Please let me know if you have any ideas.
Many thanks,
Milton
Uneven fuel consumption!
Moderators: GAHorn, Karl Towle, Bruce Fenstermacher
Uneven fuel consumption!
I am wondering if anyone out there knows what's going on with my 1955 c-170B. I am using more gas out of my right tank than out of the left one by aprox.2:1
Please let me know if you have any ideas.
Many thanks,
Milton
Please let me know if you have any ideas.
Many thanks,
Milton
I do have vented caps on both sides and when I bought the plane several months ago it was draining both tanks evenly. The last couple of flights I noticed a sudden change in consumption from the right tank (more!)
Maybe the cap on the left tank needs replacement or it's vent is clogged???
Any ideas?
Maybe the cap on the left tank needs replacement or it's vent is clogged???
Any ideas?
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Mike Smith
- Posts: 175
- Joined: Mon Jun 02, 2003 2:53 pm
I have a vented camp on only one side and have the same thing going on in my 1950 170A. It burns more out of the left, so I usually switch to the right then back to both, then back to the right ... and so on. I would guess that the single vented cap is the culprit, but I don't know.
Sometimes, if the air is really smooth and I'm verrrrry diligent about keeping the ball centered it will burn evenly. This leads me to suspect that nothing is really out of whack and the airplane just has a tendency to burn more out of the left than the right.
In every airplane I've flown that has a right and left tank, one usually burns a little faster than the other. This has been true for both props and jets. Every once in a while I'll see one that burns evenly ... but that's not the rule.
Mike
Sometimes, if the air is really smooth and I'm verrrrry diligent about keeping the ball centered it will burn evenly. This leads me to suspect that nothing is really out of whack and the airplane just has a tendency to burn more out of the left than the right.
In every airplane I've flown that has a right and left tank, one usually burns a little faster than the other. This has been true for both props and jets. Every once in a while I'll see one that burns evenly ... but that's not the rule.
Mike
Mike Smith
1950 C-170A
1950 C-170A
Mike, what jet have you been flying that has a "both" fuel selector possibility? (Douglas's maybe?)
Generally and usually when this occurs (as it does in most high-wing Cessnas) the cause is merely due to vagaries of fuel head-pressure and venting.
Since the 170A & B have essentially the same system, I believe it goes like this. The right cap (usually and req'd by AD note) is vented. The two tanks share a common vent line to the "gooseneck" vent tube.
When all is working properly, the tank with the better ventilation will draw down more readily, and as the time of most full tanks occur immediatley after a fueling stop, right rudder being used more during a climb, the right tank has a higher head pressure and so empty's slightly faster until the climb ceases and/or the two tanks equalize in head pressure. Then both tanks feed relatively equally.
Sometimes the tanks are filled so completely that the cross-over vent line contains fuel as well. In such cases, the left tank will sometimes siphon fuel over from the right tank, which is 1) better vented and 2) has it's fuel shifted toward the inboard ventline connection due to right rudder application and in B models, dihedral. This allows the left tank to feed fuel more readily until that siphon suction is broken by the falling fuel level of the right tank. Notice that the cockpit indication is that the fuel has been feeding FROM the right tank, but in actuality the right tank has been feeding both the engine AND the left tank, which has also been feeding the engine. But the indication leads the pilot to believe that only the right tank has been supplying fuel to the engine.
Airplanes with dual-vented tanks can probably attribute uneven fuel feed indications primarily to the cross-over vent line siphoning action as well.
Don't be too concerned with it unless it gets so far advanced that one wing gets noticeably heavy, or one tank refuses to feed at all after the opposite tank has lowered below 3/4 or 1/2. You might need to confirm the fuller tank is feeding by selecting it, if it remains full even when the opposite tank gets to 1/2.
Keep in mind that the fuel caps have a gasket that seals against the filler-neck that can become dry and cracked, and may alter the cap-venting. A dry/cracked gasket may behave like a vented cap, so whichever cap is truly vented may not always be the tank that feeds first.
Hope this explanation helps.
(BTW, the method to check the function of the crossover vent line is NOT to blow air into it, which may damage it and the hose connections, and may contaminate the fuel with spiders and mud-dauber nests. The best method is to use a "siphon-blow-gun" connected to the gooseneck. Open the fuel cap and listen for the flow of air into the filler neck as the siphon blowgun is operated. If you don't have such a tool (commonly used to wash down an engine) then a long hose attached to the gooseneck at one end, and a funnel attached to the other end as a "bullhorn" can be spoken/whispered into while an observer listens at each fuel cap filler. DO NOT inhale from such a set up(it can cause pneumonitis which can be fatal), and do not use any electrical noisemaker, etc. to substitute for the whispered voice. Gasoline fumes settle and a spark can be spectacular. I've heard of a wind-up bedside clock being used successfully.)
