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Eric Greenwell

Phoenix 912 engine loses 500 rpm during climb

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Eric Greenwell
My engine has 440 hours on it with no problems. I run it mostly on mogas, and it spends maybe third of it's flight hours with the engine idling while I'm soaring it. Here's the summary of what happened on two flights - Oct 7 & 8, 2017

+ It started and ran normally on the ground, including the 3000 rpm ignition check
+ Full throttle while holding it with the brake yielded about 4800 static rpm (normal), then I began the takeoff.
+ Takeoff and climb were normal for the first 20 seconds or about 300+ feet AGL
+ I felt the engine running rough, and the RPM dropped to 4600 rpm
+ I reduced the throttle to 4400 rpm, where the rough running became smooth, and I could then power up to full throttle with no problems.
+ At 3000' AGL or so, I went to idle to begin soaring, and the engine performed flawlessly, as it always has.
+ There were other full throttle climbs during both flights, but at higher airspeeds, so the pitch angle was only 7 degrees instead of 12.
+ The Dynon Skyview data log showed the EGTs stayed with 20 deg F during the roughness, and with 30 deg F the whole flight

I thought that the nose up attitude during the climb after takeoff might have caused contaminants in the fuel to get picked up, but when I drained samples from each tank after the last flight it was pure mogas. A friend wondered if it could the plugs, because I do a lot of soaring with the engine idling. A possibility? I've been doing the idling while soaring for the 3 years I've owned it, without any problems.
 
A few days later, I replaced all 8 plugs, and went flying with a half page of potential tests in hand. It worked perfectly on the takeoff and 5000' climb at 5000 rpm, so I stuffed the list of tests in the side pocket and went soaring. 3.5 hours of idling later, I did a 5000 rpm, 60 knot climb for 3 minutes - still working great. At that point, it appeared the symptoms were caused by the plugs.
 
The next day, I did a takeoff, and the engine ran rough very soon after liftoff, dropping the rpm to about 4400. Reducing the throttle didn't help any, so I continued my turn back to the runway, and landed uneventfully. Two full throttle run-ups on the ramp went to 4800 rpm (normal for the engine), with the engine running smoothly. I could see nothing wrong in the engine compartment.

I'm totally perplexed, so I'm waiting for my mechanic to call back to see what he thinks is the next step. If anyone has suggestions, or even wild thoughts, I'd love to hear them! It's disappointing that 440 hours of flawless operation is suddenly undone, and I don't know why. And the disappointment of having the glider out of service, delaying several plans.

Any ideas about the cause? And what tests might help find it? At this point I don't mind doing another flight to carry out some tests to pin down the problem. With the 30:1 glide, I only need 200 feet AGL to turn back to the airport safely, so the risk period is about 10 seconds at most.

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Jim Lee
15 hours ago, Eric Greenwell said:
Comments below in italics.
 
My engine has 440 hours on it with no problems. I run it mostly on mogas, and it spends maybe third of it's flight hours with the engine idling while I'm soaring it.
 
Besides the obvious exceeding max rpm or temps, there is nothing worse for the 912 than running at idle for extended periods.  Any engine for that matter.  We are taught to run the rpm up every 30 seconds to clear the plugs and valves when at extended idle in any airplane with Lycoming or Continental engines.  Same for the Rotax.
It is super important to keep the operating oil temp around 190 degrees F.  We are lucky to have a cowl flap which allows us to regulate the oil temp in flight, especially at high altitude, cold temps, and descents (or extended idle) where the oil temp will get way too cool without the cowl flap.  Without continued engine run at 190, you will see a fouled engine and possible corrosion due to water not being burned off in the cylinders and in the engine.  That is why the Skyview does not even give you the green arc in oil temp until you have achieved 190.
 
