I have a bit of a head scratcher and any input would be appreciated. Every now and again my fuel flow drops suddenly by 2-3gph before bouncing back to the previous setting. When in cruise this results in the engine almost cutting out. Very scary. I initially thought my fuel pump was going out but I started trouble shooting and can now report symptoms. My A&D isn’t too sure what’s going on so I thought I’d throw it out here to see if anyone else has any ideas.

Process
1) fuel flow drops by 2-3gph and bounces back to the original setting. If left alone this continues to happen every 5-10 seconds
2) It generally starts to happen 20-30 minutes after take off and it’s almost always on the left tank. It doesn’t happen on every flight and it is not predictable or, so far, replicable on demand. It’s happened once on the right tank but that was after I switched from the left tank ... could be isolated to the left tank but can’t be 100% sure
3) I’ve had it happen at full power during climb, with fuel flow at Full power the effect on the engine is not noticeable, leaned out in cruise though it will almost cut the engine
4) turning on the aux pump does not stop the bouncing, the aux pump boosts fuel flow by around 3gph so it solves the engine power issue but not the bouncing
5) switching tanks DOES solve the issue
6) I’ve noticed it happening the most when tanks are completely full. We initially thought there was a venting issue resulting in back pressure but the fuel caps are fine and functional and there is no oil canning on wings...although I don’t think I’d see this anyway since I have bag fuel tanks (not wet wing)
7) I initially though it was icing but induction heat does not fix it

I’ve linked a Dropbox video below (forum won’t let me add video) of the behavior for illustration. Before taking this video I was leaned out at 11.5gph and the bounce felt like it was going to cut the engine. I richened up the mixture to solve the cutting problem but did not switch tanks so I could film it for my A&P

www.dropbox.com/s/zivi2rqjygzezn3/IMG_4188.mov?dl=0

Any thoughts before we pull apart the entire fuel system would be appreciated.




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You’re aware of the “frothy header tank” issue in the early 205/206/210s, right? If it’s scary but otherwise benign and is solved by switching tanks, I’d bet that’s what this is.

skiermanmike wrote:You’re aware of the “frothy header tank” issue in the early 205/206/210s, right? If it’s scary but otherwise benign and is solved by switching tanks, I’d bet that’s what this is.

Never heard of it. Any reference you have would be helpful. Thanks for the quick reply btw!


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Ozzyblue wrote:

skiermanmike wrote:You’re aware of the “frothy header tank” issue in the early 205/206/210s, right? If it’s scary but otherwise benign and is solved by switching tanks, I’d bet that’s what this is.

Never heard of it. Any reference you have would be healpful


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Someone will chime in with a more technical explanation, but my understanding is that there are two header tanks roughly underneath the pilots feet. The fuel return line in the 205s goes to that header tank, not the main tank and sometimes - typically when it’s hot or after a long sustained climb - the combination of vibration and warm fuel returning from the engine will cause the fuel in the header tank to become frothy, i.e. full of bubbles.

When the fuel pump starts to pump those bubbles, you get those fuel flow fluctuations, and since the other header tank has not been frothed by the warm returning fuel, switching tanks causes the problem to go away.

When I bought my 205, the seller warned me that this would eventually happen to me and it would scare the pants off me. When I got checked out in the plane, the CFI (who had a lot of early 206 time in the bush) also warned me about this problem. They both said hot, long sustained climbs or long high-altitude cruise was the typical scenario, and it has, in fact, happened to me a few times. The solution is to switch tanks, and the problem goes away.

skiermanmike wrote:

Ozzyblue wrote:

skiermanmike wrote:You’re aware of the “frothy header tank” issue in the early 205/206/210s, right? If it’s scary but otherwise benign and is solved by switching tanks, I’d bet that’s what this is.

Never heard of it. Any reference you have would be healpful


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Someone will chime in with a more technical explanation, but my understanding is that there are two header tanks roughly underneath the pilots feet. The fuel return line in the 205s goes to that header tank, not the main tank and sometimes - typically when it’s hot or after a long sustained climb - the combination of vibration and warm fuel returning from the engine will cause the fuel in the header tank to become frothy, i.e. full of bubbles.

When the fuel pump starts to pump those bubbles, you get those fuel flow fluctuations, and since the other header tank has not been frothed by the warm returning fuel, switching tanks causes the problem to go away.

When I bought my 205, the seller warned me that this would eventually happen to me and it would scare the pants off me. When I got checked out in the plane, the CFI (who had a lot of early 206 time in the bush) also warned me about this problem. They both said hot, long sustained climbs or long high-altitude cruise was the typical scenario, and it has, in fact, happened to me a few times. The solution is to switch tanks, and the problem goes away.

Wow, you described the exact scenarios I’ve had it happen in, long sustained climbs and leaned out high alt cruise. Thanks SO much for the explanation, it’s the first I’ve heard of it but that makes a lot of sense. I wonder if there’s something about the left header tank that makes it more susceptible to this behavior? I’ll show this to my A&P this week. Thanks mate!!


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Here’s a mention of the problem I just found in a quick search. In that article they framed it as mostly affecting turbo’ed 210s, but my understanding is that it was all the early 206s, 205s (which is a 210), and 210s, and not necessarily just the turbocharged ones either.

https://www.avweb.com/ownership/a-lette ... 10-owners/

See the section under “the loudest silence in the world”

skiermanmike wrote:Here’s a mention of the problem I just found in a quick search. In that article they framed it as mostly affecting turbo’ed 210s, but my understanding is that it was all the early 206s, 205s (which is a 210), and 210s, and not necessarily just the turbocharged ones either.

https://www.avweb.com/ownership/a-lette ... 10-owners/

See the section under “the loudest silence in the world”

Thanks again. Read the whole thing and now have a laundry list of fuel issues to check out. That vapor lock issue described sounds exactly like what I’m dealing with. It does sound like the turbo 210’s were more susceptible but since my 205 is basically a 210 it would make sense that it would have similar issues given similar circumstances. Gonna have to look in to the modification mentioned to the exhaust/fuel line relationship.

Thanks again, this was exceptionally helpful.


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