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Unexplained RPM drop / EGT Rise

Airzen

Well Known Member
Hi Folks,

I am scratching my head about an oscillating RPM drop and EGT rises on a recent flight. The aircraft experienced a series of oscillating drop on RPM (and power) on a climb out.

This is on an RV-10 with a fuel-injected IO-540, with dual PMAGs. This flight was on a hot day ~95-100 Def F OAT at KSZT. The airplane was performing flawlessly to KSZT prior to this flight - KSZT was a fuel stop and this episode happened on the outbound flight.

Here is the snapshot from Savvy:

image:
NKxFi8b.png



A few observations:

1) The fuel pressure is above 20 throughout the episode. (The fuel boost pump was on during initial climb out, turned on again after a few secs as it dropped to 15 momentarily 12:45 to 13:00 mark)
2) EGTs on All the cylinders rise and fall together.
3) The RPM oscillations and the EGT oscillations seem to be correlated.
4) I have not yet seen a similar behavior after this episode. [The aircraft returned to KSZT, spent a night and departed the following morning after a thorough inspection / run-up etc.]

Due to #1 I am inclined to believe that Fuel pressure was not a problem. Is vapor lock / bubbling fuel still a possibility and cause?

Due to #2 It appears whatever the reason is it affected the entire ignition system (both Mags) and that too in-synchrony. I am unable to figure out what - given that both the mags are independent power sources.


Any suggestions / thoughts on where I should be looking?


Thanks,
Ashish
 
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I suspect a batch of bad gas with water in it. Looks like the engine is intermittently losing a significant RPM on each event. Also look at the downward trend on MAP over the chart.

I would check the two filters for debris (one is in the servo) and do a test flight on the fresh fuel. Did you check for water via the sump drains after topping off? If you find a lot of debris in the filters, it would be prudent to check all of the restrictors as well.

Gulping slugs of water out of the tanks will produce results like this and explains why the problem disappeared after a while - eventually all of the water was removed from the tank. It settles to the bottom over time and gets worked out fairly early in the process of using the fuel in that tank. Substituting water for fuel will drop RPM and raise EGT or lower it depending upon how lean it is and what your mixture was set at.

Most storage tanks have a layer of water and sediment on the bottom, but pickup is a few inches up so you don't get any of it. Truck shows up and dumps 5000 gallons of fuel into the tank, mixing that water and sediment with the fuel. It takes some time for it to settle back out and if you filled up during that time, you get the water and sediment in your tanks and over time it settles to the bottom of your tank and sucked into the engine. Could also be a poorly maintained tank that gets new water every time it rains. Good pumps have filters for the sediment, but the water goes right through them.

Larry
 
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Gosh, I have had an issue somewhat similar but not exactly but as hard to explain.

On our way to Oshkosh, one stop before Wisconsin in Springfield Ohio which we had a very short stop for fuel, we seem to have an issue at the runup with our Left PMAG. At the runup the right mag was OK but the left mag would drop so many RPM that I thought the engine would die so I would turn the key quickly to prevent it from dying. At the time, this did not happen if the ship power to the PMAG was turned on so I thought for sure the generator was bad. With that, we proceeded with the last leg and in Osh we got a loaner from Brad and replaced the left mag. The runup test after the install was all good with no indication of any issues.
At the end of the week as we were heading home we stopped in Ohio again for our first fuel stop, a different airport but very close to Springfield, the left mag again seemed to be failing at the runup. Now this was with the loaner Mag installed so it could not have been the mag to be the source of issue. During the failure mode at both incident, the EGT on all four cylinder would rise so it is not a case of one or two cylinder failing.
For the rest of the trip and two more fuel stop, I could not get the Left mag to fail again both in the air or on the ground.
Back home, I reinstalled my original mag back on and at the runup all was good. I have checked the sparkplug wire and sparkplugs and no issue have been found.
I have gone over the engine log and at all EGTs rise about the same during the time of failure and the RPM drop seem to be around 300

So, right now my suspicion points to either hot fuel on the ground or an intermittent coil failure.
 
I would send the data to Savvy Aviator (Mike Busch). He’ll probably look at it and tell you right away what it is. Gotta love recording engine monitors. A few minutes of data review vs weeks of trouble shooting!
 
water / MAP etc

I suspect a batch of bad gas with water in it. Looks like the engine is intermittently losing a significant RPM on each event. Also look at the downward trend on MAP over the chart.

I would check the two filters for debris (one is in the servo) and do a test flight on the fresh fuel. Did you check for water via the sump drains after topping off? If you find a lot of debris in the filters, it would be prudent to check all of the restrictors as well.

