Fuel smell while descending.

simtech

simtech

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Simtech
When I descend I smell fuel in the cockpit. only descending, never in level flight. Today I was at the plane to drop of the keys as my mechanic is not in town right now. So I flipped the fuel pump on for a bit thinking I might start to smell fuel.. I didn't. What possibilities are there..fuel valve...primer...but again I only get the smell in a descent so it seems odd.

My mechanic might not get to it until next week so im grounding my plane until he looks at it:sad:
 
Are you switching tanks,in preparation for landing,could be the selector valve.
 
I didn't as I was already on the fullest. Its happened twice now. The last flight I hadn't even turned on the fuel pump yet. Was descending out of 5500 in a 500ft per minute descent and I started to smell it. I didn't say anything to my wife as I didn't know what she would think. Then she piped up and asked if I smelled the fuel. so It was rather noticeable.
 
simtech said:
When I descend I smell fuel in the cockpit. only descending, never in level flight. Today I was at the plane to drop of the keys as my mechanic is not in town right now. So I flipped the fuel pump on for a bit thinking I might start to smell fuel.. I didn't. What possibilities are there..fuel valve...primer...but again I only get the smell in a descent so it seems odd.

My mechanic might not get to it until next week so im grounding my plane until he looks at it:sad:
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The vent or a tank fitting is leaking into the wing when nose own, and the fumes are coming in through the root?:dunno: Can you find any blue stains anywhere?
 
Henning said:
The vent or a tank fitting is leaking into the wing when nose own, and the fumes are coming in through the root?:dunno: Can you find any blue stains anywhere?
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No I haven't seen any blue stains. But honestly I haven't looked hard around the tanks or under them. The selector valve leaked at some point as the carpet has some blue around it but that stain has been there since I owned the plane and this just started.
 
simtech said:
No I haven't seen any blue stains. But honestly I haven't looked hard around the tanks or under them. The selector valve leaked at some point as the carpet has some blue around it but that stain has been there since I owned the plane and this just started.
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Are there inspection ports or fairings inboard of the tanks you can remove to look? Remember, it doesn't take much fuel at all to create an odor in the cabin. You may just be looking for a small streak or splotch.
 
Henning said:
Are there inspection ports or fairings inboard of the tanks you can remove to look? Remember, it doesn't take much fuel at all to create an odor in the cabin. You may just be looking for a small streak or splotch.
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I don't think so, think they are only outboard. I do have the wing root seals? Wait...yes I do..the ones that go to the struts. Ill look there. Now its 6 hours out of annual and I smelled once about 2 hours after but didn't think much of it. This last flight I figured since I smelled it again Im getting it looked at and wont fly until we find the problem.
 
Struts? This isn't the airplane in your avatar?

Leaking vent crossovers will take fumes into the cabin. They're often at the front of the tank and if there's enough fuel in the tanks, they'll leak a bit when the nose is down. Most commonly it involves short sections of rubber hose used to join the sections of aluminum vent tubes together, and those hoses most frequently never get looked at because they're in places that take a lot of time to get at. Once in a while an aluminum vent tube will corrode and end up with tiny pinholes that let fuel out in miniscule quantites; I just replaced two such tubes in a Cardinal.

I have also found cracked tanks that will let a bit of fuel out in certain attitudes, depending where the crack is located.

I once had a fuel stink in a 150. Couldn't isolate it, mostly because it was rather intermittent. Finally made an elephant nose out of three feet of 1/4" nylon tubing and a bit of thin rubber diaphragm material. Cut the rubber and made a small funnel, small end taped to the tubing and the other end trimmed to fit closely over and around my nose. Poked around with the other end of the tube while sniffing, and behold! a primer line fitting at the firewall was seeping just enough to stink but not enough to drip. The fuel evaporated faster than it could accumulate. I have since used that doodad many times to pinpoint leaks. Definitely not OSHA approved, I'd bet.

Dan
 
Dan Thomas said:
Struts? This isn't the airplane in your avatar?

Leaking vent crossovers will take fumes into the cabin. They're often at the front of the tank and if there's enough fuel in the tanks, they'll leak a bit when the nose is down. Most commonly it involves short sections of rubber hose used to join the sections of aluminum vent tubes together, and those hoses most frequently never get looked at because they're in places that take a lot of time to get at. Once in a while an aluminum vent tube will corrode and end up with tiny pinholes that let fuel out in miniscule quantites; I just replaced two such tubes in a Cardinal.

