Death By Fuel Tank Selector Valve

Discussion in 'Firewall Forward / Props / Fuel system' started by GESchwarz, Sep 10, 2011.

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  1. Sep 12, 2011 #41

    Joe Fisher

    Joe Fisher

    Joe Fisher

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    Reading this thread has me thinking about an almost fool proof fuel system for low wing airplanes. The 1" connection between the tanks would assure flow the check valves would make the system use fuel from fullest tank. The electric pump would have fuel at all times with the head in the tanks. The shutoff valves are ganged to gather just on and off. Each system has its own filter. The engine driven pump has its own system. The check valves at the carb. or throttle body allow flow from the highest pressure source. I have seen engine pump failures that aloud the boost pump to pump fuel over board rather than to the carb.
     

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  2. Sep 12, 2011 #42

    Dan Thomas

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    It still won't fix the flying-with-one-wing-low problem. If the higher tank runs dry, the pump will still suck air rather than fuel from the lower tank. That would require determining which tank is empty and shutting off that valve. Check valves only prevent crossflow, and if they have any springs at all holding the valve disc, even light ones, that causes a pressure drop across the valve and the potential for vapor lock downstream rises considerably. The boost pumps in the tanks would have to run continuously.

    And two tanks plumbed together (via the ultimate teeing at the carb, even with check valves) require a single venting system to keep tank pressure equal. With separate vents, the tank pressures are often unequal and the tank with the higher pressure will flow faster and run dry sooner. I've had this happen on several airplanes that had leaking fuel caps (low pressure over the wing holds the fuel back) and in a Glastar, which had the lines teed together immediately upstream of a single shutoff valve. It had separate tank vents and no matter what we did, we couldn't get them to flow evenly until we plumbed the tops of the tanks together to equalize the pressures. That's the reason for this:

    FAR 23.975 Fuel tank vents and carburetor vapor vents.

    (a) Each fuel tank must be vented from the top part of the expansion space. In addition—
    (1) Each vent outlet must be located and constructed in a manner that minimizes the possibility of its being obstructed by ice or other foreign matter;
    (2) Each vent must be constructed to prevent siphoning of fuel during normal operation;
    (3) The venting capacity must allow the rapid relief of excessive differences of pressure between the interior and exterior of the tank;
    (4) Airspaces of tanks with interconnected outlets must be interconnected;

    Dan
     
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  3. Sep 13, 2011 #43

    GESchwarz

    GESchwarz

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    Here's the fuel system of the Beechcraft T-34 Mentor. I like it because it has no L/R selector and it has a sump tank which is always full.

    I do have some questions. Perhaps one of you may have the answers...

    What type of valve is that between the wing tanks and the sump tank?
    What is the geometry of a "siphon break vent"?
    What is the purpose of the "Fuel Overboard Vent Line"?
     

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  4. Sep 13, 2011 #44

    rdj

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    The standard Rotax 914 engine uses two electrical fuel pumps I believe. Typically one fuel pump is driven by the alternator, and the other by a second battery system. From the lists I monitor, I see folks occasionally trying to retro-fit mechanical fuel pumps to these engines for the dead-alternator/weak-standby-battery scenario.

    BTW, I've often wondered if the fuel switch in a plane shouldn't be more like it is on many motorcycles: On/Off/Reserve. The 'Reserve' position draws from a lower pick-up tube that accesses some number of gallons between the 'On' pickup and 'Unusable'. When the big noisy fan stops, you quickly reach for the 'Reserve' position. Using 'Reserve' typically means either your flight planning needs improvement and you had better start looking for a place to put down soon, or you totally forgot to put gas in the thing from the git-go. I believe these two stupid pilot tricks are far more common in the NTSB reports than mis-placed fuel selectors. Of course, 'Reserve' might not be timely enough if the fuel ceases shortly after takeoff rotation, but then I've gotta go with Dan's analysis--either your float bowl is huge, or you're in too much of a hurry.
     
  5. Sep 13, 2011 #45

    Dan Thomas

    Dan Thomas

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    Those are valves at the tanks. Small shutoffs, wired open. They're used to shut the tanks off for downstream maintenance without having to drain the tanks. I wish all light airplanes had them.

    A siphon break is a small check valve that lets air into any line whose pressure drops below ambient. In this case it would keep the vent from siphoning fuel overboard (while sitting on the ramp) if the vent line got full of fuel. Siphon breaks are now found on the new outside faucets on new houses; we have them and they're a bit of a pain. They prevent water flowing back from the hose (which might have its end laying in a puddle of dirty water) and sucking that water into the city's water system if the pressure fell off. It's happened. They're also sometimes found on water heaters so that negative pressure doesn't collapse the tank. If the house is on higher ground, the water in the city's mains will flow downward and place suction on houses up higher.

    Dan
     
  6. Sep 13, 2011 #46

    GESchwarz

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    Thanks Dan. Yeah, it would be very short sighted to not install vaves for maintenance.

    I noticed a couple of features in the pictorial view.

    The fuel tank vent lines are routed along the forward edge of the tank. Is there a reason they are not routed along the aft edge?

    The vent outlet is on the belly of the plane. I suppose it could drain fuel on a hot day without the vacuum break.

    The main tank outlet ports are not on the bottom of the tanks, rather they appear to be at mid level on the inboard face. This would leave a lot of unusable fuel. But in the other drawing it shows the outlet port in a sump-like depression. It's hard to say what the true configuration is. I'd go with the port at the very lowest point.