Gulping slugs of water out of the tanks will produce results like this and explains why the problem disappeared after a while - eventually all of the water was removed from the tank. It settles to the bottom over time and gets worked out fairly early in the process of using the fuel in that tank. Substituting water for fuel will drop RPM and raise EGT or lower it depending upon how lean it is and what your mixture was set at.

Most storage tanks have a layer of water and sediment on the bottom, but pickup is a few inches up so you don't get any of it. Truck shows up and dumps 5000 gallons of fuel into the tank, mixing that water and sediment with the fuel. It takes some time for it to settle back out and if you filled up during that time, you get the water and sediment in your tanks and over time it settles to the bottom of your tank and sucked into the engine. Could also be a poorly maintained tank that gets new water every time it rains. Good pumps have filters for the sediment, but the water goes right through them.

Larry

Thanks for your suggestions, Larry. The tanks were sumped before the flight and no water was observed. In fact, after landing the FBO themselves poured avgas in a bucket to check for any debris/water. None was found.

Also, the decrease in MAP is due to altitude. Here is MAP and Altitude plotted together:

C42Idks.png


Ashish
 
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Thanks for the suggestion.

I would send the data to Savvy Aviator (Mike Busch). He’ll probably look at it and tell you right away what it is. Gotta love recording engine monitors. A few minutes of data review vs weeks of trouble shooting!

Thanks for the suggestion. I dropped Mike a note. I'll update if I hear back.
 
vapor

Thanks for the suggestion. I dropped Mike a note. I'll update if I hear back.

In spite of the pump running, my vote is fuel vapor. All the symptoms fit. You were climbing, so not lean of peak. Bubbles in fuel due to heat and altitude and higher vapor pressure, Bubbles causing leaner mixture to all cylinders and consequent higher EGTs. It will be interesting to see what Saavy says. I run mogas/avgas mixture and this has happened to me, causing me to use more avgas in the mix, solving it during these very hot months.
Ed
 
Thanks for your suggestions, Larry. The tanks were sumped before the flight and no water was observed. In fact, after landing the FBO themselves poured avgas in a bucket to check for any debris/water. None was found.

Also, the decrease in MAP is due to altitude. Here is MAP and Altitude plotted together:

C42Idks.png


Ashish

seems there are clear and distinct interruptions in power at somewhat regular intervals. Can't really tell from the data if they are complete stoppage of combustion as the cause or just power reductions. The RPMs hit a sharp bottom and return, which makes me think short intervals of complete loss of combustion.

In my experience, vapor lock doesn't present like this. It is typically a more general rough running as opposed to sharp spikes of power loss. When it happens after a short turn around, it is usually worse at TO and starts to get better as cooler fuel goes through the system and the fuel transport material starts to cool down.

From the chart, it looks like EGTs rise, then fall followed by the sharp drop in RPM. This would seem to point to a loss or reduction in fuel flow to the servo. I am struggling to come up with a reason, but don't see vapor lock behaving like this. It is usually a more broad problem and not defined spikes like this. That of course doesn't mean it didn't happen that way for you. Also, if the boost pump is in the cabin, it is not common to get serious vapor lock issues with the boost pump on and running 100LL.

If both ignitions were cutting out, the EGT wouldn't rise; It would drop and then come back when restored. If one ignition dropped, EGT would rise and RPM would only drop around 100-200 RPM. Fuel starvation will often show a rise and then fall of EGT, depending upon how abrupt the reduction was. You should look more closely at your data to see the EGT behavior relative to RPM. I don't have vertical lines in your pic, so hard to analyze.

My read of the data says that this is fuel interruption, but can't speculate on the cause beyond the water theory. If the water was fully mixed with the gas, you wouldn't see much of it if you drained them immediately after filling. It takes some time for the water to separate and collect at the bottom. Also, the FBO checking 2 hours later is not definitive, as by that time the water would have settled back to the bottom of the tank and they wouldn't see it in their sample. A better question would be "Was the storage tank topped off anywhere near the time that I pumped my gas?"

Larry
 
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Hi Folks,

I am scratching my head about an oscillating RPM drop and EGT rises on a recent flight. The aircraft experienced a series of oscillating drop on RPM (and power) on a climb out.
Ashish

Maybe this is obvious, but did you hear or feel these rpm variations and see them on your instruments, or did everything seem ok and you saw these variations in the data postflight?
Reason I ask is that I recently saw wide rpm excursions on my GRT EIS data, but the flight was completely normal...
 
Hi Folks,

I am scratching my head about an oscillating RPM drop and EGT rises on a recent flight.
...
Any suggestions / thoughts on where I should be looking?

I'd like to see the fuel flow, oil temp, and altitude (among other parameters). If you post the URL from this specific Savvy flight, we'll be able to zoom in/out, select parameters, etc.
 
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