Momentary RPM drop

Somedudeintn

Somedudeintn

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somedudeintn
I've had this happen to me twice now in 2 different da-20's. The first time I was a little over an hour into my flight and I was in a level cruise.

What happened was that the engine RPM made a momentary drop, it was only a second or less and then returned to normal RPM. It was an audible change and I never caught it on the tach while flying. The second time it happened was on an initial climbout, but it was the same thing. I looked at the video for that flight and the tach only registered a drop of about 100 rpm, but I'm thinking the change in pitch I heard was more than this.

Is this something that anyone else has experienced? Any ideas what could be happening?
 
Two primary possibilities, you had some carb ice that cleared itself, of you got a slug of water through the system. oh, wait, you have Fuel Injection, don't you? Not going to be ice, but you could be sucking air at an injector pump seal.
 
Henning said:
Two primary possibilities, you had some carb ice that cleared itself, of you got a slug of water through the system. oh, wait, you have Fuel Injection, don't you? Not going to be ice, but you could be sucking air at an injector pump seal.
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Since ignition is 90% of all engine hassles, I'd suspect that first. A weakening mag or a P-lead that's chafing and intermittently grounding on something.

Next would be a sticking valve.

A slug of water could kill that engine dead for a second or two or much longer, not just drop the RPM a bit. In some systems, the surface tension of the water makes it almost impossible for it to pass through the metering orifices. That's why airplanes have so many tank drain points and very fine screens in the strainer and carb or FCU fuel inlet. Small amounts of water won't go through those screens when they're wet with gasoline.

Dan
 
What did maintenance say?
 
I noticed a slight rpm drop on two different occasions. Shortly after those episodes my left mag failed. still not sure if it was related but running smooth now.
 
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simtech said:
I noticed a slight rpm drop on two different occasions. Shortly after those episodes my left mag failed. still not sure if it was related but running smooth now.
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Too many magnetos don't get the attention they need. They are very old-school systems that date from around 1900. They use breaker points that erode. As the points erode, their resistance goes up, weakening the primary coil flux. As they erode, the magnetos timing advances and a mechanic who isn't paying attention will simply reset the timing without thinking about why it shifted 5 degrees in the last year. As the points or their cam wear, the internal timing goes off (E-gap) which weakens the spark terrifically. They use capacitors whose values can change or that can short or open, either of which kill the spark. They use a distributor that relies on a carbon brush that wears out and rotor bearings that wear out and a rotor finger that burns away. Some use nylon distributor gears that are famous for getting brittle in the heat and with age and breaking down. They used sealed ball bearings that can dry out or get crankcase condensation in them that rust them out. Their bearings use oil seals that age from years and from heat and from nasty stuff in the oil, and that eventually leak oil into the magneto and foul everything up in there.

Some folks think the mags are good for the TBO of the engine. They're not.

So many times an engine acts up and everybody starts start blaming the fuel or water in the fuel or a dirty fuel filter or tank venting or tank unporting or something. Or the carburetor. Or the fuel pump. Yet in most cases it's weak spark from a bad mag. Or a worn sparkplug. Or a Champion plug of any age. Or lead or carbon fouling from pilots who have no idea how to manage mixture. Or really old ignition leads. Or chafing P-leads. Lots of stuff to check.

Dan
 
Henning said:
Two primary possibilities, you had some carb ice that cleared itself, of you got a slug of water through the system. oh, wait, you have Fuel Injection, don't you? Not going to be ice, but you could be sucking air at an injector pump seal.
Click to expand...

If it's fuel injection it could be some goobers in the injectors themselves
 
James331 said:
What did maintenance say?
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I never heard back from them. I did get a phone call asking for some more details on the issue, but to be honest I did not follow up with them after the fact either. I'll ask next time I'm at the airport.
 
I probably should know this, but I don't.... Do the Rotax engines in LSAs have mags?

If not, what do they have, and what are the common failure points?


Sent from my iPhone using Tapatalk
 
Yeah. I'd say Ice.