    The fuel strainer is upstream of the engine driven pump and the filter is downstream of the engine driven pump.

    The sump pump has no filtration. I suppose the sump pump can survive ingestion of FOD, whereas the engine driven vane type pump is more sensitive to FOD. Perhaps the sump pump is an impeller type.
     
  7. Sep 13, 2011 #47

    Hot Wings

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    BTW, I've often wondered if the fuel switch in a plane shouldn't be more like it is on many motorcycles: On/Off/Reserve. The 'Reserve' position draws from a lower pick-up tube that accesses some number of gallons between the 'On' pickup and 'Unusable'.

    There just might be some merit to this. My first VW's didn't have a fuel gauge and I NEVER was left stranded due to lack of fuel. I can't say the same thing when I moved up to ones that came with a fuel gauge. Either they stuck, quit working, or I simply tried to guess where empty really was when trying to stretch a tank of gas to the next payday.

    My tank has an electronic equivalent. A little float on a hinge with a magnet attached that closes a reed switch (on the outside) when the tank gets to 1.5 gallons. The flashing idiot light lets me know it's time to double check to see if my flight planning was accurate, or find an option if it wasn't.
     
  8. Sep 13, 2011 #48

    Dan Thomas

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    The fuel tank vents are near the front because that's usually the highest point of the tank, the last place air is forced out and the first place it wants to go when it comes in. If we had the vents farther back they'd be lower and we'd lose a lot of fuel when the tank pressures rose. When gasoline is agitated it gives off lots of vapor, and when the sun warms the wings the vapor pressure of the fuel rises. Those vapors need to escape without forcing fuel out of the vent.

    A vent on the belly would have some drop to it and would indeed siphon fuel, especially if the tanks were full and someone removed the cap and let air in.

    Main tank outlets shouldn't be right at the lowest point. That's where dirt and water accumulate, and we don't need that stuff getting into the system. The tank drains are at the lowest point. In the Cessna 172 the outlet is maybe a half-inch above the bottom of the tank on its inboard end. It really doesn't leave much fuel behind (because of dihedral) but it sure prevents contaminants going where they shouldn't. A gascolator can hold only so much water, and some tanks are prone to trapping more than the gascolator can hold. The rubber fuel bladders, for instance, often develop wrinkles in the bottom that can keep water from reaching the sump drain until the airplane is jiggled some to move it along to the sump. I'd rather keep that out of the outlet to the engine.

    The tank pump usually has a screen around its inlet. It's required by FAR 23 for certified airplanes. Any bit of hard stuff, or a scrap of rag, could stop the pump impeller. Simple to put a screen around it. The tank outlet, if there's no pump, also requires a finger strainer. A single big bug could otherwise plug the outlet.

    FAR 23.951 thru 23.999 is highly informative. I don't like endless regulation any more than anyone else, but most of this stuff is either common sense or scientifically sound. And much of it learned the hard way: by killing people. It's not something lawyers come up with to make life miserable.

    Dan
     
  9. Sep 14, 2011 #49

    Turd Ferguson

    Turd Ferguson

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    Yes, completely independent sources of power for each pump makes such a system workable. While a failure in the system may require landing as soon as practicable, that's better than having a failure mode that causes an immediate forced landing.
     
  10. Nov 16, 2011 #50

    aeromike49

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    Think about the chance of a leak somewhere and if there was only a both selection and off then a leak could possibly drain all the fuel. A pump connected to both tanks feeding a single surge tank would also be asking for trouble if there was no way to shut off one tank or another. An airplane actually ran out of fuel because of an automatic fuel balancing pump that did in fact balance the 2 fuel tanks down to EMPTY - there was a fuel leak which with 2 engines running and a leak at one engine consumed all the fuel and the plane made a dead stick landing from 33,000 feet. There are good reasons to be able to shut off each fuel tank and both and off is not a good idea in my opinion. Think about a fuel leak, a fire, or damage to one fuel tank. All of these things could be considered rare - but somehow these things do occur.
     
  11. Jun 5, 2017 #51

    ToddK

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    It seems to me a valve with an intagrated circut switch tied to the igntion might prevent a lot problems. If the valve is shut, the engine won't start.
     
  12. Jun 5, 2017 #52

    TFF

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    Loose power and the engine will not run.
     
  13. Jun 5, 2017 #53

    BJC

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    I hang my ignition switch on the fuel valve handle, and never remove it without opening the valve.


    BJC
     
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  14. Jun 5, 2017 #54

    Tiger Tim

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    How about putting the ignition switch in such a place that the key can't be put in when the fuel valve is closed and vice versa? The catch: don't lose the key!
     
  15. Jun 5, 2017 #55

    BJC

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    The fuel valve needs to be able to be closed ASAP in certain situations. I would not want to need to remove the key before operating the valve.


    BJC
     
  16. Jun 6, 2017 #56

    pictsidhe

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    You could add a switch to the fuel valve to inhibit electric start if fuel is off.
     
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  17. Jun 6, 2017 #57

    Tiger Tim

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    Yeah, there is that. Since physically blocking the ignition switch is about the only reasonable mechanical answer I could come up with, I think a safety fuel selector is one of those solutions that just creates more problems.
     
  18. Jun 6, 2017 #58

    autoreply

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    Have the fuel valve closed position only block the ignition position of the key, but not L/R/both.
     
  19. Jun 6, 2017 #59

    BJC

    BJC

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    That could work.


    BJC
     

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