Fuel dropping from drain tube after flight

[Davidv]

After the past few flights I’ve noticed a small blue puddle on the hangar floor coming out of one of the rubber tubes. I know one of these tubes appears to be coming out of the density controller unit and I don’t think it’s that one.  It doesn’t drop out immediately but rather in the hours after sitting.  The consistency is slightly thicker than fuel, feels like it’s mixed with a bit of oil.  I’ve attached a picture of the stain below.  I’ve heard this could be fuel pump related but I warned to see if anyone else has experienced a similar occurrence and what the outcome was.  

It’s also worth noting that there’s a chance this has been going on longer than the last few flights since those two tubes have a tendency to escape back into the lower cowling and this liquid could have been caught in there.  I know we will have to take off the lower cowling to fully diagnose.

I also haven’t seen any anomalies in flight, all fuel flows and pressure normal.  

Thanks for any knowledge you can send my way.

[Davidv]

Quick update on this if anyone has any thoughts( @carusoam after I posted this I read a lot of what you responded to in similar posts - very helpful).  The rubber tube no longer is dripping fuel after the flight.  I spoke with some other people including someone at lycoming who was pretty sure it was the sniffle valve just doing its job.  However, after my long flight on Friday I noticed it was definitely dripping some fuel in flight (when previously mostly dry in flight).  The bottom of the tube was wet and I can see some streaks down my belly that cut through the normal belly oil.  We all know that nothing cleans oil better than gas right? :).  Any thoughts? Bad fuel pump diaphragm?  My fuel flows are all normal.  30.6 gph on takeoff on my Hoskins and pretty steady at 38-40 psi in flight.  Although, I’m not sure where in the fuel system these measurements are taken...

[carusoam]

Something to consider...

On the newer Models... The sniffle valve is purpose built... (the O has a fancy one, the Bravo may also...)

It may be possible that the sniffle valve may need some cleaning... and verify it’s operation.

The oddity to consider... while the engine is running... outside air is being drawn into the sniffle valve... until it closes...

Having fuel leave in flight, probably isn’t happening... if it is... that is a bigger challenge that desires follow-up...

 

Excess priming fuel is what usually is being drained through the tube...

 

The other common fuel leaks from similar tubes in the area are the two fuel pumps... The electric pump is quirky... when it leaks, the pilot has already completed the walk-around and it’s not able to see it...

 

Chase the tube that is leaking fuel to identify what it is really attached to, and when it is leaking...

1) If it is leaking continuously in flight, I would be suspecting the mechanical fuel pump more than anything...

2) If it leaks prior to start-up, after priming, I would be suspecting the electric fuel pump...

3) If it leaks from the normally operating sniffle valve... that would not be continuous.  Left over fuel from start-up May get dumped the next time the valve opens, after shut-down...

4) Try cutting back on the amount of prime being used...  the IO550 uses OAT and pump time for guidance... above 40°F the amount of prime is cut in half compared to below @80°F it probably can get away with half again...

I grow more surprised as all the drains aren’t marked or color coded somehow... 

PP thoughts only, not a mechanic...

Best regards,

-a-

[Davidv]

Thanks @carusoam, just as a precaution before heading back to FL next week on a 6 hr flight I’m going to uncowl with a mechanic to see if anything looks out of the norm with the fuel pump.  However, if it was really dripping during the flight I would assume I would be seeing some anomalies on my fuel pressure?  I supposed it’s possible that the residue on the gear door and belly streaks could have been from right after startup due to excess priming, but weird that it would still be wet immediately after shutdown.

[carusoam]

Big Fuel leaks are still a small amount compared to engine operations....

So they won’t necessarily stand out in Pressure or FF instruments...

PP thoughts as usual...

+1 reviewing under cowl with mechanic.

Best regards,

-a-

[philiplane]

fuel stains running down the belly indicate a leak in flight. They will not show up on fuel flow since the leaks usually come from components before the fuel flow transducer.