In my CS 235 I had it run really rough when I pulled the throttle back after takeoff, so I went with "UNDO WHAT YOU JUST DID" and pushed the power back. A few minutes later all was well. It didn't occur to me to turn on carb heat.
 
RalphInCA said:
I probably should know this, but I don't.... Do the Rotax engines in LSAs have mags?

If not, what do they have, and what are the common failure points?


Sent from my iPhone using Tapatalk
Click to expand...

Well the DA-20 can be a Rotax 912 or it can be an IO-240 (DA20-C1).
 
I had this about a month ago in my arrow. Also not running as smooth as normal during cruise. My flying buddy didn't notice it, but I did. Cleaned the plugs. All good. That 100 Lotsa Lead just isn't nice to plugs. Especially the bottom ones.
 
I zoomed in on the video when it happened, its not great quality, but you can see the drop. You can't hear the drop on the audio, I'm guessing that since the camera was mounted to the ceiling of the plane it picked up on more wind noise than engine noise.

https://www.youtube.com/watch?v=MKwS-Aijt-k&feature=youtu.be

It happens at 6 seconds on the video.
 
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Arrow67 said:
I had this about a month ago in my arrow. Also not running as smooth as normal during cruise. My flying buddy didn't notice it, but I did. Cleaned the plugs. All good. That 100 Lotsa Lead just isn't nice to plugs. Especially the bottom ones.
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REM38 or REM40 plugs are a pain where lead is concerned. REM37BY are far better. They have extended electrodes that not only get the spark into the air/fuel mix a lot better, but they don't get fouled by filling up with lead.

airplaneandaquariumpics001.jpg


I have used them with great success on flight school airplanes. SOme of those would foul up in 25 hours just by doing too many circuits.

They won't fit all engines. The plug manufacturers have application charts.

Dan
 
Somedudeintn said:
I've had this happen to me twice now in 2 different da-20's. The first time I was a little over an hour into my flight and I was in a level cruise.

What happened was that the engine RPM made a momentary drop, it was only a second or less and then returned to normal RPM. It was an audible change and I never caught it on the tach while flying. The second time it happened was on an initial climbout, but it was the same thing. I looked at the video for that flight and the tach only registered a drop of about 100 rpm, but I'm thinking the change in pitch I heard was more than this.

Is this something that anyone else has experienced? Any ideas what could be happening?
Click to expand...
 
Did you ever discover the cause of this rom drop? I have had the exact same occurrence: a momentary, 1 second rpm drop that occurs only on climb out. It has happened on several flight. Further, I have downloaded the engine monitor data (and had it looked at by Savvy) and there is no change in EGTs, MP, fuel pressure, fuel flow, oil pressure, etc. the only thing that changes is a 120-150 drop in rpm.
 
Rick5678 said:
Did you ever discover the cause of this rom drop? I have had the exact same occurrence: a momentary, 1 second rpm drop that occurs only on climb out. It has happened on several flight. Further, I have downloaded the engine monitor data (and had it looked at by Savvy) and there is no change in EGTs, MP, fuel pressure, fuel flow, oil pressure, etc. the only thing that changes is a 120-150 drop in rpm.
Click to expand...
I wish there was an engine monitoring system that tracked sparkplug firing. Electrical hassles are the biggest and most common hassles.

I bet the data could be taken right off P-leads. A shorted plug, or an open plug, is going to generate a different wave profile (compared to a good plug) in the mag's primary winding as it discharges. The old-school automotive shop performance analyzers did just that, displaying the info on an oscilloscope screen. A computer chip could do it all and plot the info in the system's memory, identifying the bad plug, or the weak mag, or leaking or open plug lead. Whatever.

upload_2022-4-20_14-51-9.jpeg
 
Rick5678 said:
Did you ever discover the cause of this rom drop? I have had the exact same occurrence: a momentary, 1 second rpm drop that occurs only on climb out. It has happened on several flight. ...
Click to expand...
Carb ice?
 
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