Excessive Blue Dye in 100LL

dfw11411

Pre-takeoff checklist
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dfw11411
Not a problem, but I am curious. About a year ago I discovered that the fuel from the starboard tank would have a very dark blue tint when I sumped. Port tank is always a normal light blue tint. When I sump, I drain about 1-1.5 oz from each. Usually, the second test from the starboard tank shows the normal light blue tint, but sometimes it takes two or three tests to clear the dark blue. I’m convinced the Avgas is just fine and this is just an appearance issue.

* I always fuel from busy GA fields, never from remote or lightly used fuel tanks. Most of my fuel comes from a busy Class D with an active flight school.​
* I’ve never seen water or other fuel contaminants in this aircraft.​
* I always put fuel into both tanks when refueling.​
* Each tank holds 52 gallons usable. I recently ran them down to 5 gallons each then filled both. Same excessive dye issue a few weeks later.​
* I usually fly once every 2-3 weeks. The dark blue doesn’t show every time it sits, and I have not noticed that the issue relates to the length of time the plane sits on the ground. It never shows dark before a return flight or after refueling.​
* Neither tank has been worked on, but three years ago they drained both tanks to calibrate the fuel level sensors for an avionics upgrade. The issue began almost two years after the calibration, so I doubt that’s involved.​
* Neither fuel tank cap is kept locked, but Leprechaun sightings are rare in Texas. The aircraft is hangared and secure.​
 
About a year ago I discovered that the fuel from the starboard tank would have a very dark blue tint when I sumped.
Are you sure its the avgas dye changing or could it be something is dissolving in the avgas after a period of time and darkening the sample?
Perhaps the next time you sump a dark sample take it to your local refueler and ask. Maybe they'll get it checked?
 
I had that happen in the C206 and didn't figure it out until I had an engine failure over mountainous and wooded terrain. As I was preparing to land in a small area of clearing, the engine restarted, then stopped, then restarted again, maybe three times in all. I made to an airport and had the everything inspected and but they found nothing wrong.

What apparently happened was that fuel was leaking from the fuel bladder then dissolving some of the adhesive on the tape inside the wing, then the concentrated blue "goo" was re-entering the bladder and sinking to the bottom where it was sucked into fuel system, blocking off the flow until there was enough flow to dissolve the clot. I later realized that there were little blue droplets in the fuel that was sump tested and that those droplets could be dissoved with stirring. Both bladders were replaced (and were cracked) and the problem went away.

(Although there was no need for a "get out of jail card" I filled out a NASA form and they actually called me back to discuss the events.)
 
This is a sign of evaporation, which will concentrate the dye. If your sump drain is leaking, evaporated fuel residue will build up on the exterior, and when you drain a sample the next time you will get a concentrated load of dye in your sample.
 
Fuel is likely seeping VERY slowly out of or around the sump and the dye is drying. When you sump, it dissolves, and ends up in the sample.

This is a sign of evaporation, which will concentrate the dye. If your sump drain is leaking, evaporated fuel residue will build up on the exterior, and when you drain a sample the next time you will get a concentrated load of dye in your sample.

I think this is the issue. Easy enough to check and fix. And, yes, I sometimes did notice some blue stains below the valve, just not always. Thanks!

I had that happen in the C206 and didn't figure it out until I had an engine failure over mountainous and wooded terrain. As I was preparing to land in a small area of clearing, the engine restarted, then stopped, then restarted again, maybe three times in all. I made to an airport and had the everything inspected and but they found nothing wrong.

What apparently happened was that fuel was leaking from the fuel bladder then dissolving some of the adhesive on the tape inside the wing, then the concentrated blue "goo" was re-entering the bladder and sinking to the bottom where it was sucked into fuel system, blocking off the flow until there was enough flow to dissolve the clot. I later realized that there were little blue droplets in the fuel that was sump tested and that those droplets could be dissoved with stirring. Both bladders were replaced (and were cracked) and the problem went away.

(Although there was no need for a "get out of jail card" I filled out a NASA form and they actually called me back to discuss the events.)

Wow! Reads like a Sir Arthur Conan Doyle mystery. I'll keep this in mind, but I've already told my A&P to service/replace the valve as that sound far more likely.
 
Wow! Reads like a Sir Arthur Conan Doyle mystery. I'll keep this in mind, but I've already told my A&P to service/replace the valve as that sound far more likely.
Does your aircraft have fuel bladders and if so what year is it? If no bladders that would eliminate the tape adhesive possibility.
 
Each wing has an inboard tank using the sheet metal skin of the wing and a fuel cell (bladder) as the outboard tank. The outer tank drains into the inner tank. The bladders are circa 2008.
 
This is a sign of evaporation, which will concentrate the dye. If your sump drain is leaking, evaporated fuel residue will build up on the exterior, and when you drain a sample the next time you will get a concentrated load of dye in your sample.
I had the same thing happen. Once I replaced the quick drain, it never happened again.
 
Each wing has an inboard tank using the sheet metal skin of the wing and a fuel cell (bladder) as the outboard tank. The outer tank drains into the inner tank. The bladders are circa 2008.

If it’s possible in your setup you could try to keep the outer bladder tank empty and compare.

1- Take your left and right fuel samples as normal. Keep each in a wide jar to exaggerate the color differences. Label each.

2- Fly the plane or drain fuel to completely empty the starboard side inboard and outboard.

3- Fill only the inboard tank.

4- Sample the inboard after a period of hours or a week or more. Label jar. Then empty or fly it dry.

5- Fill outer tank (even if it means filling the inboard). Sample after a few hours/days and label.

Compare all 4 samples to rule out one or the other starboard tank.
 
Each wing has an inboard tank using the sheet metal skin of the wing and a fuel cell (bladder) as the outboard tank. The outer tank drains into the inner tank. The bladders are circa 2008.
So a bladder leak could be the problem but shouldn't be after 16 years. Who knows, though? Quality has eroded in so many products recently.
 
I know my bladder ain't what it used to be :skeptical:

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99.999% slow leak from the sump valve (crud stuck in the o-ring?).
Easy way to prove. Fill tanks (only mains, if possible, keep aux empty), sump, confirm same color in both. Come back x days later, sump again, if one tank is much bluer that valve is leaky. Remove some fuel from tank thtough the filler to confirm what's in the tank has the proper color.
 
You've got a leak at starboard tank, probably the Curtiss valve.
 
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