Fuel left/right/both on the ground and run-up?

Discussion in 'Hangar Talk' started by Pedals2Paddles, Nov 24, 2014.

  1. Pedals2Paddles

    Pedals2Paddles Cleared for Takeoff

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    I'm writing a checklist for the 172. I want to test operation of the fuel flow in left, right, and both prior to departure. All the "canned" checklists leave it on both the entire time. Here's what I'm thinking. Tell me if this won't work for whatever reason.

    Set left prior to start, leave it on left while warming up. The few minutes on the ramp at 1000rpm I would think should be enough to establish that fuel flows on the left setting. Set right prior to taxi. Taxi to wherever you're doing your run-up. This few minutes should be enough to verify you have fuel flow on the right setting. Set to both for the run-up. This should establish that fuel flows in both. Always before the run-up. Never change tanks just before takeoff!

    Am I stupid (as it relates to this topic)
     
  2. MauleSkinner

    MauleSkinner Final Approach

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    You'd be amazed at how long it'll run on just what's in the line between the valve and the engine...I wouldn't bother with it myself.
     
  3. BillTIZ

    BillTIZ Final Approach

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    I've never thought about it for a Cezzna. I just leave it both when the engine is running.

    The T-41 it is advised to run off the right tank for cruise to make room to pump from aux tank to right man in. Fly off the left tank while transferring fuel into the right. I only do that when I have fuel in the aux for a very long leg. Otherwise I just leave it in both.

    If you want to know how long it takes to determine the fuel is flowing properly. Time it when you switch to OFF to engine quit. Then double it for the test.
     
  4. coloradobluesky

    coloradobluesky En-Route Gone West

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    It will work against you to do anything but leave it on both. There are way too many accidents where it was left on right or left and the pilot had an off field landing with one side empty and one side full. Way too many.
     
  5. James331

    James331 Ejection Handle Pulled

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    That.

    Try turning the fuel off and seeing how far you get through your run up :yikes:
     
    Last edited: Nov 24, 2014
  6. Pedals2Paddles

    Pedals2Paddles Cleared for Takeoff

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    On my to-do list now. Thanks guys.
     
  7. MAKG1

    MAKG1 Touchdown! Greaser!

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    Your Cessna will work fine on both even if one tank is completely empty, though you're vulnerable to unporting if that happens.

    The only reasons to use left of right are to prevent crossfeeding if fueling on a slope, or to correct a fuel imbalance.
     
  8. flyingron

    flyingron Touchdown! Greaser! PoA Supporter

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    Or you are complying with the mandatory placarded instruction on some models to not use BOTH above 5000' to avoid vapor lock.

    Oddly enough, because of the vent line interconnect between the tanks (which will flow fuel just as well as air), it's very difficult to correct imbalances when the tanks are filled above the level of the vent line.

    I'd make sure the HANDLE moves on the ground but that's about it. On my plane (pretty much anybody using the Brittain/Osborne tip tanks), you'd be well advised to flow a little fuel from the tips on the ground if they had been run dry previously. These are prone to locking as well (though you can usually clear it with some prolonged use of the boost pump).
     
  9. James331

    James331 Ejection Handle Pulled

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    I've always just switched to left or right on shutdown, verify fuel on and back to both on my startup. Aside from that I rarely ever touch the selector.
     
  10. RotorAndWing

    RotorAndWing Final Approach

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    Exactly.
     
  11. flyingron

    flyingron Touchdown! Greaser! PoA Supporter

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  12. bnt83

    bnt83 Final Approach PoA Supporter

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    Does it move? Can you feel the detents? If you can't feel the detents is there any chance the valve could walk from and ON position to an in-between position?


    This is what they look like when the detents are weak... (full of crap)

    [​IMG]

    You can see the detents on that top plate. There is a spring & ball bearing in the thing where the knob screws on and that ball rolls around the plate when you turn the knob.


    Even this doesn't mean that is functioning properly. Testing "Both" doesn't mean much since it could feed from one or the other. The oblong cam pushes the medium sized balls against the big balls to open the port. I think it is possible for the medium balls to fall out of position if very badly worn parts in which case, the affected port won't open.

    Great care is required when servicing these. I try to make the balls fall out of position in any possible attitude, then verify fuel flows from the engine fuel hose after installation with the handle in each position. Cold soak the airplane for a leak check has been part of the process, mostly because every valve I have done so far is during winter months since it seems they start leaking in cold weather. Heated hangars can "hide" leaky fuel components.

    Not all valves are built this way but this covers many Cessnas.
     
