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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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"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. |
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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:
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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Craig,
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. 1. Is the fuel port inboard on the C172? 2. Does selecting "both" draw fuel from both tanks, even if one is empty? 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. Why does is operation of the A/P prohibited when there is a fuel imbalance? 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. Thanks, Alex |
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I have an oblique answer to this question. I'm building an airplane that seats four and is a high wing monoplane. The fuel tanks are in the wing root, just like most high wing Cessnas. The plans show that the fuel outlets should be placed near the wingroot but in different locations fore and aft from each other. One tank has an outlet near the wingroot by the trailing edge, the other tank has the outlet near the wingroot but towards the leading edge. Since it's a homebuilt, you have some flexibility in how you do things. Why not put in two ports for each tank - one fore one aft. That way you can slip and nose down and get fuel no matter which tank is selected? Try talking with the designer of the plane about this - it might be worth considering. Jose -- (for Email, make the obvious changes in my address) |
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