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 11, 2011 #21

    Aircar

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    The NTSB report on John Denver's accident identifies the fuel tank selector as the primary reason for his death --it required an awkward over the shoulder manipulation of a non standard placed lever and he lost orientation and control during the switching it was deduced.

    Motorcycles also have a fuel shut off presumably because of the possibility of fuel draining by gravity when parked (loss of fuel could strand you in either a motorcycle or aircraft if the carby float was not functioning or some other leak or flooding of the intake occurred -- maybe, just surmising, the fire on start up possibility or even hydraulic lock if a cylinder filled with fuel is behind the requirement ) On a Cessna single the fuel select/shut off is out of sight and in a position that is easily forgotten (somewhere down near the seat mount on the floor I recall --it was a favourite trick of one instructor to stealthily turn the cock off and wait for the student to forget to check and run out of fuel while taxying or in run up .

    Meant to drive home the checklist but also showing how well hidden the thing is --easily overlooked.

    Having a discrete sump in the actual wing tank to do a water check imposes another restraint on where a common connection can be and contamination of one tank (eg refilling only one side) could make having a selector a safety feature (there was a massive AVGAS fuel contamination problem in Australia about ten years ago as a result of an error at the refinery --gumming up of filters and corrosion etc )

    Just a comment on the high wing gravity feed and "wing low" thing -- I was once 'checking the hand ling' on a Cessna 172 -- at some height -- testing the slipping behaviour to see whether it ran out of rudder or aileron first only to find that it ran out of FUEL before either (!) -- a quick jump on to the primer and a cough or two restored the noise but this points out at least one reason why Cessna singles say 'no slipping' (the other might be that if you do a full blooded side slip with full flap in a C 150 it will snap roll so fast it will make your head spin.....only a knot or two under normal approach speed and the ASI goes U/S with a lot of yaw )

    The problem of fuel assymetry is similar to the water ballast dumping (and filling) case in sailplanes and there have been instances of uneven dumping and cross feeding in banked flight due to sustained r turning (and sinking relative to the air and not always co ordinated flight)

    Now ...about inverted fuel systems......
     
  2. Sep 11, 2011 #22

    Turd Ferguson

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    A Caravan has 2 pilot operated fuel selector valves and after the Hermans Air Service crash (where a commercial pilot managed to start, taxi and takeoff with both fuel selectors in the off position) a fuel selector OFF warning system was designed and installed on all Caravans. It's one of the best designed features on the airplane and I'm not aware of any fuel selector related accidents or incidents since.

    So ultimately, I think we can have both. A selector valve or valves, for obvious safety reasons, to comply with the regs and it can be a design that considers the shortcomings of the human in the cockpit.
     
  3. Sep 11, 2011 #23

    autoreply

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    Hmm. I'm allergic to all those "automatic" things because according to statistics they can fail too and there are many of them. According to Murphy for example, one of the fuel pumps will fail and lock up, instead of freefloat. Trap doors can wear down or be wrecked by fuel. And so on.

    If we're talking about low wings, why not have 2 independent systems. Each wing has a single vent, a single fuel pump and a single filter and both fuel lines join only in a small (1/2 gallon) sump, which gravity-feeds into the engine.

    Contrary to the system from Langford for example, no single component failure can leave you without fuel, unless most systems and the resulting unbalance if one of the sides fails to pump isn't a problem for most aircraft
     
  4. Sep 11, 2011 #24

    Mad MAC

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    I have sourced breakaway valves off a parted out bell 222 for a project. They make sense on helicopter fuel tanks that have meet the part 27 etc, drop tests but on wing tanks and the like, they possibly don't come out on the right side of the risk/benefit analysis.

    Its interesting to note part 23 & 27 single engine turbo props/shaft are required to have fuel systems without tank selectors (there is something about Cessna caravans, getting an exemption on this rule I think).

    I know one of these systems (low wing) uses jet pumps to transfer the fuel through the wing to the sumps (not have a system schematic of it).
     
  5. Sep 11, 2011 #25

    Vigilant1

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    And a line between the tanks (controlled with a normally-closed crossflow valve) could provide that last bit of redundancy. If either fuel pump fails, open this crossflow valve and raise the "full" wing to let the "stranded" fuel get to the tank with the good pump.

    I recall reading about a fuel system that uses a small header tank with a pump (with a backup) that constantly fills it from the same wing (left?). The acft has FI, and the bypass fuel not used by the engine always goes back to that same wing tank. The pilot uses a transfer pump to periodically move fuel to the left "main" tank from the right "aux" tank. You could put it on a timer: Green light in cockpit indicates 20 minutes have passed, press button one time to activate transfer pump for 30 seconds moving 2 gallons of fuel.
     
  6. Sep 11, 2011 #26

    Turd Ferguson

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    I'm guessing you would use electrically powered fuel pumps to transfer fuel to the "sump". If that's the case, loss of electrical power would leave you unable to get fuel to the engine.
     
