Momentary slight loss of rpm/power in cruise

[bmcconnaha]

Today on a long leg of an XC, about 2.5 hours in, I was level in flight and the engine had quite a hiccup.  I downloaded the data from the event and took a screenshot.  I’m hoping someone can throw some ideas at me. Here is the link to the data.   below is the note I sent to savvy.  Event happens at 2:26:15.  Engine has 27 hours on it. 

https://apps.savvyaviation.com/flights/4917481/b4e89af4-5822-4ea5-9481-d4c00690a41a

“Level in cruise, engine had been running perfectly for almost 2.5 hours. I had just switched takes about 2 gallons ago. I felt a sudden deceleration coupled with a loss of roughly 100rpm. I did notice a drop in fuel flow and rise in EGT. I did reduce power and start a descent to an airport I was close to. There was a mag checks and mixture adjustments after that event, so I’m not sure there is much to be gained from after the event from the data. On the ground, I removed the cowl, let it cool and looked carefully. No abnormalities were observed. I installed the cowl and took on some fuel. I dumped both tanks and drained the gascolator and switched to the other tank and drained again. I proceeded with a careful runup, followed by a short field style takeoff and climbed in a circle above the airport. The engine ran fine, so I proceeded the 150mi home, and it continued to run fine. That was the next flight in my flights (s40-coe). Couple of observations. I took on a lot of fuel the last three days and didn’t drain the gascolator. Also, I got task saturated briefly with talking to Seattle center and I missed the fact I had my fuel pump turned on for about five to ten mins after switching tanks. It was on during the event. Thank you”

[carusoam]

B,

FF is pretty steady in the flight... at 13.2gph

Seems kind of high for an M20J?

Something happens as if the mixture gets pulled back...

FF drops to 12.9gph, EGTs climb along with the change...

FF then wobbles unsteady above 15gph....

Are these FF numbers correct?

can you share more than this one flight?

I am expecting FF closer to 10gph for an IO360...

Looks like the mixture was trying to flood the engine, after a momentary lean state...

Check to see if your mixture knob is working correctly...

Confirm that you were really using 13gph in an IO360.

Are you at home base now?

PP thoughts only, not a mechanic...

Best regards,

-a-

 

 

[carusoam]

RPM is controlled by the gov...

The gov is mechanical, so it may lag while trying to keep up...

The FF suddenly drops as the RPM cuts about 75rpm...

The FF drop and rpm drop may just be a sign of something larger... as if the ignition got turned off...(?)

PP observations only... I haven’t seen my first cup of coffee yet today...

Best regards,

-a-

[carusoam]

Adding to the observations...

CHTs were steady around 350°F....

But, preceding the RPM spike... the CHTs started heading lower...

Was there a change in OAT, or descent occurring for this?

Great data set...   OAT and Altitude were steady during this portion of the fight...

 

[carusoam]

Let’s see if the mags were telling us something....

Looking at the graphs... I typically try to see where mag one was run, then compare to mag two.... then try to see where the prop was pulled...

See if you can explain what was happening during the run-up...?

I see a couple of blips, but they don’t seem to match what my mind is expecting...

 

Here’s the chart...  maybe someone else may see something...

got to go find my coffee...   

Best regards,

-a-

[carusoam]

Getting a closer look...

Trying to see where the two independent peaks of the run-up are....

For some reason, I don’t see what is usually defined peaks... most often near identical in height...

It looks like right around 00:11 minutes the run-up occurs...

Possibly done so quickly, data didn’t get collected...

If the run-up is done like in flight school... click, listen, click again....   it takes 15 seconds for things to settle out, and data to collect.... wait for the EGTs to stop moving....  the twin peaks seem to be melded into one blur... 

or one mag is behaving like two?

-a-

[carusoam]

OAT and Altitude referenced above...

The JPI does a nice job of sewing together a lot of data...

-a-

[ArtVandelay]

If really running that rich, maybe just the spark plugs getting clogged with lead.

For comparison, at that altitude I run around 9gph with a TAS of about 150k.

[carusoam]

Closing the loop by inviting @kortopates to the conversation...

Best regards,

-a-

[Mooneymite]

Water?

In the fuel....

Edited May 13, 2021 by Mooneymite

[N201MKTurbo]

I would check the point gap on the mag. The point cam followers wear and you lose gap. You can usually readjust the gaps and the internal timing without resetting the cam position. I do it every other annual or any time I have the mag off.

[N201MKTurbo]

Just now, Mooneymite said:

Water?

