Should I put a Fuel Drain Here?

Discussion in 'Firewall Forward / Props / Fuel system' started by wsimpso1, Jun 24, 2019.

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  1. Jun 24, 2019 #1

    wsimpso1

    wsimpso1

    wsimpso1

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    I thought that I had my fuel system resolved and going together, and then I came across something. Dave Prizio's book cautions us to ALWAYS put a fuel drain point at the lowest spot in out system... Well, I need a couple more hours at a buddy's mill to finish them, but things are getting crowded down there. Let's look at the system in the attached photo.

    Wing tanks are in the outer panels of my wings, they have quick drain points at the very lowest spots in the tanks, and a pickup above the quick drain point.

    The fuel line from each wing follows the lower wing skin then the fuselage belly skin, then near centerline up to a duplex selector valve and a pair of Facet 40109 pumps in parallel.

    One of the Facet pumps is powered all of the time, and they run 32 gph.

    The Facet pump moves fuel into a 10 gallon header tank that has a sump of 18 fluid ounces with a drain point below and a pickup above. The header tank overflows back to the selected tank via the duplex selector valve. So, when the header tank fills, it overflows to the wing selected and fuel circulates. And fuel keeps flowing at 32 gph from the tank selected to the header tank.

    The low spot is near center line on the way from wing tank to selector valve. There is no sump here, just 3/8 x 035 aluminum tube. 32 gph in a 3/8 x 0.035 aluminum tube is about 28 in/sec. The entire line from wing tank to the header tank is 100" long, and 7 cubic inches of fuel. The portion that is below the wing tank is 55" long and less than 4 cubic inches - less than 2 fluid ounces. That line containing my low spot will be completely flushed to the header tank every four seconds during operation and I can sequester 18 fluid ounces of water there before the pickup will get much of it. It seems like a real stretch that the fuel line needs a drain between the wing and header tank to me...

    So, let's hear if the cognoscenti feel I can skip these drains on the low spot, or if difficult package or not, it still needs to find a spot in my tight package here.

    Billski
     

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  2. Jun 24, 2019 #2

    Vigilant1

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    That makes sense to me, too, particularly if the pickup for the wing tank is high enough that we can be sure any condensation/water already in the fuel will stay below that pickup.

    I don't know what the applicable reg says, or what an inspector would be guided by.

    FWIW, most Sonexes fitted with the Aerocarb/Aeroinjector have no drains in the fuel system, and no gascolator. The "carburetor" is at the lowest point of the fuel system, and as soon as the mixture is taken out of full lean (engine running or not), fuel begins to dribble out. If there's water in the tank, it'll be the first bit of dribble.
     
  3. Jun 24, 2019 #3

    BBerson

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    Check if the Cherokee or 172 has a drain at low point. I don't think so.
    My Grob has a drain at low point but no drain at tank.
     
  4. Jun 24, 2019 #4

    Marc Zeitlin

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    So the first question is what kind of fuel delivery system are you using on the engine? Carb? Mechanical FI? Electronic FI?

    Gascolators, drains and fuel system traps were (IMO) used in aircraft because with a carburetor, it's very important not to have water in the fuel line - if the float bowl in a carb fills with water, you're done - the engine will stop, and until the float bowl is drained, you're not getting it started again.

    But with FI, that's not the case. a little bit of water in the fuel will just pass through and cause the engine to stumble a bit, if even that, but won't stop it. And since there's no place for the water to collect (other than your 18 oz sump in the header and the low points of the wing tanks, which you'll check before each flight), I don't see an issue here. Performance Engineering's FI system manual clearly states that no gascolator is necessary, and a gascolator was always supposed to be at the lowest point in the system to collect water/debris.

    Assuming you've got accessible/cleanable/replaceable filters for each of the three tanks, as well as a drain for the header, I wouldn't bother with a drain in the 3/8" line between the two tanks. But that's just me, and I might be missing something...

    My $0.02.
     
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  5. Jun 24, 2019 #5

    gtae07

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    My understanding is that Billski is using EFI. In that case I definitely see no need for a drain there.

