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#41
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Dave Butler wrote in message ...
If you're paying attention and know your fuel burn you can predict the moment of running out within plus/minus a few minutes. When the fuel pressure gauge drops to zero, you still have some time while the engine continues to run smoothly. Switch tanks when the fuel pressure drops, the pax will never know. Of course, you've run some on the other tank previously, so you know the fuel in the other tank is sweet. Bingo! Dave's right on the money. My plane's flight manual cautions against running a tank dry (I believe there's an outside possibility of fuel pump cavitation). Using fuel pressure method works great. I do this on long cross-country flights where maximizing my range is important. In my Cherokee, the fuel pressure will begin to drop with about 1.5 gallons usable in the tank (close enough for me). At this point, assuming no unusual fuel burn, I have 1.5 hrs of fuel left in the other tank, which in the real world means I have 1/2 hr. to be on the ground. This old pilot thinks everyone should do this at least once so that they know how their plane behaves in this situation. It also provides a chance to make a direct measurement of exactly what your tank capacity is. Agreed. I do this on every flight where I'll be flying at maximum range. By running a fuel tank almost dry, I know if my fuel burn is normal and can complete the flight as planned. If the tank runs out before the clock says it should, I still have a bunch of gas to make it to an alternate (assuming I'm not flying in some of the more remote parts of the Northern Territories of Canada :-). By using this method, I can usually guess the fuel burn on a long cross-country flight to within 1 gallon. This kind of knowledge makes flying in sparsely populated areas a lot more comfortable and allows me to get better utilization of the amount of fuel my plane is able to carry. Note : Do not take the above to mean that I fly my tanks to dangerously low levels. My personal limit is to land with a minimum of 1 hr. in the tanks. In 16 yrs. I've never landed with less, but by gauging my fuel burn precisely, I can often reach my destination with 1.2 hrs. remaining, thereby maximizing my range, yet minimizing my risks. John Galban=====N4BQ (PA28-180) |
#42
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"Corky Scott" wrote in message ... snip The designer implied that this is a standard aviation design (the different locations for the fuel outlets). Dunno about others but in the few I've had occasion to help build- The PA-12 has outlets only at the aft end of the tank. This worked great for our use, banner towing, as most our flying was at high AoA (though, since the gauge was at the FORWARD end of the tank they didn't work after 30-45 minutes). The PA-18 has pickups at both ends, then feeds into a Y fitting, that then goes to a header tank (small 1 gal) then to the selector valve. Worked even better. |
#43
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Tony Cox wrote:
"Frank" wrote in message ... Tony Cox wrote: Some models of 172 *require* you to select a single tank above 5000', due (supposedly) to vapor lock problems. No 'supposedly' about it. Ours locked at 7500' on a humid day because one of my partners didn't believe it could happen and didn't follow the procedure. Fortunately an airport was within glide range. Strangely enough, by the time he landed the engine was running fine again.... -- Frank....H Its happened to me too. I said 'supposedly' because I've not heard a plausible explanation as to why selecting one particular tank rather than both should affect the chance of vapor lock. Is the fuel selector close to a heat source? The explanation I got had to do with pressure (or lack of it) above 4500'. This problem is exacerbated by high humidity. It seems that the plumbing allows a condition to develop whereby there is not enough pressure from gravity to push the fuel through the plumbing without forming "bubbles". It has to do with the venting. Closing off one tank (ie selecting left or right) solves this. This admittedly vague explanation is only intended to point out that pressure differences are the culprit. No one has ever shown me in great detail just how it happens. Nonetheless, I'm a believer. -- Frank....H |
#44
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"Frank" wrote in message
... Tony Cox wrote: Its happened to me too. I said 'supposedly' because I've not heard a plausible explanation as to why selecting one particular tank rather than both should affect the chance of vapor lock. Is the fuel selector close to a heat source? The explanation I got had to do with pressure (or lack of it) above 4500'. This problem is exacerbated by high humidity. It seems that the plumbing allows a condition to develop whereby there is not enough pressure from gravity to push the fuel through the plumbing without forming "bubbles". It has to do with the venting. Closing off one tank (ie selecting left or right) solves this. This admittedly vague explanation is only intended to point out that pressure differences are the culprit. No one has ever shown me in great detail just how it happens. Nonetheless, I'm a believer. My 'vapour lock' occurred at 7500' on a cold November day with low humidity (ceilings were well above us). If anything, switching to one tank rather than both should actually *lower* the fuel pressure slightly - faster flow = greater friction loss. So I'm not really impressed at all by the explanation. Luckily, engine power was restored when passing through 3500', shortly (as it happens) after fiddling with the fuel selector as part of the emergency checklist. Incidentally, that was the last time I assumed that the POH for one plane was the same as another. -- Dr. Tony Cox Citrus Controls Inc. e-mail: http://CitrusControls.com/ |
#45
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Tony Cox wrote:
"Frank" wrote in message ... snip The explanation I got had to do with pressure (or lack of it) above 4500'. This problem is exacerbated by high humidity. It seems that the plumbing allows a condition to develop whereby there is not enough pressure from gravity to push the fuel through the plumbing without forming "bubbles". It has to do with the venting. Closing off one tank (ie selecting left or right) solves this. This admittedly vague explanation is only intended to point out that pressure differences are the culprit. No one has ever shown me in great detail just how it happens. Nonetheless, I'm a believer. My 'vapour lock' occurred at 7500' on a cold November day with low humidity (ceilings were well above us). If anything, switching to one tank rather than both should actually *lower* the fuel pressure slightly - faster flow = greater friction loss. Right, lower fuel pressure means less oomph to get it through the pipes....Seems consistent with the 'theory'. I don't know enough about it to say whether it's right or wrong. So I'm not really impressed at all by the explanation. Luckily, engine power was restored when passing through 3500', shortly (as it happens) after fiddling with the fuel selector as part of the emergency checklist. Incidentally, that was the last time I assumed that the POH for one plane was the same as another. I know the explanation wasn't very satisfying, I'd love to hear it explained better too. But, as you have apparently also experienced, the phenomenon is real. So there is something going on and pressure differences due to venting/plumbing seems to explain it better than temperature related ones. -- Frank....H |
#46
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"Frank" wrote in message ... Tony Cox wrote: My 'vapour lock' occurred at 7500' on a cold November day with low humidity (ceilings were well above us). If anything, switching to one tank rather than both should actually *lower* the fuel pressure slightly - faster flow = greater friction loss. Right, lower fuel pressure means less oomph to get it through the pipes....Seems consistent with the 'theory'. I don't know enough about it to say whether it's right or wrong. To get maximum oomph, it would seem that 'both' should be selected. Which is just what they tell you not to do! I know the explanation wasn't very satisfying, I'd love to hear it explained better too. But, as you have apparently also experienced, the phenomenon is real. So there is something going on and pressure differences due to venting/plumbing seems to explain it better than temperature related ones. I just draw a blank on this. Unless there is a heat source near the selector. Then I can rationalize an explanation. If there's some venting/plumbing explanation, I'd expect some admonition to "select the fullest tank" or "don't fly above 5000' without more than 10 gallons in a tank". Makes no sense as is. tc |
#47
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Tony Cox wrote:
snip I just draw a blank on this. Unless there is a heat source near the selector. Then I can rationalize an explanation. If there's some venting/plumbing explanation, I'd expect some admonition to "select the fullest tank" or "don't fly above 5000' without more than 10 gallons in a tank". Makes no sense as is. tc As far as I can tell there is no heat source anywhere near the selector. One day I'll get the real story, until then....I'll just select L or R above 5000' and chalk it up to one of those mysteries of the universe. -- Frank....H |
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