Here's the summary of what happened on two flights - Oct 7 & 8, 2017

+ It started and ran normally on the ground, including the 3000 rpm ignition check
 
We do a 3000 rpm ignition check because at 4000, the plane is straining at the bit.  A 3000rpm check will catch most issues, but Rotax says to do the ignition check at 4000rpm.  So if someone is looking for the max permissible drop in rpm, or irregular engine performance, the check must be done at 4000 and the rpm drops noted and compared to the standards.

+ Full throttle while holding it with the brake yielded about 4800 static rpm (normal), then I began the takeoff.
 
4800 is on the low end, 4600 is absolute least static rpm allowed (during a static test) to avoid high manifold pressure.  4800 is fine, and results in max cruise speeds at 5000rpm.  The 912 loves 5000rpm better than any other continuous rpm.  If max speed is not desired, then a finer prop pitch which allows 5000rpm continuous is preferable to a continuous cruise at say 4600rpm.

+ Takeoff and climb were normal for the first 20 seconds or about 300+ feet AGL
+ I felt the engine running rough, and the RPM dropped to 4600 rpm
+ I reduced the throttle to 4400 rpm, where the rough running became smooth, and I could then power up to full throttle with no problems.
+ At 3000' AGL or so, I went to idle to begin soaring, and the engine performed flawlessly, as it always has.
+ There were other full throttle climbs during both flights, but at higher airspeeds, so the pitch angle was only 7 degrees instead of 12.
+ The Dynon Skyview data log showed the EGTs stayed with 20 deg F during the roughness, and with 30 deg F the whole flight
 
20-30 degree F EGT?  Makes no sense.  Should be closer to 1000 degrees.

I thought that the nose up attitude during the climb after takeoff might have caused contaminants in the fuel to get picked up, but when I drained samples from each tank after the last flight it was pure mogas. A friend wondered if it could the plugs, because I do a lot of soaring with the engine idling. A possibility?
 
Absolutely.  Fouled plugs and valves are common after extended idle.
 
I've been doing the idling while soaring for the 3 years I've owned it, without any problems.
 
It is too common that extended idle during soaring is used instead of a shut down.  It is strange that more people don't report problems from this extended idle.  I am sure that I have more in-air shut downs while soaring than anyone.  I have never had the engine fail to start.  But I always restart within range of an airport or a good field to land in if it doesn't start.  This is much more preferable than idle soaring for both the engine and the soaring pleasure.
 
A few days later, I replaced all 8 plugs, and went flying with a half page of potential tests in hand. It worked perfectly on the takeoff and 5000' climb at 5000 rpm, so I stuffed the list of tests in the side pocket and went soaring. 3.5 hours of idling later, I did a 5000 rpm, 60 knot climb for 3 minutes - still working great. At that point, it appeared the symptoms were caused by the plugs.
 
Since you did another 3.5 hours at idle, you are back to square one with the engine.  Your previous spark plug change has now been negated.  It could also be that a spark plug ceramic has been cracked during the gap adjustment which can also cause the symptom you describe.
 
The next day, I did a takeoff, and the engine ran rough very soon after liftoff, dropping the rpm to about 4400. Reducing the throttle didn't help any, so I continued my turn back to the runway, and landed uneventfully.
 
Excellent Eric!!
 
Two full throttle run-ups on the ramp went to 4800 rpm (normal for the engine), with the engine running smoothly. I could see nothing wrong in the engine compartment.

I'm totally perplexed, so I'm waiting for my mechanic to call back to see what he thinks is the next step. If anyone has suggestions, or even wild thoughts, I'd love to hear them! It's disappointing that 440 hours of flawless operation is suddenly undone, and I don't know why. And the disappointment of having the glider out of service, delaying several plans.
 
You understandably have a high expectation for your 912 Eric.  The 912 is a super engine which performs well for many hours, unlike the Continental or Lycoming engines which rarely run for over 400 hours without problems.  I would never fly a C or L powered SEL over the Gulf of Mexico without a raft onboard, and yet I have done it 3 times behind the 912 with only a PFD (and the glide ratio of the Phoenix).  It is too bad that the timing of this problem interferes with your plans.