I have also found cracked tanks that will let a bit of fuel out in certain attitudes, depending where the crack is located.

I once had a fuel stink in a 150. Couldn't isolate it, mostly because it was rather intermittent. Finally made an elephant nose out of three feet of 1/4" nylon tubing and a bit of thin rubber diaphragm material. Cut the rubber and made a small funnel, small end taped to the tubing and the other end trimmed to fit closely over and around my nose. Poked around with the other end of the tube while sniffing, and behold! a primer line fitting at the firewall was seeping just enough to stink but not enough to drip. The fuel evaporated faster than it could accumulate. I have since used that doodad many times to pinpoint leaks. Definitely not OSHA approved, I'd bet.

Dan
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Great Idea....:thumbsup::thumbsup:..

I have heard of homemade stethascopes,, but stinkascope...;):D
 
If you don't have any luck finding the leak, ask your local hvac guy to bring his combustible gas leak detector out to the airport. They work very well on 100LL. Don't fly until you find the source. I would try to simulate on the ground, your descent attitude plus a little extra due to sloshing in flight. We have had at least three RV-10's with fuel leaks in flight. One exploded after turning off the runway, one caught fire in flight and the engine quit on rollout on the third.
 
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Dan Thomas said:
Struts? This isn't the airplane in your avatar?

Leaking vent crossovers will take fumes into the cabin. They're often at the front of the tank and if there's enough fuel in the tanks, they'll leak a bit when the nose is down. Most commonly it involves short sections of rubber hose used to join the sections of aluminum vent tubes together, and those hoses most frequently never get looked at because they're in places that take a lot of time to get at. Once in a while an aluminum vent tube will corrode and end up with tiny pinholes that let fuel out in miniscule quantites; I just replaced two such tubes in a Cardinal.

I have also found cracked tanks that will let a bit of fuel out in certain attitudes, depending where the crack is located.

I once had a fuel stink in a 150. Couldn't isolate it, mostly because it was rather intermittent. Finally made an elephant nose out of three feet of 1/4" nylon tubing and a bit of thin rubber diaphragm material. Cut the rubber and made a small funnel, small end taped to the tubing and the other end trimmed to fit closely over and around my nose. Poked around with the other end of the tube while sniffing, and behold! a primer line fitting at the firewall was seeping just enough to stink but not enough to drip. The fuel evaporated faster than it could accumulate. I have since used that doodad many times to pinpoint leaks. Definitely not OSHA approved, I'd bet.

Dan
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It is the plane in my avatar. Under the wing where the strut goes up into the wing a panel comes off. Im sure mostly to view the strut housing in the wing but it did have some space around it. I just took it off for the annual last month.

haha Good job with the OSHA approved sniffing tool. I thought at one point it could be the primer because even after 3 pumps it doesn't feel all that stiff( lets here it 6pc, hahaha). Was wondering if it was leaking but couldn't explain while descending. Then I thought maybe its the gascolator since we took the fuel screen out and fuel bowl..But when we put it back on and turned on the pump it didn't leak. So...but again only while descending. that's what makes it strange. This last flight the tanks were rather low. 5 gallons in one and maybe 8 in the other after the flight..

RV10flyer, that is not cool! That's why I grounded it. I will not fly a plane that is leaking fuel. In my primary training the 150 I flew one day was leaking from a sump drain. I couldn't get it to stop for nothing. I scrubbed the flight and wrote the plane up. A few more people flew it after me with the write up. No thanks....obviously it turned out fine but im not gonna try it.
 
You may try jacking it up a bit without a tail stand/weight to get a good nose down attitude.
 
Fuel vent hoses are one possibility but they're ahead of the spar and I don't believe you can see them without removing the tanks.
 
I assume you reduce power to descend? What about airspeed? Attitude difference between level cruise and 500fpm descent isn't the only factor to consider. What kind of airplane?
 
Issue is reported frequently in Cherokees. Can't recall if it's anything to worry about or not.
 