    I don't have any drains in my system other than the low points in the wing tanks. I'm using EFI too, and anything in the lines will blow through the first few seconds after turning the pumps on.

    Even in RVs with carbs, nobody (that I know of) has ever put a drain in other than the tanks and a gascolator. There's not one on the plans other than those, either.
     
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  6. Jun 24, 2019 #6

    BJC

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    Will you have a firewall-mounted gascolator on your RV?


    BJC
     
  7. Jun 24, 2019 #7

    gtae07

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    No gascolator. No point on an EFI system like this. I have pre-pump and post-pump filters and water drains in the lowest corners of the tanks.

    From the SDS installation manual (emphasis theirs):
     
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  8. Jun 24, 2019 #8

    BJC

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    Thanks. That is what a well known kit developer recommended to me.


    BJC
     
  9. Jun 24, 2019 #9

    wsimpso1

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    EFI

    Thanks for weighing in Marc! That is where I was but with only thinking and no experience, well, I needed a sanity check. I am running a simple model of the system with different amounts of water in each tank as initial conditions, then various scenarios of checking sumps and running the pumps.

    Billski
     
  10. Jun 24, 2019 #10

    wsimpso1

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    No Gascolator for me. I am convinced they are inappropriate for EFI equipped planes.

    Billski
     
  11. Jun 24, 2019 #11

    Dan Thomas

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    All 172s and most other Cessna singles have a drain port in the bottom of the selector valve, which is the lowest point in the system. Cessna didn't put a drain valve in there until 1996; they had a plug in it and the inspection schedule demanded removal of the plug and draining of any debris and wter at 100-hour intervals. I put drain valves in those ports instead. I found plugs that had obviously never been out in more than 30 years, and plenty of garbage in the valves. Sometimes I had to drain the tanks and remove the valve and disassemble it to get that plug out. Seized in there tight.

    But the 172's fuel flow is much lower than Billski's, and a straight 3/8" line doesn't have the dead spots that the bottom of a selector valve does.
     
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  12. Jun 24, 2019 #12

    BBerson

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    That drain valve sounds like a good idea for water. But it won't drain a plug of gunk. And it might be a fire hazard if a crash scrapes the drain off.
     
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  13. Jun 24, 2019 #13

    Dan Thomas

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    In a carburetor, even a few drops of water is enough to stop the engine. The metering jets in a carb are small, and with gasoline on them, the surface tension of the water can be enough that it will refuse to flow through the jet. The outlet from the bowl to the jet is only a fraction of an inch above the bottom of the bowl, too, so the bowl certainly doesn't need to fill up.

    All aircraft carbs and mechanical fuel injection servos have very fine inlet screens. They're a last-chance filter. They are so fine that they; too, aren't supposed to pass water when they're wet with gasoline. For that reason, I would have a gascolator just to make sure that no water reaches that screen. It wouldn't take much water to fill that screen cavity and stop the fuel flow. A good gascolator has a screen fine enough that water can't get through it.

    Those carb and FI screens are another 100-hour inspection item.

    The Mr. Funnel filtering funnels use a fine screen to filter water out during refuelling.

    Remember that water in gasoline can take three forms: free water, which is what we find when we drain the sumps; entrained water, which is usually invisible unless the weather is cold and it appears in the fuel as "snow," and which could get past all the low spots and end up plugging filters if the fuel stays cold enough, or once in a header, they can melt and accumulate as water; and dissolved water, which is normally no hassle at all.
     
    Last edited: Jun 24, 2019
  14. Jun 24, 2019 #14

    Dan Thomas

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    A crash breaks the plug off, too. In a forced approach, one of the pre-touchdown checklist items is to shut the fuel off. That plug is in the outlet sections of the selector. And if the valve is drained before every flight, no gunk accumulates. That takes a long time.

    In the later Cessnas they used a valve that didn't protrude below the belly skin. But a crash is still going to bend and break stuff hard enough to get at it. .025" skin isn't much protection.
     