Any ideas about the cause? And what tests might help find it? At this point I don't mind doing another flight to carry out some tests to pin down the problem. With the 30:1 glide, I only need 200 feet AGL to turn back to the airport safely, so the risk period is about 10 seconds at most.
 
I can't remember the length of the runway you use Eric, but on a normal 6000'+ runway, a return to the runway straight ahead or the turn back should be possible at any time and there is no risk period as long as the climb out is established downwind of the centerline and not tracking the runway centerline.
 
You should have your mechanic go through a complete inspection and annual service of the engine.  Change spark plugs, inspect the float bowls and balance the carbs, change the oil, borescope the cylinders, do a compression test, etc.  The engine needs to be returned to a known operating condition.  Only after this is done can engine runups or in flight testing be done to assure that the engine is performing as it should.

 

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Eric Greenwell

Hi Jim - some answers and clarification...

Idling issues: I looked into this a couple years ago, couldn't find any good evidence against the practice, and persuaded myself the harm, if any, was minimal. I do keep the oil temperature at 190 degF, the throttle opened 1/4 turn, and the rpm varies from 2200 (thermalling) to 2400 (cruising at 60 knots). It's always run very smoothly at idle, even after 3+ hours, and  responds to more throttle, up to full throttle, with no hesitation or missing. The plugs have always looked normal when replaced, and my mechanic, Jim Scott of Aircore, always says the oil looks normal when he changes it, with the biggest contaminant being lead when I've had to use 100LL during the 50 hour period. If you do have reports or studies about the issue, I'd really like to see them.

Ignition check RPM: I did not know about the 4000 rpm check, but will remember it now.

Max static rpm: 4800 is normal my glider on the ground; in the air at 60 knots it goes to at least 5200 with full  throttle, and I always dial it back to 5000. Cruise is at 5000 (TAS 110 knots), and that requires backing off the throttle quite a bit. So, I don't think it's down on power.

EGT temperatures: during the flights on Oct 7 & 8, the EGT was in the 1300 degF range; the 20-30 degF was the spread between the two EGT signals. I should have written "within" instead of "with".

Plug fouling, compression: As mentioned, I've never noticed any plug fouling. The compression was correct at the last annual, about 50 engine hours ago, so at least no valve leakage at that time.

Cracked ceramic on the plugs: I've gapped 100's of plugs over the years, in old (60's!) cars that needed new plugs every 10-15,000 miles, and in the sports cars I raced for seven years without cracking any of them, so I think it unlikely it happened this time; and, of course, the problem began using plugs gapped by a professional.

Runway situation: KRLD has two 4000' runways with limited overun, so there is a short period (10 seconds?) where an off-airport landing might be required.

I'm discussing my options with Jim Scott, but no choices made yet. I don't want to fly across the Cascades (or anywhere, really) the way it ran last time,  so Jim coming to RLD is the most likely option right now. He strongly suspects it's a heavy float, and he does have a set of the very newest version (two(!) versions newer than the ones Rotax paid to have installed 2 years ago). I'm sure he'll want to do all the checks and tests you've mentioned.

 

 

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Eric Greenwell

I and another pilot spent several hours a week ago, testing things and doing WOT run-ups, and the only thing we found was a speck of what looks like red rubber (like the red rubber on the hose insulation) in the left float bowl.

hihjadmdomlkmipi.jpg.8eecd659f5834784021a1d8047d58e9b.jpg

The run-up after that was perfect - no rough running, no RPM sagging. The next two day, I did another full power runup, a takeoff, and two 5 minute, full power climbs; the following day, I flew a two leg, two hour total trip to another airport. During the runup, climbs, and the out and return flight, the engine performed smoothly just like it did for the first 450 hours. So, I'm persuaded it's fixed, but I'll be extra cautious for the next few flights, keeping an airport in easy reach while flying.