Generally and usually when this occurs (as it does in most high-wing Cessnas) the cause is merely due to vagaries of fuel head-pressure and venting.
Since the 170A & B have essentially the same system, I believe it goes like this. The right cap (usually and req'd by AD note) is vented. The two tanks share a common vent line to the "gooseneck" vent tube.
When all is working properly, the tank with the better ventilation will draw down more readily, and as the time of most full tanks occur immediatley after a fueling stop, right rudder being used more during a climb, the right tank has a higher head pressure and so empty's slightly faster until the climb ceases and/or the two tanks equalize in head pressure. Then both tanks feed relatively equally.
Sometimes the tanks are filled so completely that the cross-over vent line contains fuel as well. In such cases, the left tank will sometimes siphon fuel over from the right tank, which is 1) better vented and 2) has it's fuel shifted toward the inboard ventline connection due to right rudder application and in B models, dihedral. This allows the left tank to feed fuel more readily until that siphon suction is broken by the falling fuel level of the right tank. Notice that the cockpit indication is that the fuel has been feeding FROM the right tank, but in actuality the right tank has been feeding both the engine AND the left tank, which has also been feeding the engine. But the indication leads the pilot to believe that only the right tank has been supplying fuel to the engine.
Airplanes with dual-vented tanks can probably attribute uneven fuel feed indications primarily to the cross-over vent line siphoning action as well.
Don't be too concerned with it unless it gets so far advanced that one wing gets noticeably heavy, or one tank refuses to feed at all after the opposite tank has lowered below 3/4 or 1/2. You might need to confirm the fuller tank is feeding by selecting it, if it remains full even when the opposite tank gets to 1/2.
Keep in mind that the fuel caps have a gasket that seals against the filler-neck that can become dry and cracked, and may alter the cap-venting. A dry/cracked gasket may behave like a vented cap, so whichever cap is truly vented may not always be the tank that feeds first.
Hope this explanation helps.
(BTW, the method to check the function of the crossover vent line is NOT to blow air into it, which may damage it and the hose connections, and may contaminate the fuel with spiders and mud-dauber nests. The best method is to use a "siphon-blow-gun" connected to the gooseneck. Open the fuel cap and listen for the flow of air into the filler neck as the siphon blowgun is operated. If you don't have such a tool (commonly used to wash down an engine) then a long hose attached to the gooseneck at one end, and a funnel attached to the other end as a "bullhorn" can be spoken/whispered into while an observer listens at each fuel cap filler. DO NOT inhale from such a set up(it can cause pneumonitis which can be fatal), and do not use any electrical noisemaker, etc. to substitute for the whispered voice. Gasoline fumes settle and a spark can be spectacular. I've heard of a wind-up bedside clock being used successfully.)
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Mike Smith
- Posts: 175
- Joined: Mon Jun 02, 2003 2:53 pm
George,
I guess I should have qualified my response with ... if you've chased all the likely culprits and everything seems "normal", then this is my experience. Some things, IMHO, are not worth the trouble one puts into chasing them down. This follows the old saying, "if it ain't broke, don't fix it."
On the jets, T-37, T-38, U-2, B-737, B-727. The one I'm currently qualified on now, the 737, is more often than not in a process of getting out of balance. Most flights over 50 minutes usually require fuel balancing by the crew. The T-37 was usually good unless we were doing some acro or spins, the T-38 was more often in balance than not and the U-2 has no guage for the wing tanks so we had to balance by "feel". We usually turned the roll axis of the autopilot off at altitude and watched to see which wing fell first. Then we'd balance accordingly and repeat the process to see if we got it right. I was an engineer on the 727 and it was fairly common for me to do the fuel balancing procedure.
So, the big point of my dissertation is KISS. Why spend an inordinate amount of my limited recreational flying time chasing an abnormality when in fact it may not be that "abnormal" for this or many other aircraft. Safety should never be comprimised, but one should also consider "is it really that big of a deal for me to do my pilot stuff and check my fuel state every 10-15 minutes and then adjust it accordingly?"