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  15. Jun 24, 2019 #15

    Pops

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    I always added the drain valve to the bottom of the selector valve on my C-172's. Those selector valves are not easy to take out with a seized plug.
     
  16. Jun 24, 2019 #16

    proppastie

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    Yes at low point....if large dip might not have to have a sump. Simple T with Churtis valve.
     
  17. Jun 24, 2019 #17

    Dan Thomas

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    A lot of work, but when I was looking after the flight school fleet, I wanted stuff done right. In any case, airplanes built in the 1970s--and we had four or five of them--had selectors that needed some preventive maintenance anyway. The 172's selector has six O-rings in it, two of which are seats for the steel balls that are the shutoff valves. O-rings get old and shrink and crack and the selector doesn't shut the fuel off properly anymore; that's a hazard in an inflight fire or forced landing, besides being a pain when the gascolator is apart for cleaning. My philosophy was to anticipate trouble before it occurred and fix it so that the airplane didn't break down far from home on some long cross-country. That's expensive and inconvenient, a lot more so than overhauling a selector valve.

    I figured that the risk of gunk in that valve eventually stopping the fuel flow was a much bigger risk of any valve damage in an accident, and Cessna seemed to agree when they started production again in 1996.

    Many airplanes were built with plugs in drain ports in fuel tanks, a lot of Citabrias among them. I found plugs that I replaced with drain valves. Cessna had to issue two service bulletins, one for airplanes with separate tanks and one for those with integral tanks, demanding the installation of drain valves in the tanks. The airplanes were built with plugs in the ports. Amazing how we have to learn stuff the hard way.
    http://www.sumpthis.com/04191999letterwithappendixes/seb9224.htm
    http://www.sumpthis.com/04191999letterwithappendixes/seb9225.htm

    I used to fly an Auster in the 1970s, towing gliders. I stopped flying it when its maintenance wasn't being done well at all. It had a leaking tank, and the guys replaced it with a surplus tank that had no drain valve. Water accumulated and the engine quit shortly after takeoff with the pilot that replaced me, and the subsequent hard landing damaged the airplane. They started restoring it, then the work stalled. I bought it and continued the work until our son was born and Mom stayed home. One paycheck. Had to let the airplane go.

    So you see, folks? Water in fuel bothers me when it's that close to home.
     
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  18. Jun 25, 2019 #18

    Pops

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    When I bought a 1956 C-172 the fuel selector valve was leaking around the shaft at the top of the valve. Bought the overhaul kit and took it apart. There is short tube spacer that has a thin washer on top of the spacer that puts pressure on the O-ring for the seal around the shaft. The tube spacer was missing.
    Bought the project in Tenn, the fuselage was in a chicken house and wings and engine in a wood work shop and after going over the log books better after I got it home , realized that it was the same C-172 that I wanted to buy in 1965 when it was based at the local airport were I was living in PA, but with a young family just couldn't afford it, but I still had pictures of it from 1965. Completely restored it and flew it for several years. 1956 Cessna 172  #120170217_10380472-1.jpg 1956 Cessna 172  #120170217_10410192-1.jpg 1956 Cessna 172  #420170217_10435684-1.jpg
     
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  19. Jun 25, 2019 #19

    Toobuilder

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    Billski

    I'm trying to imagine the outcome of the worst possible case scenario, and I can't see where a drain would help. Even if somehow you picked up a load of water that somehow migrated from the wing to the lines and settled there, so what? Worst case is the lines freeze solid while parked and you don't refill the header at engine start. At least you are still safely on the ground and it becomes a nuisance, not a safety issue. If the water is NOT Frozen and you sumped the tanks, so what? A few ounces of water will shoot right through an EFI system without any real issue.

    I'd vote to leave the drains off.
     
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  20. Jun 25, 2019 #20

    wsimpso1

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    Yeah, that is where I am too. If you drain at the tank sumps, it will be OK, even if you run the pumps before draining the existing tank sumps. And ice will shut you down on the ground either way - roll it into a heated space, drain the tank sumps and run the pumps to clear things...

    I am leaving it with its tanks sumps.

    Billski
     
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