Power Loss on Climb Out

[FlyingMonkey]

Following trying to troubleshoot my high CHT problem, I replaced the CHT probe and heat soaked the engine on the ground very well.  I got the oil temp up to about 200°F, and the CHT stabilized at about 205-210°F.  I decided to fly it to test it in the air.  No issues on the ground, run-up, etc.  Once I got to full power for takeoff and beginning the climb I didn't see the CHTs going up as they had before.

However, on climb out at about 200-300ft I had a momentary loss of engine power, just a dip in rpm for maybe a second.  I lowered the nose and power came back.  I reduced power to idle and landed on the remaining runway without incident (but with a slight pucker factor).

A couple of days earlier I had done a full rpm static run-up to 5000rpm+, no issues.  back in the hangar I pushed the tail to the floor and drained fuel to see if there was any fuel restriction or fuel system vent obstruction.  I let the fuel drain into a jug for about a minute, no loss of fuel flow at all.  That should have been more than long enough to drain the lines, carb bowls, and gascolator and uncover any issues.  I don't think there are any fuel bottlenecks.

So here's my working hypothesis:  I have a failing fuel pump.  It's good enough to keep up in level flight or on the ground, but under a full power climb, the deck angle increases, the pump height gets closer to the fuel tanks, gravity head pressure decreases, and the pump can't keep up.  This might even account for my higher CHTs if the cylinders are running lean as a result of mild fuel starvation.  My fuel pump is about 8 years old.

Thoughts?

[Tom Baker]

I'm not sure where you were draining fuel, but the carb bowls will not drain. I'm not sure how much drainage you would get from the engine side of the gascolator. 

[FlyingMonkey]

2 minutes ago, Tom Baker said:

I'm not sure where you were draining fuel, but the carb bowls will not drain. I'm not sure how much drainage you would get from the engine side of the gascolator. 

Okay...but what do you think of the fuel pump idea?  Is there something else you'd check first?  I was draining from the gascolator valve.

[Garrett Gee]

1 hour ago, FlyingMonkey said:

Okay...but what do you think of the fuel pump idea?  Is there something else you'd check first?  I was draining from the gascolator valve.

About a year after I got my airplane, I started to have some low fuel pressure warnings right after rotation and during initial climb.  Never got a power loss, but the fuel pressure alert on the Dynon will make you pucker.  I started going through the logs and saw the fuel pump had not been replaced, even though Rotax had a 5 year replacement interval.  I am not (typically) about removing a good part from service because of a time limit, but it seemed logical in this case.  My pump was about 9-10 years old at the time, and replacing the fuel pump solved the problem. Not saying that is the issue, because your scenario is a little different with the stutter, but just my 2 cents.

[FlyingMonkey]

16 minutes ago, garrettgee2001 said:

About a year after I got my airplane, I started to have some low fuel pressure warnings right after rotation and during initial climb.  Never got a power loss, but the fuel pressure alert on the Dynon will make you pucker.  I started going through the logs and saw the fuel pump had not been replaced, even though Rotax had a 5 year replacement interval.  I am not (typically) about removing a good part from service because of a time limit, but it seemed logical in this case.  My pump was about 9-10 years old at the time, and replacing the fuel pump solved the problem. Not saying that is the issue, because your scenario is a little different with the stutter, but just my 2 cents.

Good info, thanks.  I’m going to replace the pump regardless, it’s probably at the end of a normal service life at this point.

[FredG]

Any chance you had high(er) vapor pressure winter blend gas in the airplane (which is more volatile than summer blend fuel and more prone to vapor lock)?  Heat soaking the engine on the ground followed by a full power take-off with winter blend gas is a bit of a vapor lock stress test.  

According to the EPA, " The federally mandated dates for summer-grade gasoline and reformulated gasoline, where required, are May 1 to September 15 for refiners and terminals, and June 1 to September 15 for gasoline retailers."  ( https://www.eia.gov/todayinenergy/detail.php?id=11031 )

Edited May 18, 2023 by FredG

[FlyingMonkey]

16 minutes ago, FredG said:

Any chance you had high(er) vapor pressure winter blend gas in the airplane (which is more volatile than summer blend fuel and more prone to vapor lock)?  Heat soaking the engine on the ground followed by a full power take-off with winter blend gas is a bit of a vapor lock stress test.  

According to the EPA, " The federally mandated dates for summer-grade gasoline and reformulated gasoline, where required, are May 1 to September 15 for refiners and terminals, and June 1 to September 15 for gasoline retailers."  ( https://www.eia.gov/todayinenergy/detail.php?id=11031 )

I thought about vapor lock.  If it happened I don’t think it’s a blend issue…I’m in Georgia and I don’t think the seasonal blends here are hugely different.  And I’ve flown this airplane in this area for a decade and never had a similar issue.