  7. Sep 11, 2011 #27

    autoreply

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    As it would in any other aircraft with two of those pumps. Which is by the way the reason you have considerable juice in the batteries.

    Comparing it with the KR2 setup, simply make one mechanical and one electrical.
     
  8. Sep 11, 2011 #28

    Dan Thomas

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    The fuel shutoff is to stop fuel feeding an engine fire in flight. If you had a fire start and it burned through the fuel line and had no shutoff, you'd be toast long before you could get the thing on the ground. The FARs that govern certified airplanes are there because a whole lot of stuff was learned the hard way by pilots who died or were permanently and sometimes grotesquely maimed. I've seen more than one guy who was badly burned; if one was a pilot who was asked if the thought a fuel shutoff was more dangerous than not having one, I know what he'd say.

    We also have checklists for starting, runup, takeoff, climb, cruise, landing, and shutdown. They're there for a reason. Flying small airplanes is much more complicated and risky than driving a car, and we have to be willing to learn to do it right. Lazy pilots want everything done for them; they should take the airlines instead.

    Dan
     
    Last edited: Sep 12, 2011
  9. Sep 11, 2011 #29

    Dan Thomas

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    Several things here that need comment:

    1. There are only so many places in a small airplane to put controls, and the 172 and its larger brethren must have the fuel selector in a low spot as far forward as practical so it's under the floor. The lever that works it is on the center console, not a stupid place at all for the pilot who is properly taught what it does and how to use it. We can't have everything right in front of us just because we can't be bothered to remember it or use a checklist.

    2. If a 172's engine quit in a slip, it meant one of two things: Either the fuel was below the Unuseable Fuel level (which is around 2 gallons per side and is there to let the engine get fuel "in the attitude most critical for flight," as the regs put it, or else the pilot had either Left or Right selected instead of Both. The selector placard says Both must be selected for takeoff and landing simply because of such maneuvers; you cannot run the engine dry on Both in a slip unless the total fuel is too low anyway. Gravity feed makes sure fuel is flowing from one side or the other.

    3.The Cessna 150 will not flick into a snap roll from a slip with or without flaps. I have hundreds of hours instructing in various Cessnas and others and have never had any hassle with that. Skidding at low speed, on the other hand, can result in a spin, but that phenomenon is common to most light airplanes, not just Cessnas.

    4. The "Slips Prohibited" thing is an aviation myth. In some older Cessna 172s (those that had 40° flap) there was a placard required that said "Avoid slips with flaps extended." Not "Prohibited." The reason is that in an aggressive slip with 40 flap, the elevator might oscillate a little; nothing dangerous at all. It's the tail moving in and out of the flap downwash. As I said, I have hundreds of hours in these things and slipped with full flaps all the time (and still do, after 19 years) and have never had any scare at all.

    Dan
     
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  10. Sep 11, 2011 #30

    Turd Ferguson

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    The carvan left and right fuel selectors are simply an ON/OFF valve that allows fuel to flow from the left and right wing tanks (respectively) to the reservoir tank under the pilot seat floorboard. From there the fuel is fed to the engine, an electric boost pump provides head pressure for start and critical flight regimes, an ejector pump handles the chore during less critical times. There is also an emergency only firewall shutoff valve. A quick scan of Part 23 doesn't seem to indicate any of this is prohibited on SE turbine airplanes.

    I'd be interested in your source on the Caravan getting an exemption. It's my understanding the fuel system was fully compliant with the current FARs when certificated. Since it was to be the first US single engine turbine to go into production, the FAA wasn't too keen on handing out exemptions.
     
    Last edited: Sep 11, 2011
  11. Sep 12, 2011 #31

    Turd Ferguson

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    I'm not aware of any other aircraft operating with such a system - where loss of electrics would cause the engine to stop. I guess a homebuilder is free to do as he pleases but the FAA would never certify such a system, for obvious reasons.
     
  12. Sep 12, 2011 #32

    GESchwarz

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    I agree with the recent comments about why we have a shutoff. Most all requirements are written in blood, meaning that lost of people were hurt or killed before the rule was made. Once the rule and the fix is in place, the problem goes away and everybody forgets or otherwise becomes ignorant from one generation to the next. The rule then is challenged and broke and history starts to repeat itself.

    The key is to know your system, and to design and build it in such a way that it is easy to operate correctly.

    The way John Denver died was absolutely predictable. The design was really bad, and the plane was new to him, and he was not careful.
     