 

 

 

Rubber from carburetor.JPG

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Eric Greenwell

More on the rpm loss issue...

A few weeks ago, I had an rpm loss like the others, except it wasn't as much of a loss, and it cleared up more quickly with the usual power reduction and leveling off. About 5 flights since then with no further problems.

April 23, during the annual inspection, the mechanic found a red bit of debris similar to the first bit, but this time in the right carburetor bowl. The next day, at home, a friend made a call to Lockwood Aviation, and confirmed his suspicion that there are red rubber components in the mechanical fuel pump. We removed the pump, took the top off, examined the diaphragm and check valves, but every thing looked normal. So, we still don't know where the red bits are coming from. They look too big to get past the gascolator filter and the inline filter.

I realized recently that all the rpm loss events occurred during full or nearly full power climbs after takeoff, so definitely a nose-up attitude. Perhaps there is a mechanism that could reduce power that's triggered by a nose-high attitude? For example, if there was water or other heavier-than-fuel contaminant in the carb bowl, would it be more likely to enter the orifice going to the carburetor body?

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Eric Greenwell

This problem has returned a year after it first occurred.  An EAA member here that also uses the 912 suggested the propeller changing to coarser pitch would explain the symptoms. Is it possible for the blades to shift a few degrees, maybe because something internal is loose, not greased enough, or worn? I don't use it in the feathered mode very often, but I usually twist the blades by hand during the pre-flight, and often cycle the prop with the control lever. Everything seems normal, but is there something that could be inspected to rule out the possibility?
 

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Eric Greenwell

The problem was found and corrected last week. It was just luck that I got the information needed to solve the problem: during a ground test at full throttle, a video recording of the exhaust pipe caught two puffs of black smoke about 20 seconds after the engine RPM dropped by 500 and a strong vibration started. The video recording was intended to measure the effect of the exhaust diverter on the exhaust pipe.

After seeing the video of the puff of smoke, my mechanic (Jim) consulted the Lockwood people, and all agreed a sticking exhaust valve was the most likely cause of the RPM loss and vibration. Jim did the valves on all the heads, and the engine runs smoother and about 300 rpm faster at full throttle on the ground. The #1 cylinder exhaust valve was the culprit, with thick black, almost "gooey' deposit about 1/2" long on the valve stem from the valve head up. The valve was tight in the guide, and the guide was difficult to ream out, but both valve and guide were in good condition, once the deposit was removed. The other exhaust valves were in good condition, but perhaps with more deposit on them than normal.

I did not get a picture of the #1 valve, but the attached image shows the worst of the other valves. On #1 the entire deposit was as black and thicker the black ring on the pictured valve.

Jim was not sure what caused the heavy deposit on the #1 exhaust valve, but offered several suggestions:

  • The extended idling I do while soaring (gliding) might not be keeping the valve hot enough to prevent deposits from forming. I estimate I've idled in flight about 200 hours out of the total 600 hours of engine time.
  • Possibly, lead from using 100LL might cause a sticking valve, but most of my flying is with E0/91 AKI fuel, so that isn't likely.
  • He suggested I test the E0 mogas I've been using; I did so, and it tested good (no alcohol) so it's probably not the fuel
  • The only time he's seen such a thick, almost gooey deposit, was in plane with tanks sealed with the wrong product for fuel with ethanol. Not likely my problem, as none of the other 50+ planes have had any problems.

Of course, I want to avoid this problem in the future. One way is to do my soaring with the engine off, propeller feathered, as was intended by the designer! There can be difficulties with that approach, but I think I can cut the engine idling time in half with only a bit more effort while soaring. Another possibility is some additive that reduces the build-up of carbon on exhaust valves, but I don't know what it is. Any ideas?

Thanks to those that responded with ideas about the problem. Even though none of them solved the problem, they did get me and others think more carefully about the situation.

Valve.jpg

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