Oiling Comanche Fuel Bladders

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Kevin Holbrook

Pre-takeoff checklist
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Questions for @Dan Thomas or any Comanche experts...

My PA-24 Comanche is equipped with four fuel tanks--two 30 gallon mains and two 15 gallon aux tanks. For my missions, I almost never need the fuel in the aux tanks. Maybe once a year, or less. In a prefect world, I would just leave the aux tanks empty most of the time.

However - for maximum fuel bladder life, the owner's manual recommends that they are kept full, or, if they aren't to be used for an extended period of time, to oil them to keep them from drying out.

Questions:
-when not flying, the plane lives in a hangar. Do they still need to be kept full?
-how are you supposed to oil the insides of the bladders? With what kind of oil? The owners handbook does not specify this.

Thanks in advance...
 
First time I have heard of it. Comanches are 60+ years old and oiling might have applied to earlier polymers. Bell206 might have more experience here.
 
Kevin Holbrook said:
Do they still need to be kept full?
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If your goal is max bladder life, then yes. The issue is the bladder interior lining dries out over time.

Kevin Holbrook said:
how are you supposed to oil the insides of the bladders? With what kind of oil? The owners handbook does not specify this.
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Check in the maintenance manual. Its not really a common procedure even within the same OEM model line. The required oil is either a type of preserve oil or engine oil. But to access the interior the bladder needs to have a hand sized access plate in order to do so. Not really worth the effort in most cases unless storing the aircraft for extended periods which would trigger other preservation tasks. Regardless, my guess would be if you flew once a month or once every two months with full aux fuel you'd stay ahead of the game and get a respectable life out of the bladders vs leaving them dry all the time.
 
Can you fill the aux for parking/hangar/storage and transfer to mains when flying?
 
Fill the aux, and underfill the mains for the flight in mind. The fuel sloshing in the mains will keep them good, and after a flight, if you do not fill the aux, they will have had recent fuel in them.

I would take off with a main, change to first aux when passing an airport, next aux near another airport and unused main at destination airport, if needed.
 
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Bell206 said:
But to access the interior the bladder needs to have a hand sized access plate in order to do so. Not really worth the effort in most cases unless storing the aircraft for extended periods which would trigger other preservation tasks.
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Stoopid question: would mixing a few ounces of oil with the fuel and then using up the fuel in the bladders leave enough of an oil film behind to protect them?
 
Bladder deterioration I’ve encountered are usually at the top.

IMHO this is due to not completely topping off when hangared.
 
AV8R_87 said:
would mixing a few ounces of oil with the fuel and then using up the fuel in the bladders leave enough of an oil film behind to protect them?
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No. They're both hydrocarbons which will cause the oil to dissolve into the gasoline. Its actually the reason they use oil products to preserve engine fuel systems, fuel cells, etc.
 
On recommendation from the tank manufacturer(verbal, not in writing), I used MMO. I would rub it around the neck area as far as I could reach. As stated above, the leaks are mostly in the upper area of the bladder. Leaving a tank empty is just asking for problems. Tanks half full is much better than empty, even though the neck area needs extra attention.
 
I would just put a few gallons in each aux and every so often, slosh it around (rapid rolls, slips, rudder side to side).

Or as mentioned, occasionally fill the aux tanks, but not the mains.
 
I have wondered how, keeping a few gallons, of gasoline in a tank keeps a bladder from deteriorating? I remember getting 100LL on my hands years ago, smarter now, and it really dried my hands out. I operated a C340 for 17 years, sold it almost a year ago, we rarely, maybe 6 times, had any fuel it the nacellle bladder and it never leaked. I believe the bladder was around 25 years old when we sold the plane.
 
Plano Pilot said:
I have wondered how, keeping a few gallons, of gasoline in a tank keeps a bladder from deteriorating? I remember getting 100LL on my hands years ago, smarter now, and it really dried my hands out. I operated a C340 for 17 years, sold it almost a year ago, we rarely, maybe 6 times, had any fuel it the nacellle bladder and it never leaked. I believe the bladder was around 25 years old when we sold the plane.
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Bladder rubber and human skin are vastly different. Your skin has fat in it that keeps it soft and flexible, and Avgas leaches that out, leaving the white stain on your hands. That stain is the fat. It's not lead, as unleaded Mogas will also do it.

Bladders don't have fat. They have polymers that dry out over time, losing the more volatile elements through evaporation. Fuel tends to slow that down.
 
Dan Thomas said:
Your skin has fat in it that keeps it soft and flexible, and Avgas leaches that out, leaving the white stain on your hands. That stain is the fat. It's not lead, as unleaded Mogas will also do it.
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So, like, take a bath in gasoline to lose weight?
 
is the weight penalty of keeping them full intolerable or otherwise undesirable?
 
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