Water would cause a complete loss of combustion. The CHTs and EGTs would both drop. The rise in EGT indicates that one mag quit. 

[Mooneymite]

4 minutes ago, N201MKTurbo said:

Water would cause a complete loss of combustion. The CHTs and EGTs would both drop. The rise in EGT indicates that one mag quit. 

I suspect you are correct, but a small mixture of water in the avgas might not put the fire out completely.

Strange that everything returned to normal, so I'm wondering about a momentary cause.

Edited May 13, 2021 by Mooneymite

[bmcconnaha]

Guys, engine is a 27 hour 390.  I’m running give or take 125-150 ROP.  Still a little rich while breaking in.  The CHTs naturally cooled as I flew from Redding CA where it was warmer, into Oregon and then WA and. Please don’t pay much attention to the data before and after the event, I put in the note to savvy that I was Descending after that event and I enrichend the mixture. I was close to an airport.  One side of the ignition system is a Lycoming electronic mag, it's been a while since I've done an in flight mag check, in fact I haven't done one in this plane, but I don't recall a fuel flow reduction to that extent, nor an RPM drop.  I'm not ready to rule out the ignition system, but I find it unlikely.  As for the governor, I wouldn't expect a rise in EGT like that with a drop in rpm.  I leaning more towards a fuel system issue or contamination of the fuel, wheather that's debris, or water, or a potential governor problem, but I'm thinking that's unlikely.  Governor installed was a freshly overhauled unit with the 390. 

[bmcconnaha]

2 hours ago, N201MKTurbo said:

I would check the point gap on the mag. The point cam followers wear and you lose gap. You can usually readjust the gaps and the internal timing without resetting the cam position. I do it every other annual or any time I have the mag off.

One mag is electronic, engine and mags have 27 hours on it.  Both in flight and on the ground mag checks after that event were ok, plane ran smooth on both.  

[bmcconnaha]

4 hours ago, ArtVandelay said:

If really running that rich, maybe just the spark plugs getting clogged with lead.

For comparison, at that altitude I run around 9gph with a TAS of about 150k.

This is an IO390.  Trues out at 163-165.  Still slighty rich, but only 150 or so rich of peak.  Definitely not enough for plug issues, plus all 4 EGTS rose uniformly.  A plug issue would show up as a single EGT rise.  

[PT20J]

Fuel flow is regulated by the servo which only knows mixture control setting and air flow. You'll get the same fuel flow for the same rpm and throttle position whether the engine is running or windmilling -- the servo won't know the difference. A 75 rpm drop is not enough to cause such a drastic drop in fuel flow; it has to be the other way around. Decreased fuel flow and increased EGT are symptomatic of the engine leaning out. Something caused a decrease in fuel flow.

Skip

[bmcconnaha]

4 minutes ago, PT20J said:

Fuel flow is regulated by the servo which only knows mixture control setting and air flow. You'll get the same fuel flow for the same rpm and throttle position whether the engine is running or windmilling -- the servo won't know the difference. A 75 rpm drop is not enough to cause such a drastic drop in fuel flow; it has to be the other way around. Decreased fuel flow and increased EGT are symptomatic of the engine leaning out. Something caused a decrease in fuel flow.

Skip

My thoughts exactly.  My mechanic and I plan to start with the gascolator, clean that out and work our way forward.  I'll remove the cowl and we can take a look and see if we see anyrhing off with the fuel system.  Very strange it ran so well after this event on the next leg, but I'm glad it did.  There isn't much at the small little airport I stopped at in the way of services

[A64Pilot]

I’m going to bet that you will never find the smoking gun as I think it was air or a vapor bubble, I haven’t looked at the data just read your description and it sounds to me like a momentary interruption in fuel flow. 

As the boost was running we should be able to eliminate the mechanical pump, did you happen to notice a drop in fuel pressure during the event?

Try checking the fuel tank vent, if it’s partially clogged it could cause a vapor lock somewhere in the fuel system before the pumps. Of course it could have cleared itself too of the spider nest or whatever may have been there

As you had just switched tanks, is it possible that maybe you didn’t get the valve all the way over?

 

On edit. in trouble shooting I try to concentrate on was anything done just prior to the event that may be connected? You switched tanks so I’d start there.

Another edit, it actually could be water

Edited May 13, 2021 by A64Pilot

[bmcconnaha]

6 minutes ago, A64Pilot said:

I’m going to bet that you will never find the smoking gun as I think it was air or a vapor bubble, I haven’t looked at the data just read your description and it sounds to me like a momentary interruption in fuel flow. 