I did replace the steel gascolator bowl with an aluminum one, and replaced the heat shield for the gascolator with a new one (the same size & shape) I made from T61-T6 aluminum sheet.  I doubt these changes cause a vapor issue, but I suppose it’s possible.
 

The gas I used was purchased 05/10/2023.

[cdarza]

Check your carb bowls for any contamination -  Ive had momentary loss of power/engine vibration from dirty fuel in the bowl as a result of corrosion in the bowl.   And less likely the cause but still something to check - are your vent balance tubes properly attached.  

[FlyingMonkey]

7 minutes ago, cdarza said:

Check your carb bowls for any contamination -  Ive had momentary loss of power/engine vibration from dirty fuel in the bowl as a result of corrosion in the bowl.   And less likely the cause but still something to check - are your vent balance tubes properly attached.  

I just had the bowls off within the last week for my annual, they were super clean.  I can check them again, but I don’t think that’s it.

[Tom Baker]

9 hours ago, FlyingMonkey said:

I just had the bowls off within the last week for my annual, they were super clean.  I can check them again, but I don’t think that’s it.

Didn't you just do your hose replacement? That is a perfect time to introduce debris. 

[FlyingMonkey]

5 minutes ago, Tom Baker said:

Didn't you just do your hose replacement? That is a perfect time to introduce debris. 

I did, but there are no rubber fuel hoses anymore, they are all now professionally-made teflon hoses with AN fittings (no more barbs) and they were all blown out with compressed air before installation.  As I mentioned, I did a full rpm WOT ground run up and it ran great.  I would think if there were a debris problem it would manifest whenever going to WOT.

For completeness I'll drop the bowls and check when I replace the pump.

[Anticept]

If you have a long runup on a hot day, this very much can introduce vapor.

One of the things about the pre-injected CT designs is that there isn't enough head pressure with low fuel tanks to push fuel past vapor in the lines. I confirmed this by opening the gascolator after draining the lines and had to fill over 5 gallons before flow finally started. Tested again by adding just 5 gallons even and nothing. I had to add just a bit more for it to work.

Then I disconnected the hose up next to the firewall.

I only had to add a gallon and it started gushing.

The line routing in a flight design is a terrible trap in multiple places for air. Both the LS and the SW have lines that dip down then come back up to route to the fuel shut off, in line fuel filter, and any flowmeters if installed. This makes that area an air trap. Air traps are terrible things in engineering and cause a lot of problems, anyone working in pipelines will tell you that air traps act like huge restrictions to flow and it takes substantially more pressure to displace them, especially once there is fluid *past* the air trap.

Your SW has fuel lines that go out from the A pillar into the engine bay before looping back inside. So you not only have an air trap, but also are warming the fuel a little early on.

I'm not experimental yet, and this is on my list of things to do, but maybe you can get a head start.

The return line that goes down to your gascolator: Splice in a little see-through line that's only 6mm or 1/4 inch transparent hose. Take it back into the cabin and up the A pillar to your left tank and attach it to the top of your fuel level indicator. During operation, watch the line and see how much air and fuel flow back. It won't flow fast, there should be a restrictor orifice in the distribution block. I am willing to bet the longer your run up, the more air bubbles will start to flow through it, and I bet there will be a huge one on takeoff.

What I am hoping is this would accomplish 2 things: It would allow head pressure to displace air more easily by giving it somewhere to go from the gascolator other than trying to push it through the carbs, thus stabilizing fuel pressure, AND air that does get pushed to the distribution block should very quickly squeeze through the restrictor and exit up through your vent line to your left fuel tank.

I should note as well, the fuel pump that we use now on Rotax has the inlet on the top and outlet on the bottom. This makes it quite difficult for the pump to purge air thats introduced into it. If we give the air somewhere to go with this vent line, it should be able to eject that air quickly before it is a problem.

[FlyingMonkey]

Corey, thanks dor the long discussion on vapor.  I have noted that since I changed hoses my new upper hose behind the panel (the one just before the fuel valve) has a slight dip then comes up before curving back down to get to the valve.  I wonder if that is my problem...

I can have a new hose made that's shorter and mostly avoids that, but I might also need a 90-180° AN fitting on the valve to make it work right.  I never liked the existing routing, so that might be worth it.

[Anticept]

The most important thing will likely be my recommendation to install a venting hose.