    Last edited: Nov 25, 2014
  13. Stewartb

    Stewartb Final Approach

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    Do a test. Turn the valve to OFF. Start the engine and warm up normally and time how long it takes to quit. If your airplane is anything like mine you'll get bored waiting. I left a maintenance hangar once. Warmed it up and taxied for takeoff. Did a run up and started onto the runway when it quit. The mechanic had turned the valve off and I hadn't checked it. Another 30 seconds and I'd have run out of fuel in a very bad place. That was the last time I didn't check my fuel valve as part of my takeoff checklist. The point is, even after an extended winter warmup the engine ran a long time with the fuel selector in the off position. Your test would be better run at a safe cruise altitude with cruise power full flows by selecting R, L, and both for a few minutes each.
     
    Last edited: Nov 25, 2014
  14. Pedals2Paddles

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    So let me ask you this then. If you switch it off and wait for the engine to start dying, how long does it take after turning the fuel back on for the engine to have what it needs to run?
     
  15. Stewartb

    Stewartb Final Approach

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    In my 180? 2-3 seconds. But I always operate on Both in any airplane that has that option.

    Cub drivers with stock systems routinely run one tank dry to maximize range. My Cub had a L/R/Both system so I passed on that routine.
     
  16. bnt83

    bnt83 Final Approach PoA Supporter

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    Some fuel valves (Cessna 177/177B ) don't have an OFF position. It exists, but you have to remove the knob to turn it that far. I think a few other models that do the same.
     
  17. JOhnH

    JOhnH Touchdown! Greaser!

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    At the risk of hijacking this thread, I have a related question?
    I was reading a Bonanza AFM the other day and referencing a plane with two tanks, it said to first run 10 gals off the left tank, then 22 gals off the right tank, then 22 gals off the left tank. Why is that? Why not just do like the Cessna and leave it on both? I could see this contributing to the moniker "Dr. Killer" as a post above mentioned off-field landings due to forgetting to switch.

    Now, back to the original thread, in five years in a C172, there were two times I noticed one side (left) fuel gauge going down with the other side (right) staying steady. I switched to the right tank to see what would happen, being ready to switch back immediately if necessary. But the right tank gauge started going down with no noticeable effects. When they became balanced (according to the gauges), I put it on "both" again and all was normal. I was going to have this checked out, but I sold the C-172 and am looking to buy a Bo, which is the reason for my question above).
     
  18. bnt83

    bnt83 Final Approach PoA Supporter

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    I do the 1/2 throttle thing lined up on the runways with the brakes locked, check all instruments release the brake and apply full throttle for that reason.
     
  19. bnt83

    bnt83 Final Approach PoA Supporter

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    Many Cessna do not have a BOTH.

    Some fuel valves also change which tank is connected to the fuel pump return line.

    Reading the fuel system description and operation should answer most questions.
     
    Last edited: Nov 25, 2014
  20. Stewartb

    Stewartb Final Approach

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    The OP does, so the responses are appropriate for the question.

    JohnH, I'd suspect your old plane had a vent issue.
     
  21. bnt83

    bnt83 Final Approach PoA Supporter

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    If one vent is full of gas the fuel will flow from the tank that has an open vent. If you switch to the tank that is not feeding, it can clear the gas from that vent line then it should feed pretty close to even when switched back to BOTH
     
  22. jsstevens

    jsstevens Final Approach PoA Supporter

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    Most (all?) low wing planes do not have a both setting. This is because of not having gravity feed the fuel to the engine.

    John
     
  23. JOhnH

    JOhnH Touchdown! Greaser!

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    Ok thanks. That makes sense.
     
  24. flyingron

    flyingron Touchdown! Greaser! PoA Supporter

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    MOST not ALL.

    The stock Navion fuel valve has two positions ON and OFF (just like a Cessna 152).

    Mine has four positions (left tip, MAINS, right tip, OFF).

    As JS points out, if you can't pump out of a line connected to an empty tank. You can demonstrate this with two straws and a glass of your favorite beverage. Put one straw in the liquid and the other outside the glass and try to suck from both of them at the same time. The certification rules preclude this (even if it is obvious that it's a dumb idea to start with).

    The Navion gets around this by actually having a third tank that both main tanks drain into and it pumps from that.


    As for uneven feeding on Cessnas, the CPA has a info sheet on this. What happens is that the only normally open vent is on the left tank. The right tank vents via a line between the two tanks. This line isn't selective and fuel will crossfeed through the vent line when the levels are high enough. You can fly the 172 around with the fuel selector on RIGHT and still watch the left tank level go down. The CPA sheet will tell you how to adjust the left tank vent to even the flow out if it bothers you.
     
  25. simtech

    simtech En-Route

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    I'm not totally sure on a Bo, I've only been in one once. But my cherokee is left and right only, no both. My poh says, with full tanks, run one side for an hour. Run the next side for 2 hours. Now you have 1.5 hours of fuel left in the first tank and .5 hours in the second tank. My gps has a timer in it to remind me when to switch but honestly I always keep it in my mind. I flew only high wings for the past 3 years and I thought going to a low wing would be hard to remember the fuel pump on for take off and landing and when switching tanks. But I've been really good at it after ingraining the checklist in my head.
     
