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"Tanks on both" checklist item
Good day all,
With regards to fuel tanks in a C172, why does the takeoff and landing checklists both call for the fuel selector handle to be in the "both tanks" position? The only time that fuel is set to one particular tank is on the ground. I've never used fuel from only one tank in flight. Why would someone do that? Also, why is the fuel selector set to one tank during refueling? Is it to minimize crossfeeding? Thanks, Alex |
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#3
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"Koopas Ly" wrote:
With regards to fuel tanks in a C172, why does the takeoff and landing checklists both call for the fuel selector handle to be in the "both tanks" position? The geometry of the fuel tanks and the locations of the fuel ports in the tanks cause the actual amount of usable fuel in each tank to vary depending on aircraft attitude. By way of example, let's say that you've only got 5 gallons in the right tank, and the wind conditions on landing require a pronounced slip with a bank to the right. If the fuel port is inboard and all the fuel goes sloshing outboard, and only the right tank is selected, you've just starved your engine of fuel. But as long as you have some fuel in both tanks and Both selected, you ought to be fine even in a prolonged uncoordinated condition. The only time that fuel is set to one particular tank is on the ground. I've never used fuel from only one tank in flight. Why would someone do that? Cessnas have the notorious characteristic of not drawing fuel evenly from both tanks in the Both position. This can result in fuel imbalance. In a Cessna 172 you might never notice any difference in the flight characteristics, but in a 182 which has more fuel capacity in each wing than the 172 has in total, you can develop an imbalance which becomes fairly uncomfortable. By selecting one tank or the other, you can burn fuel (mostly) from the full tank in order to reestablish lateral balance. Read your autopilot limitations carefully as well. My POH prohibits operation of the autopilot when fuel imbalance exceeds 90 lbs. That's only a 15 gallon difference between left and right, so I try to stay ahead of the situation. Also, why is the fuel selector set to one tank during refueling? Is it to minimize crossfeeding? If you're parked not quite level, and start refueling with the fuel selector in the Both postion, if you start by topping off the high wing, some of that fuel may crossfeed into the low wing while you switch sides and go about filling the low wing. If you don't recheck the high wing, you might not notice that the first tank is no longer full, and that you are a few gallons short of the fuel load you planned for your trip. |
#4
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"Koopas Ly" wrote in message om... Good day all, With regards to fuel tanks in a C172, why does the takeoff and landing checklists both call for the fuel selector handle to be in the "both tanks" position? The only time that fuel is set to one particular tank is on the ground. I've never used fuel from only one tank in flight. Why would someone do that? Also, why is the fuel selector set to one tank during refueling? Is it to minimize crossfeeding? Thanks, Alex There are several 'parts' to the answer here. The first is that in certain failure scenarios, the ability to switch tanks is useful. Imagine in flight, you suddenly see a leak from the right tank. In this situation, you obviously want to land quickly, but with the individual tank selection ability, you can switch to burn fuel from the leaking tank, then switch to the other when this runs out, and this tank will not be loosing fuel. In a sense this is a 'left over' from larger multi-tank installations. The second relates to a problem that Some versions of the Cessna have in flight, where in certain atitudes, there can be fuel feed problems. Some are placarded to use single tanks at altitude to avoid this, since when the problem occurs, switching to the other tank cures it (at least temporarily). This was to do with a low pressure area forming over the fuel cap, and the fuel caps were redesigned to prevent it. The 'cross feed' answer is correct on fuelling. If the selector is left to both, especially if the plane is not level, and the upper tank is filled first, fuel can drain into the lower tank, which is then filled, and the result is a fuel load significantly below what is expected. The same problem, can also "rear it's head" in flight. If a pilot flies the plane out of balance (or it is not rigged quite square), there can be a very significant tendency to cross feed. Manually using the selector to draw fuel from the 'heavy' wing, can allow this to be compensated for. Best Wishes |
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"Koopas Ly" wrote:
