Loosing rpm in cruise flight ... Prop moving but not cranking ...Hmmm .... sounds like you made a good decision to land immediately. Please let us know what you find.

Lets get back to basics here. What do we need for the engine to run?

1. COMPRESSION
2. CORRECT AIR/FUEL RATIO
3. SPARK
4. Strong enough battery to crank motor (My addition)

Your rear engine didn't "suddenly lose compression" during the test flight. Your engine didn't take the vax, did it? ;) So lets rule out compression.

You could have an issue with the air/fuel ratio.

You could have a magneto issue.

If the rear engine won't swing the prop to start, you either have:
A. A weak battery - VERY common - Doesn't explain loss of power in flight though
B. A weak starter - Doesn't explain loss of power in flight though
C. Hydraulic cylinder lock - One or more cylinders are filled with either oil or fuel such that it can't be compressed and the prop won't swing. COULD possibly relate to the loss of power in flight. You reported the engine dripping fuel, so I'd look closely at this one.

I would charge the battery and pull the lower plug on all cylinders. Make sure the prop moves freely with the plugs removed. Does any cylinder have oil or fuel leaking out when you remove the plug? There's your culprit.

No liquid? Now check your mag timing - Mechanics are notorious for not tightening the screws properly on the rear mags due to inaccessibility. Loose mags slowly coming out of time would exhibit exactly the type of behaviors you describe. If firing too early, could also prevent engine from turning over (maybe).

Mags OK? Now check your mixture control.

Do all this and report back. If you don't solve your issue, I will continue to provide assistance.

Best wishes on solving your issue quickly. :D

Thanks for all the tips. I will pass on the findings since it all sounds pretty strange.

Your 1967 does not have the engine access panel in the rear bulkhead which allows access to the rear engine from inside the cabin. This leads me to suspect the mags even more, because without that access panel, the rear mags are very difficult to tighten properly. Use your hand and see if the mags are loose at all. Even if they seem tight, I would still check the mag timing if you don't find any other issues with the engine.

Tim, i also fly a 337B! SN 661!
I know you just spent a bunch of money but i would invest in an engine monitor.
I have a jpi edm 790.
I would not fly without it.
Its really helped me manage my flying and also identify problems on runup (usually a fouled plug)
It might also be a good idea to put gauges on your fuel system.
You want to measure metered and unmetered fuel.
You will need this continental adaptor. Continental Tee MS 51523B4
Attached is a chart showing what the numbers should be.
Third row down
https://ibb.co/VL1kXN5

Concur. My avionics shop built a very precise test rig with inline AN pressure transducers to digitally measure the metered and unmetered fuel flows, The rig is housed in a small Pelican case and you can read the displays in the cockpit, making it a one-man job. I do this every annual on my NA G model. Have also done the local twin baron. If you are near central OK, we’d be happy to help you out with yours.

Essentially, you hook up the rig and run the motor at a few select power settings and record pressures. Shut down, adjust as necessary, and run it again. The Continental M-0 manual explains all the steps pretty well. If everything is working normally, a few runs gets the fuel metering and idle speed dead on perfect.

LJ

Rear engine power loss- update
 

Today I did a ground run test on the rear engine. The battery charge level was good and there was no problem on the start up. Run up checks as usual were completed without issues. I waited for the engine and the oil temperature to get warmer running the engine around 1800 rpm's. As I moved the prop lever to cycle the prop a drop in rpm's appeared however returning the lever to "fine" pitch didn't change the rpm's as expected and it stayed at the lower rpm reading. I moved the prop lever again to cycle the prop and the rpm's would drop even more once the prop lever moved further towards the "coarse" position and no return to the high rpm setting. Moving the prop lever only caused a drop in rpm's and no increase in rpm's. GOVERNOR fault.
In discussing the matter with my AME, a governor fault fits with my situation since the oil would be thicker at the start of my flight making the seals in the governer work as normal, but as the flight progressed the hotter oil got thinner and the seals inside the governer failed to move the oil around. Since the front prop is pushing air towards the rear prop this would cause the prop to twist to a coarser pitch. The subsequent drop in rpm's would then cause me to increase power to the front engine which leads to forcing more air towards the rear prop, twisting the blades more causing more rpm drop and the cycle continues until the engine quits or shut down. This wouldn't have happened on a side by side engine configuration without a front engine pushing air causing the rear prop blades to twist when a governor fails. Since the run up checks found no issue with the governor there wouldn't be a reason to suspect a problem with this part.

Cheers.

You could always swap front/rear governors, same part number.
Heres the gaskets youd need TEMPEST® AA9144 FAA-PMA Prop Governor Gasket

Thanks for the information about the gaskets I may need it. Would anyone have the part# for the actual rear prop governor? One repair shop in Canada has provided a quote to overhaul my governor of $2000.00. Checking around for perhaps a better price.

Woodward Prop Governor 210443D

Hi jhogan, thanks so much for the assistance. BTW I own serial#650.

Here's a used one on Ebay for cheap: https://www.ebay.ca/itm/284792413069

FYI if you have a prop-unfeathering and synchronizing option the prop governor is part#210529.:)

Tim,

Call Aircraft Specialties of Oklahoma for the governor. They likely have options on the shelf for you. I paid 2k for a 210443 with no core to give back. About 1600 for an overall of that P/N.

Also, if yours can be overhauled, H&S prop in Michigan can do it and has been very good to me. They are doing my 210443 now. For the record, they're also one of very few shops that will attempt to locate ferrules in order to overhaul a threaded type prop.

Gray