Right now, the only place that this air can go is through the carbs. It just keeps getting recycled until it purges. So on a long run up, you're cooking everything firewall forward, making the gasoline more volatile.

The rotax recommended installation is to have a return hose to the fuel tanks. The restrictor doesn't allow much fuel to return to the gascolator, but air will purge VERY quick as soon as it hits it. The accepted method is to return to the gascolator and honestly, I think traditionally ventilated cowlings would work fine with this (think open space above the engine like 172s), because the fuel lines would be exposed to incoming air to cool them too.

Not having any return line is prohibited, because the return like contributes to pressure regulation.

Finally, as one more point to make: the fuel system goes from top down. That means any vapors that form are applying upward pressure on the system. The fuel not only has to deal with pushing past all the lengths of fuel lines and equipment, but it also now has to deal with the resistance offered by vapors, as well as rising back up and over the engine before it finally reaches a fuel pump.

Add in the pitch up attitude on takeoff, and you've further reduced head pressure.

The long term solution would either be to use a less volatile fuel, or to install a fuel pump (A project I am struggling to find any useable pump that fits rotax's parameters and isn't some 15 pound power sucker)

EDIT: I JUST FOUND A CANDIDATE.  Facet 477060E

I know some piper aircraft use this type (but not brand) of fuel pump so I think it's worth a try.

They have the ability to flow fuel through them when turned off too.

These have integrated fuel filters. I would replace the fuel filter in the cockpit with one of these. Any fuel pump without a filter would probably have to go after the gascolator to reduce the risk of damage from debris.

I'd also put a bypass valve in while experimenting just in case the thing acts up so you have a way to send fuel directly so you can get back to safety.

https://www.pegasusautoracing.com/2021/046.pdf some more pumps and part numbers.

[FlyingMonkey]

There's a vent hose off the fuel pump on my CTSW.

I have not heard of anybody having vapor issues with CTs, so I'm not sure it's worth adding electric pumps and other shenanigans to the fuel system to solve a very rare issue.

I'm not an engineer, this is just my impression.

[Tom Baker]

There have been plenty of vapor issues with the CT's.

Does your new fuel lines have the integrated fire sleeve that is molded to the line, or does it have the traditional fire sleeve with band-it clamps on the ends? 

[Anticept]

I have six CT's here at OSU that have all had issues with vapor. One was a CTSW.

I am pretty sure you are having vapor issues as well.

I have also had several others on this forum call me to ask for advice.

I have also worked on a sport cruiser which DOES have a boost fuel pump and has never had any of the issues with fuel pressure alarms or dips in power on takeoff. That airplane went to Illinois, but there's another one here that I don't work on, and I asked him questions about his. No alarm issues as long as he uses the boost pump.

The reason why I think it's vapor? Mix in about 15-20% avgas and it's a quiet happy engine as far as fuel pressure goes. Vapor pressure for avgas is limited to 7psi. Autofuel can be more than twice that *while at room temperature*.

I went down to SC earlier this year in March/April too and it was a good hot day compared to the freezing temps in ohio. My father wanted to go fly. We couldn't even leave the pattern, I had constant fuel pressure and fuel flow alarms and no throttle setting seemed to fix it. I'm on the ramp in that sun tearing the airplane apart thinking somethings messed up before remembering to try addressing the vapor with avgas first. In went 3 gallons a side... happy as can be, no more alarms.

When I say vent hose, I am not referring to the one on the fuel pump. That's for the dry side of the diaphragm. I said vent hose to refer to modifying the return line to send air back to the fuel tank. By default, these airplanes ship from the factory with a return going to the gascolator. Vapor has no where to go but out the carbs, and the newest fuel pumps can't purge vapor effortlessly like the old pierburg did.

[FlyingMonkey]

2 hours ago, Tom Baker said:

There have been plenty of vapor issues with the CT's.

Does your new fuel lines have the integrated fire sleeve that is molded to the line, or does it have the traditional fire sleeve with band-it clamps on the ends? 

It's teflon, then stainless braided sheath, and finally a molded integrated silicone firesleeve. 

I guess I just haven't heard of others having loss of power in flight due to vapor issues.

[Tom Baker]

20 minutes ago, FlyingMonkey said:

It's teflon, then stainless braided sheath, and finally a molded integrated silicone firesleeve. 

I guess I just haven't heard of others having loss of power in flight due to vapor issues.

That style hose may not provide as much resistance to becoming heat soaked leading to vapor issues compared to the traditional fire sleeve.