  26. Sac Arrow

    Sac Arrow Touchdown! Greaser! PoA Supporter

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    That sounds odd, I do not recall seeing that in the the POH of any of the Pipers I have flown. Which Cherokee? I've always just switched tanks every half hour, unless the destination is less than an hour away, in which case I'll fly there on one tank and back on the other.
     
  27. Jimmy cooper

    Jimmy cooper En-Route

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    As I recall, the aerostar has a fuel system that one should be very familiar with. Jeff ethyl , an authority on WW2 aircraft apparently was not an authority on the fuel system of a P38. He ran out of gas in the pattern and was killed. Do I remember this correctly?
     
  28. simtech

    simtech En-Route

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    Cherokee 180D. My gps is set for every 30 minutes but the poh says what I wrote. Its in the last paragraph of the right page (22).
     

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    Last edited: Nov 25, 2014
  29. Sac Arrow

    Sac Arrow Touchdown! Greaser! PoA Supporter

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    I'm not doubting, I just find it interesting. I generally feel start to feel the imbalance noticeably after 30-40 minutes on one side.
     
  30. write-stuff

    write-stuff En-Route

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    Don't bother. It'll just increase the wear on your fuel valve. They are expensive.
     
  31. Tom-D

    Tom-D Taxi to Parking

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    When you don't test once in a while, how would you know the selector valve actually shuts the fuel off?

    Many times I'll find that when I shut off the fuel to the engine for maintenance the valves leaks so bad the the enging will still run. most of the time it shows a drip, drip, drip..

    Best time to test is after the flight. shutter down with the fuel shut off. stops all those pesky carb leaks.
     
  32. Pedals2Paddles

    Pedals2Paddles Cleared for Takeoff

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    I was thinking about making that part of the shutdown. Maybe not every time, but with some regularity. Instead of pulling the mixture to cut off, close the fuel valve. Ensures it works. Is doing that bad for the carb / intake manifold in any way? Would it make the engine more difficult to start the next time?
     
  33. MAKG1

    MAKG1 Touchdown! Greaser!

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    It will make getting fuel samples out of the strainer in your next preflight "interesting."

    If it's a rental, the next guy will curse you into the next dimension once he figures out the fuel system isn't FUBAR.

    Some O-rings will last much longer if left in contact with fuel. I don't know if your 172 has those.

    Your shut downs will also take a while at 1000 RPM. Like, minutes.
     
    Last edited: Nov 25, 2014
  34. Pedals2Paddles

    Pedals2Paddles Cleared for Takeoff

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    Turning the fuel back to BOTH is part of the pre-flight, so the fuel sampling shouldn't be a problem. However you do have a valid point that most people just don't even look at it until something is wrong.

    Perhaps if keeping things "wet" is good, turning the fuel back on after it is verified operational would do. Now you know the OFF settings works, and you don't have to worry about it later.
     
  35. Alexb2000

    Alexb2000 En-Route

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    I had a stiff valve on my 206 since it was new. Required quite a bit of force to switch. It never leaked and worked in all positions. I discussed the difficulty in servicing these with my mechanic and decided to just replace the valve. I suspected a machining issue which I knew they couldn't solve. Do you service these beyond seal replacements?
     
  36. Tom-D

    Tom-D Taxi to Parking

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    no...
     
  37. Perezhr

    Perezhr Pre-takeoff checklist

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    Many low wing aircraft do not have a BOTH. I'm not sure about the BO, but my Mooney has LEFT, OFF, RIGHT and you have to actively manage the fuel.
     
  38. BillTIZ

    BillTIZ Final Approach

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    I do not recall any low wing aircraft I have flown, that had a "both" position in the fuel system.
     
  39. denverpilot

    denverpilot Tied Down PoA Supporter

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    And John will wink and tell you at systems class that bothering to fix it is a great money-maker for mechanics and it won't matter much at all in the end.

    On some 182s, it's possible to drain both tanks from the selector on left or right indefinitely, not just to the level of the vent line, because there's another crossover in the system, added for the lawyers.

    He goes over it pretty thoroughly in class. But generally the fuel feed system in the 182 is a totally screwed up engineering design with a bunch of bandaids on later model 182s that make the selector pretty useless other than "off" and no one recommends using that position for anything other than parking on a really slopes ramp.

    Even then, you're better off just putting the left wing uphill and knowing the right wing will be full all the way up to the cap when you come back in a couple of hours.

    You'll remember about the time you open it to stick the tank during pre-flight. Ask me how I know. ;)
     
  40. Stewartb

    Stewartb Final Approach

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    Have you built an airplane and crafted your own fuel system to feed the engine from two tanks? What did you do differently from the traditional fuel systems that you claim were full of band aids for lawyers? Educate us.