As the boost was running we should be able to eliminate the mechanical pump, did you happen to notice a drop in fuel pressure during the event?

Try checking the fuel tank vent, if it’s partially clogged it could cause a vapor lock somewhere in the fuel system before the pumps. Of course it could have cleared itself too of the spider nest or whatever may have been there

As you had just switched tanks, is it possible that maybe you didn’t get the valve all the way over?

Good points.  Plane was out on the ramp for a couple days while I was travelling.  I did check the vents before I took off, nothing was apparant.  I took off, burned ten gallons on the left tank and then switched to the right tank where I burned twenty.  I had just switched back to the left tank a gallon or so ago before this happened

[ArtVandelay]

This is an IO390.  Trues out at 163-165.  Still slighty rich, but only 150 or so rich of peak.  Definitely not enough for plug issues, plus all 4 EGTS rose uniformly.  A plug issue would show up as a single EGT rise.  

I should have mentioned my numbers are LOP...additional 5% horsepower would means your fuel flow numbers should be about 0.5 gph higher for same power (%) settings.
Nice to see 390s in the wild.

[bmcconnaha]

Just now, ArtVandelay said:

I should have mentioned my numbers are LOP...additional 5% horsepower would means your fuel flow numbers should be about 0.5 gph higher for same power (%) settings.
Nice to see 390s in the wild.

I haven't explored lean of peak with the 390 yet, as I'm trying to keep it producing as much power as I can during the first 50 hours, but I definitely plan to.  LOP would have enabled me to make it easily non stop from south of San Fran back to norther idaho.  

[PT20J]

Water is a liquid and should not cause such a drastic drop in fuel flow.

A plugged fuel vent should show up after some time as a vacuum builds within the tank as fuel is withdrawn until it exceeds the capability of the pump. It's possible that you were just on the verge of this happening when you switched to the right tank and that it happened when you switched back to the left, but the amount of time you were on the right tank would mean that the left tank would have had to hold vacuum for that amount of time and that would require a really tight seal. 

Sounds like your plan to methodically check the fuel system is sound.

Skip

 

[bmcconnaha]

For those interested.  Savvys reply:

Hi Bryan,
Glad it was a brief episode and appears to have cleared itself.
Based on the limited data we have to work with, we can't be entirely sure of what this was. But it does have all the symptoms of a mixture event as opposed to a ignition event. I believe we can rule out ignition because we would expect to see  the EGT rise corroborated by a drop in CHT, but instead of a drop in CHT we see a small rise in CHT1 & 3 and no change in 2 & 4 - and where only seeing a small rise of 4-6F. And of course we're also seeing a significant momentary drop in FF which supports the lean mixture event theory as since the 80+ RPM drop with no drop in MAP, could cause the FF drop. So although its a very brief event, what we see fits much better with a ROP mixture event showing a brief leaning of the mixture corroborated by a rise in EGT on all 4 cyl, a rise in 2 CHTs (rather than a drop in CHTs)  and by the momentary drop in FF. 
So what could cause this momentary loss FF drop/lean event?
We can't say specifically to this event, but typically when they effect all cylinders as it did here we're looking at a brief fuel contamination event caused by air or water rather than something solid like a piece of lint which would effects a single cylinder.
Its interesting that you mention the boost pump was on, since when we do get a bit of air in the line from the fuel boiling in the line at altitude, causing FF and fuel pressure fluctuations, the boost pump can be used to dampen if not eliminate this. The pump on should lessen the chance of this being from air - unless the pump introduced the air. But the stronger evidence of FF sensor showing a drop, contradicts the water theory and is more supportive of air passing through the line; despite the boost pump being on. 

Most likely we may never see this again, but if we do we'll want to see if we can isolate it to one tank (such as a leaky fuel selector) and if it always happens with the fuel pump on or not. Also I would add checking the drain of your fuel boost pump to see if it drips any fuel after operating on the ground - too the things you already looked at. The aux fuel pump drain is just forward of the gascolator drain under the pilots feet - it should remain dry after operation.

Of course keep us posted if this re-occurs but the vast majority of times we never see these kind of events repeat.

[carusoam]

When a tank vent gets clogged... mud dauber style.... the fuel pumps pull enough fuel that the wing skins oil can...

They start collapsing...

So... when you look out the window and the wing skins look funny... because they are getting crushed...

changing fuel tanks gives some reprieve... but if the other tank vent is equally blocked... you might have equal amounts of time to find the ground...

One mud dauber vent blockage is enough...

+1 for sumping, because you can...

 

Got any other flights you can share data from?

Best regards,

-a-