[Tom Baker]

2 hours ago, Anticept said:

I have also worked on a sport cruiser which DOES have a boost fuel pump and has never had any of the issues with fuel pressure alarms or dips in power on takeoff. That airplane went to Iowa, but there's another one here that I don't work on, and I asked him questions about his. No alarm issues as long as he uses the boost pump.

Maybe Illinois? I have a Sport Cruiser here in my shop that has your signature in the logbooks.

[Anticept]

Sure is. Owner initials LG.

Also I meant to say illinois. I am.not sure why I said Iowa!

[FlyingMonkey]

10 hours ago, Tom Baker said:

That style hose may not provide as much resistance to becoming heat soaked leading to vapor issues compared to the traditional fire sleeve.

That seems possible.  the braiding would certainly retain heat better than rubber once it warms up.

13 hours ago, Anticept said:

I have six CT's here at OSU that have all had issues with vapor. One was a CTSW.

I am pretty sure you are having vapor issues as well.

I have also had several others on this forum call me to ask for advice.

I have also worked on a sport cruiser which DOES have a boost fuel pump and has never had any of the issues with fuel pressure alarms or dips in power on takeoff. That airplane went to Illinois, but there's another one here that I don't work on, and I asked him questions about his. No alarm issues as long as he uses the boost pump.

The reason why I think it's vapor? Mix in about 15-20% avgas and it's a quiet happy engine as far as fuel pressure goes. Vapor pressure for avgas is limited to 7psi. Autofuel can be more than twice that *while at room temperature*.

I went down to SC earlier this year in March/April too and it was a good hot day compared to the freezing temps in ohio. My father wanted to go fly. We couldn't even leave the pattern, I had constant fuel pressure and fuel flow alarms and no throttle setting seemed to fix it. I'm on the ramp in that sun tearing the airplane apart thinking somethings messed up before remembering to try addressing the vapor with avgas first. In went 3 gallons a side... happy as can be, no more alarms.

When I say vent hose, I am not referring to the one on the fuel pump. That's for the dry side of the diaphragm. I said vent hose to refer to modifying the return line to send air back to the fuel tank. By default, these airplanes ship from the factory with a return going to the gascolator. Vapor has no where to go but out the carbs, and the newest fuel pumps can't purge vapor effortlessly like the old pierburg did.

You guys are starting to convince me of the vapor theory.  Has any other CT besides mine had any rpm loss like I have, or just fuel pressure/flow alarms?

It sounds like the easy troubleshooting here is to add a few gallons of avgas to each wing and go fly and see if the problem recurs.  If not, it's probably vapor, if it does recur, I'll replace the fuel pump and try again.

Longer term, it seems like either avgas mixing or some type of aux pump might be in my future.  Would the pump only be used during takeoff, climb, and landing, or left running continuously?  Even though that Facet pump is sold on ACS, it's marked "not for use in aircraft".  Hmm.  Experimental, right?  Do you know if the pump would "fail open" and allow flow through it if the pump fails?

Regarding venting, don't the carb vent lines running through the airbox allow air to vent from the system?  Is the argument that those are not adequate?  IIRC there is a return line going to the gascolator, but as you mentioned there's a restrictor on it.

[Anticept]

The pumps internals would have to be investigated as to its failure modes. It's why I suggest a bypass valve until all of that is investigated. You could even put a checkvalve in parallel if you wanted.

I suspect its a spring return piston pump. Same kind of design as a brake master cylinder

They're marked not for aircraft use, but so are NGK plugs.

Also, try to make sure that you get an E type pump if you order one. Those have the solid state pressure controls, the non E version is mechanical.

[FlyingMonkey]

7 minutes ago, Anticept said:

The pumps internals would have to be investigated as to its failure modes. It's why I suggest a bypass valve until all of that is investigated. You could even put a checkvalve in parallel if you wanted.

I suspect its a spring return piston pump. Same kind of design as a brake master cylinder

They're marked not for aircraft use, but so are NGK plugs.

Also, try to make sure that you get an E type pump if you order one. Those have the solid state pressure controls, the non E version is mechanical.

The one at ACS is marked 477060E so it should be correct.

I guess I'll do more troubleshooting using avgas and determine if it's actually a vapor issue then go from there.  Being a test pilot is fun! 

Honestly adding a pump, wiring, and valves to the fuel system is more re-engineering than I'd like, but it is what it is.  It would be nice to have fuel pressure and flow instrumentation in my airplane for issues like this.

[procharger]

I had an issue one time on a really hot day. Shut down for about 5 minutes took off and lost a 

some RPM on climb pulled back throttle and it caught back up within a few seconds no issues

after that. Thought I was going to put it down in the water. Once in only 7 years of flying CT.