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  #1  
Old 09-26-2022, 03:04 AM
SuperCubDriver SuperCubDriver is offline
 
Join Date: Feb 2011
Location: Germany
Posts: 511
Default Sticking exhaust valve during flight

Here is a report with an incident during departure which caught my attention:

Last Friday I was taking off from LOWZ airfield in Austria on Rwy 08 with my RV-7. When I reached a height of 500 ft the engine suddenly was shaking with a significant power loss. I immediately pushed the nose over and turned right knowing I hava to land ASAP. Two options came to mind - a short pattern or land on opposite rwy, chosed the second option as I was in a good position for it. I glanced at the instruments and saw cyl 2 is gone (not physically). Engine was still responsive but with reduced power and out of balance. I radioed I was having trouble and will come in on rwy 26. The approach was not pushed and good on speed and altitude being aware of about 2 kts tailwind on the 2500 ft rwy. I came in slightly high and the plane just didnīt wanīt to descent and become slower, somehow the engine was still producing power and I pulled hard on the throttle - no change.
Now I realized I will not make it and end up in the bushes at the end of the rwy. I made a bad decision but was lucky with it and did go around being aware I could crashland in the golf course ahead. I retracted the flaps to 10 and did a shallow climb with a slight left turn for a teardrop pattern. Then I came in high assuring to make the rwy. Again the airplane didnīt want to descent as usual, there is still the power just like on the first approach. I pulled the mixture and shut the engine down and she descendet normal towards the rwy for a reliefing landing. During rollout I pushed the mixture in again to keep the engine running for a taxi-out. I realized that the RPM was around 1200 while it should be around 650 - so it was really running with power and this explains the unsuccessful approach.

I can tell you that during these two seconds floating over the rwy realizing not to make it and having two bad options left is no fun at all. It is completely different analysing this later on the ground with all the information available.

A question for the engine gurus here:
What could have caused the power increase at idle setting?
My guess is that the sticking exhaust valve in open position acted like an"aggressive cam" with some significant valve overlap so the idle MAP was increased (less vacuum in the intake manifold).

Here are the datas from the Skyview datalog:












The whole flight lasted four minutes. You can see this in the timelines from 02:30 to 06:30.
Note the RPM decrease at 06:30 when I pulled the mixture. The fuel flow went to zero while the RPM dropped from 1500 to 1100 RPM (windmilling).
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Last edited by SuperCubDriver : 09-26-2022 at 03:11 AM.
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  #2  
Old 09-26-2022, 06:44 AM
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rv8ch rv8ch is offline
 
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Herman, so happy you made it onto the ground safely - that must have been a very exciting landing. Your MAP graph supports your theory of why the engine ran at a higher RPM - something was causing the MAP to be higher.

Any chance the intense vibration was preventing the throttle from closing properly?

You might open a ticket with the SavvyAviation gurus - this is one that will get their interest I'm sure.
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  #3  
Old 09-26-2022, 06:54 AM
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AlexPeterson AlexPeterson is offline
 
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Wow, so glad it turned out ok!

The high idle is indeed a mystery - perhaps a sticking intake valve could be bringing in more air during that cylinder's exhaust stroke? Meaning at that time, both valves in that cylinder would be open at the same time, allowing exhaust flow to draw air into the intake?
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  #4  
Old 09-26-2022, 08:27 AM
lr172 lr172 is offline
 
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Location: Schaumburg, IL
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Quote:
Originally Posted by SuperCubDriver View Post
A question for the engine gurus here:
What could have caused the power increase at idle setting?
My guess is that the sticking exhaust valve in open position acted like an"aggressive cam" with some significant valve overlap so the idle MAP was increased (less vacuum in the intake manifold).

The whole flight lasted four minutes. You can see this in the timelines from 02:30 to 06:30.
Note the RPM decrease at 06:30 when I pulled the mixture. The fuel flow went to zero while the RPM dropped from 1500 to 1100 RPM (windmilling).
Based upon the FP data, assume you have FI. Fuel flow is based upon airflow through the servo, so having 1 stuck Exh valve should have reduced fuel flow. It is possible that you were too rich and this reduction in flow leaned things up a bit which would increase power a bit if it moved you from excessively rich to best power.

My guess is that you were going too fast and this airspeed is helping to turn the prop. Note that even when at ICO you are still turning 1100 RPM. That is an indication of too much airspeed. These are slippery planes and when an FP prop is in place, pulling the throttle back during a decent is not always enough to make the plane "go down now." Some times you need to lift the nose to bleed enough airspeed to make a more aggressive descent and slips can help. In my 6, if I am doing 100 MPH, it doesn't really want to descend much without increasing speed. Drop the speed to 80 MPH and it is a manhole cover. If you are going too fast and push the nose down, there is no real way to manage things. Pushing the nose down increases the speed and just makes things worse. A descent to landing pretty much has to start at a low enough airspeed to make things work.