The only time that fuel is set to one particular tank is on the ground. I've never used fuel from only one tank in flight. Why would someone do that? Cessnas have the notorious characteristic of not drawing fuel evenly from both tanks in the Both position. This can result in fuel imbalance. In a Cessna 172 you might never notice any difference in the flight characteristics, but in a 182 which has more fuel capacity in each wing than the 172 has in total, you can develop an imbalance which becomes fairly uncomfortable. By selecting one tank or the other, you can burn fuel (mostly) from the full tank in order to reestablish lateral balance. I rent the aircraft I fly. Several times the previous renter wrote a squawk stating: "Fuel gets used only from left/right tank.". Thus far it turned out that they could not fly coordinated... Nevertheless, whenever I see this kind of squawk, I select left tank to taxi, right tank for runup, both tanks for takeoff. After having reached sufficient altitude, I fly for several minutes first on the left tank, then on the right. If this works oke, there is nothing amiss with the fuel feed. Of course, if it turns out there IS a problem, I can select the other tank (or both) and return with sufficient fuel. Beter to find out early than discovering later in the flight one of the tanks does not feed at a moment I do not expect it and possibly at a point which is a bit awkward. |
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"Koopas Ly" wrote: With regards to fuel tanks in a C172, why does the takeoff and landing checklists both call for the fuel selector handle to be in the "both tanks" position? Cessnas have the notorious characteristic of not drawing fuel evenly from both tanks in the Both position. This can result in fuel imbalance. In a Cessna 172 you might never notice any difference in the flight characteristics, but in a 182 which has more fuel capacity in each wing than the 172 has in total, you can develop an imbalance which becomes fairly uncomfortable. By selecting one tank or the other, you can burn fuel (mostly) from the full tank in order to reestablish lateral balance. On longer XC flights in my 172N, my SOP is to depart with selector on both & remain there 1/2 hour. Next 1/2 is on right only. Then, back to both for next 1/2 hr, continuing that sequence with prelanding checklist calling for both. The drawdown of fuel is much faster from the left tank than the right when using both. Also, why is the fuel selector set to one tank during refueling? Is it to minimize crossfeeding? I leave the selector on both when refueling. After topping the tanks, I rock the wings before I go pay & pee. During the following preflight, when I confirm the placement of the filler caps, the fuel level is always topped off. YMMV John If you're parked not quite level, and start refueling with the fuel selector in the Both postion, if you start by topping off the high wing, some of that fuel may crossfeed into the low wing while you switch sides and go about filling the low wing. If you don't recheck the high wing, you might not notice that the first tank is no longer full, and that you are a few gallons short of the fuel load you planned for your trip. |
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Koopas Ly ) wrote:
snip The only time that fuel is set to one particular tank is on the ground. I've never used fuel from only one tank in flight. Why would someone do that? The C172 (at least the more current model years with which I am familiar) does not draw fuel from both tanks evenly when the selector knob is set to "Both." The reasons for this anomaly are numerous. This unequal fuel draw tends to be more noticeable on longer XC flights where one tank could potentially be five-to-eight gallons lower per hour when compared to the other. Unequal fuel load translates to unequal weight distribution. Therefore, on these longer flights and only during level cruise, the pilot should be using the Left/Right selector knob to even out this imbalance. I find myself adjusting the selector knob once every thirty minutes or so. -- Peter ----== Posted via Newsfeed.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeed.com The #1 Newsgroup Service in the World! 100,000 Newsgroups ---= 19 East/West-Coast Specialized Servers - Total Privacy via Encryption =--- |
#8
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Amazing...all those answers by 172 owners and no one of them went to the net to
find the "truth". If you go to Google and fill in "Airworthiness Directive for 172 fuel system", you will get a string of answers and some really foolish conclusions (like this string) that are worth reading. One states...my plane has the placard to not fly on both above 5,000 feet, but I did and the engine quit at 8500. Sigh (mine). If you know how to get to rec.aviation.owning of the Usenet, the string is there. Note that the problem is model/version specific. Neal |
#9
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"Koopas Ly" wrote in message
om... The only time that fuel is set to one particular tank is on the ground. I've never used fuel from only one tank in flight. Why would someone do that? Some models of 172 *require* you to select a single tank above 5000', due (supposedly) to vapor lock problems. Check your POH. -- Dr. Tony Cox Citrus Controls Inc. e-mail: http://CitrusControls.com/ |
#10
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COUGARNFW ) wrote:
Amazing...all those answers by 172 owners and no one of them went to the net to find the "truth". If you go to Google and fill in "Airworthiness Directive for 172 fuel system", you will get a string of answers and some really foolish conclusions (like this string) that are worth reading. It really must be painful for you to walk among mere mortals. -- Peter ----== Posted via Newsfeed.Com - Unlimited-Uncensored-Secure Usenet News==---- http://www.newsfeed.com The #1 Newsgroup Service in the World! 100,000 Newsgroups ---= 19 East/West-Coast Specialized Servers - Total Privacy via Encryption =--- |
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