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  13. Sep 12, 2011 #33

    orion

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    Actually the slip avoidance on Cessnas with 40 deg. of flaps was a result of an investigational program conducted by NASA in the mid-1970s. The program examined several of the available flight trainers of the day (Cessna, Piper and Grumman) and worked at determining their suitability to the training mission, and their safety. The flight tests were pretty exhaustive and examined all the nooks and crannies of the aircrafts' flight envelopes. One of the findings was that on the 150's and the 172's that came with 40 deg. of flaps there was a significant occurrence of actually blanketing the horizontal surface during a slip with the flaps fully deployed. The NASA finding was cautionary however the FAA issued an AD that installed a blocking device on the flap control that prevented the flaps from being extended more than 30 degrees.

    Most owners I knew modified or removed said device (between annuals) so they still got full use of the aircraft's capabilities. I trained in one of those and one day, out of curiosity, I flew several slips with full flaps to see if I could replicate the condition. And success on the first try - a power off slip with full flaps resulted in a stable condition of descent. Glide slope trim was however unavailable (except with power) - the horizontal was truly blanketed. I was able to move the yoke the full range (stop to stop) with no effect on the airplane. But the fix was simple - relax rudder pressure and effectiveness returned immediately. I was able to replicate this with the 150's and the 172.

    After this point all new Cessnas had only 30 deg. of flaps. Too bad though, the 40 deg. flap was quite useful.
     
  14. Sep 12, 2011 #34

    BBerson

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    I suppose a study would be needed to see how many inflight fires could have been stopped using a shutoff valve compared with how many takeoff crashes happened because of the valve .
    A shutoff is not needed in my aircraft because all I need to do is shut off the engine and the fuel pump will stop pumping. The fuel lines have fire resistant sleeves also and that might help and is a passive safety measure. Fire sleeves probably cannot cause a crash like a shut off valve.
    Yes, some high wing designs might need a shutoff valve, my aircraft does not.
    The regulations should be reviewed from time to time as new safety ideas are invented.
    I don't feel that the young female commercial pilot that died in the Herman's crash was lazy. Airplanes should made as simple and foolproof as possible.
    BB
     
  15. Sep 12, 2011 #35

    Dan Thomas

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    Funny. Never found this at all. We regularly slip with full flaps (40°) and I have never encountered an AD or flap stop in any of the two 150s or 172s we've had. The only AD I can find regarding 172 flaps has to to with defective actuators.

    Dan
     
  16. Sep 12, 2011 #36

    Turd Ferguson

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    Perhaps not lazy but she had no formal training on the aircraft. Many people have found the hard way that using the "It's just a big 172" mentality is not the right attitude for flying a Caravan.
     
  17. Sep 12, 2011 #37

    Turd Ferguson

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    I'm not aware of any AD's to limit flap travel to 30 degrees either (and I have performed a lot of annual inspections on these airplanes). I'm quite sure the 150 never carried a caution about slipping with full flaps...that model never exhibited the nose bobbing phenomena when slipping with full flaps. The 172 would only do this under certain loading conditions, it is not easy to duplicate. Not aware of any instance of elevator control being completely lost, that would most certainly generate action from the small airplane doctorate.

    Cessna did limit the flap travel to 30 degrees on the 152 and post "N" model 172's so they could increase gross weight and maintain the requisite go-around climb performance with full flaps. You can get a gross wt. increase on the some 172's with 40 degrees of flaps by limiting the flap travel to 30 degrees. That is the only reason why there may be a flap limiting requirement on those airplanes - they have the STC weight increase.
     
  18. Sep 12, 2011 #38

    orion

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    Interesting - I remember the instance because our flight school had to modify their airplanes at the time and I remember the silly little piece of metal that was installed behind the instrument panel (with a simple screw - easy to remove). I was told it was and AD by the owner of the plane I trained in - but that was a while ago - I do not recall the details. But I do recall that the deflection limit was sort of a pain once you were used to the full deflection.

    The condition that the NASA tests uncovered were not characterized by bobbing or pitch attitude change of any sort - that's actually why it was considered a risk. The airplane in a full forward slip was actually very stable. The problem envisioned however was where a pilot continued the slip to near ground (possibly due to high cross wind) and then would attempt to flare. Since the tail was blanketed, there would of course be no flare and the aircraft would continue its descent until it hit.

    If I have time I'll try to dig out the old reports and see if there are any references to the instances I recall.
     
  19. Sep 12, 2011 #39

    Richard6

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    Ah - just a small note - autos do have a fuel shutoff system, or at least the ones made in the last 10 years or so. The fuel pump is in the tank, and only runs when the starter is on or when there is oil pressure.

    Richard
     
  20. Sep 12, 2011 #40

    skeeter_ca

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    Actually the fuel pump on all modern, like the last 15 yrs, runs when you first turn the key on. That primes the system. If the PCM(computer) doesn't see an rpm signal from the crankshaft sensor or camshaft sensor within about the first 5 seconds it shuts the pump down and waits. When the starter is cranked over and see's an rpm signal it reactivates the pump. If during anytime the engine quits for some reason it will shut the pump off also. Some vehicles but not all have an inertia/impact switch that will also shut down the fuel pump during an accident.
     

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