Larry
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Last edited by lr172 : 09-26-2022 at 08:37 AM.
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  #5  
Old 09-26-2022, 09:49 AM
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Janekom Janekom is offline
 
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Glad that you have pulled it off.

My guess is that with the valve stuck open, the compression is obviously less and therefore the prop has lost some of it's braking capability. I assume it is a fixed pitch prop?

Secondly I would like to know the engine hours and also if it has done more than 400 hours, why did you not do Lycoming SB-388C?
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  #6  
Old 09-26-2022, 09:53 AM
SuperCubDriver SuperCubDriver is offline
 
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Iīm having an ECI fuel injection system, this controls the fuel flow not according airflow but according the position of the throttle plate. Well - it is kind of dependent of airflow in this regard. But the actual fuel flow is controlled mechanically.
I do not agree in my case that the airspeed messed up my approach. My experience of many thousand landing approaches with all speeds possible just doesnīt match. And pulling the mixture on the second approach was just like pulling power to idle.

Note that the exhaust valve was stuck OPEN. That means when this Cyl 2 is on itīs intake stroke the exhaust valve is still open and the Cyl doesnīt need to suck air in as outside pressure is coming via the exhaust port. There might have been even some pressure from the exhaust and so increasing the MAP. As the mixture is rich on idle (when the mixture lever is set to rich) the other cylinders can burn this fuel with the additional air and so produce more power.
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  #7  
Old 09-26-2022, 09:57 AM
SuperCubDriver SuperCubDriver is offline
 
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Quote:
Originally Posted by Janekom View Post
Glad that you have pulled it off.

My guess is that with the valve stuck open, the compression is obviously less and therefore the prop has lost some of it's braking capability. I assume it is a fixed pitch prop?

Secondly I would like to know the engine hours and also if it has done more than 400 hours, why did you not do Lycoming SB-388C?
Thanks for your input.
It makes sense on the braking capability but as said it was completely different when pulling the mixture.
The engine is a low compression IO-375 with a 200RV constant speed prop.
I did the SB-388C at 1000 hours. The cylinders are having the marks saying it should be done at 1000 hours and not 400 hours.
The event happened at 1248 hours.
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  #8  
Old 09-26-2022, 11:22 AM
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Dan 57 Dan 57 is offline
 
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Hallo Hermann, yes, I'm also happy all went well in the end

Was it really a sticking exhaust valve? Have you found physical evidence for this, after the flight that is? Do you have the MP graph as well?

I've had 4 cases of sticking valves in the past few years... 3 of them on the smaller Conti C-90 engine, and one on a O-320. Well, none of them had any increase in RPM recorded, actually a slight decrease at idle on all of them. I didn't have any fancy engine monitoring on those airplanes/engines, but the symptoms were coughing, as in missing beats, and extreme vibrations forcing a massive reduction in power. Then once on the ground the faulty cylinder was identified by being cold to the touch... and the exhaust valve totally stuck in their respective guide, so as too be hammered in for the guide to be reamed. Or the cylinder replaced with a cylinder kit...
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  #9  
Old 09-26-2022, 11:32 AM
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wcalvert wcalvert is offline
 
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Herman, well done managing your issue, happy to see a good outcome.

Just a few weeks ago I had an incipient sticking valve that I noticed on taxi out. EGT was about 200 degrees on my #2. Rough idle and would settle down after a two minute warm up. I also noticed that during the rough period the idle fuel flow was nearly 5 gph, vice the normal around 2 with throttle/mixture in normal positions. Once the idle smoothed out the FF returned to normal.

So my FI system was somehow increasing FF while that valve was stuck open. (standard AFP FM150 FI).

The high fuel flow doesn't seem to be on your graphs, but I do believe there is some interaction between cylinders while a valve is stuck open that might be causing unusual FI behavior. Imagine the upset manifold pressures and non normal flow of combustion mixture and exhaust mixing to the other cylinders with the exhaust valve stuck open.

Just a thought that has gong through my head for a while. I would like to learn more about my event and get to the bottom of your issue as well.

Cheers
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  #10  
Old 09-26-2022, 01:18 PM
Steve Watkins Steve Watkins is offline
 
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Default Similar Experience

I had #3 exhaust valve stick open about a month ago leading to an unplanned landing away from home. I was fortunate to be at 8500' when it happened. Violent shaking that reduced with power reduction. EGT dropped to about 200. I had an airport about 8 miles away and made it there, but almost overshot as well. Aggressive slip aimed at the 1000' foot mark of the 4000' runway because I didn't want to come up short or have to go around. In the round out it didn't want to slow down and just kept floating. Stuck it on the mains and began braking with tail still up. Got it stopped by the end of the runway and found it to be idling at 1200 RPM. WTF? Lots of braking needed to taxi to the ramp. I don't understand the high idle with a dead cylinder. There was no compression on 3, so the engine is easier to turn, but I would think that would be offset by that cylinder not making any power?
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