Over the years I have had a few fuel issues with my PA-28. The simple ones to look for are the gascolator and then the fuel selector valve. I had both at different times. Of course evidence of a slight leak can evaporate quickly.

I got a 'Steve's gascolator' and the other time had the fuel selector rebuilt. If it just came up after annual and they had the gascolator apart, could be the problem.

Then keep an eye on the fuel primer.
 
Stewartb said:
I assume you reduce power to descend? What about airspeed? Attitude difference between level cruise and 500fpm descent isn't the only factor to consider. What kind of airplane?
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I do reduce power and descend cruise altitude around 120mph. It is a cherokee 180.
 
sourdough44 said:
Over the years I have had a few fuel issues with my PA-28. The simple ones to look for are the gascolator and then the fuel selector valve. I had both at different times. Of course evidence of a slight leak can evaporate quickly.

I got a 'Steve's gascolator' and the other time had the fuel selector rebuilt. If it just came up after annual and they had the gascolator apart, could be the problem.

Then keep an eye on the fuel primer.
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Yeah I was trying to think simple and thinking it had to be something we messed with since it started after the annual. Sb1006 was done about 7 years ago and the vent lines got replaced. Not saying it can't be them but..well.. Just throwing that out there.

I'll de cowl it next time I go out and look for evidence of leaking around the gascolator. I'm betting its in there. When we put it back together and hit the fuel pump it started to leak. So we tightened it up more and under pressure the leak stopped. Wondering if the pressure difference while descending is causing it to leak some.?? Waiting for my mechanic to get a chance to look at it, so I'll keep it posted. Thanks for the replies!
 
Instead of investigating attitude I'd investigate an increase in fuel line pressure first. Which leads back to selector, primer, and strainer. Just thinking out loud.
 
Funny you mention fuel pressure. I didn't pay attention to it while descending but I've noticed times its higher than its ever been after the annual. I'm sure those will be my mechanics first area of attack. If I can get out there before he does I'll take a gander and see if I can notice something standing out at me.
 
Another place to look is your fuel cap, pitch changes the airflow interface and can pull fuel through weak gaskets and let it seep under a skin and into the wing. It takes very little to get the smell since the air gets pulled through the cockpit.
 
I did not enrich while descending.

The mechanic looked at it today and found the strainer, primer, and selector not leaking. However he found a broken seal on a cable pass through on the firewall. So he resealed it and thought maybe I was just picking up the smell from the engine area itself as he did not find any evidence of leakage. So Saturday Ill take it up and see if I can replicate the issue. If I can, will further investigate. If I cant, ill just always be smelling for it later.
 
Could look at the tank filler necks and see if there cracked. When you fill up some fuel will get into the wing and vaporize. We have constant fuel smell in our high wing planes because of this issue. After about 5 years of smelling 100LL you get use to it.

Our wings actually have drilled holes on the underskin behind the tank to drain any fuel that gets into the wing.
 
simtech said:
I did not enrich while descending.

The mechanic looked at it today and found the strainer, primer, and selector not leaking. However he found a broken seal on a cable pass through on the firewall. So he resealed it and thought maybe I was just picking up the smell from the engine area itself as he did not find any evidence of leakage. So Saturday Ill take it up and see if I can replicate the issue. If I can, will further investigate. If I cant, ill just always be smelling for it later.
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I will add one thing that happened to me on my 75 Warrior, altho I am not sure your plane is plumbed the same way... After a engine rebuild the mechanic had his assistant build new fuel lines...

The way the Warrior was set up was the fuel lines from the tank went to the fuel selector by my left leg, it then ran to the gascolator, which is mounted low on the left side of the engine side of the fire wall, then it ran to the boost pump which is next to it.

From there it ran to the motor driven fuel pump and then a line to the carb...

Turns out the assistant was a rookie and had no idea how to build flex lines and installed the fitting wrong... The plane ran fine and no smells were present during full power take off but fuel smells did appear during landing at low power settings and the boost pump running....

During cruise, the boost pump did not run so the line from the boost pump to the motor driven one was basically a suction line with no pressure in it, but once you turned on the boost pump the fitting at the firewall / boost pump end would seep fuel and you could smell it in the cockpit...

They built a new line and the smell went away... YMMV...
 
Silvaire said:
Patent pending...:wink2:
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Most pilots already own one:

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