Cheers,
Mike
I guess I should have qualified my response with ... if you've chased all the likely culprits and everything seems "normal", then this is my experience. Some things, IMHO, are not worth the trouble one puts into chasing them down. This follows the old saying, "if it ain't broke, don't fix it."
On the jets, T-37, T-38, U-2, B-737, B-727. The one I'm currently qualified on now, the 737, is more often than not in a process of getting out of balance. Most flights over 50 minutes usually require fuel balancing by the crew. The T-37 was usually good unless we were doing some acro or spins, the T-38 was more often in balance than not and the U-2 has no guage for the wing tanks so we had to balance by "feel". We usually turned the roll axis of the autopilot off at altitude and watched to see which wing fell first. Then we'd balance accordingly and repeat the process to see if we got it right. I was an engineer on the 727 and it was fairly common for me to do the fuel balancing procedure.
So, the big point of my dissertation is KISS. Why spend an inordinate amount of my limited recreational flying time chasing an abnormality when in fact it may not be that "abnormal" for this or many other aircraft. Safety should never be comprimised, but one should also consider "is it really that big of a deal for me to do my pilot stuff and check my fuel state every 10-15 minutes and then adjust it accordingly?"
Cheers,
Mike
Mike Smith
1950 C-170A
1950 C-170A
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Mike Smith
- Posts: 175
- Joined: Mon Jun 02, 2003 2:53 pm
George,
Oh yes, the origial question. None of the jets had a "both" type of fuel selector. But they got out of balance anyway even when the engineers said they should be feeding evenly. Some fed tank to engine and some fed a "header" or "center" type of tank. I was just trying to help someone not make a mountain out of a molehill.
Oh yes, the origial question. None of the jets had a "both" type of fuel selector. But they got out of balance anyway even when the engineers said they should be feeding evenly. Some fed tank to engine and some fed a "header" or "center" type of tank. I was just trying to help someone not make a mountain out of a molehill.
Mike Smith
1950 C-170A
1950 C-170A
Hi, Mike! Yeah, well, I was just pointing out that it's not an apples to apples comparison. Since none of the jets I ever flew (I'm also rated in 737's and have experienced a gradual fuel imbalance for the same reasons as I have in other jets) ...but since none of them had a "both" selection available, then any fuel imbalance was usually due to innate differences of each engine and it's fuel burn with regard to the selected thrust. What you've just pointed out is that each engine has it's own fuel burn rate. (And with their generally greater burn rates, especially in climb or down low, any differences are likely to show up as marked fuel imbalances.) But in a single engined airplane, feeding out of "both" tanks, it's not a dumb question to ask why both tanks aren't feeding equally. And unless the pilot is given a reaonable explanation, then it didn't seem foolish to me for him question whether or not he's got a fuel system irregularity.
A comparable question in a jet (for example a Hawker) might be when transferring fuel from an aux to the mains, ...why are the mains still being drawn down by the engines ...while the aux is remaining full? Certainly that would be an alarming observation while inflight, similar to why one tank in a 170 didn't appear to be feeding even tho' the fuel valve was selected to both tanks. Without an explanation, it might certainly be a concern on a flight which required access to all the fuel.
A comparable question in a jet (for example a Hawker) might be when transferring fuel from an aux to the mains, ...why are the mains still being drawn down by the engines ...while the aux is remaining full? Certainly that would be an alarming observation while inflight, similar to why one tank in a 170 didn't appear to be feeding even tho' the fuel valve was selected to both tanks. Without an explanation, it might certainly be a concern on a flight which required access to all the fuel.
Most Boeings have a crossfeed which allows both engines to feed from both tanks (assuming all fuel pumps are turned on). Then the resulting uneven fuel burn is a result of either uneven boost pump pressures or uneven check valve settings. The only time the crossfeed would be open with all pumps on would be per irregular checklist (i.e. low fuel quanity), but still, both engines feeding from both tanks. You could also tranfer fuel in this case if you had a failed check valve, not uncommon in 737.
Karl
'53 170B N3158B SN:25400
ASW-20BL
'53 170B N3158B SN:25400
ASW-20BL
Thanks, Karl. But again of course, this is way not an apples/apples comparison.
As Mike points out, the advice is "if it's not broke ...don't fix it"... while the questions was...."is it broke?"
In the situation of the Cessna 170A/B airplanes, it's not broke if one tank appears to feed faster than another, as long as after one tank is down to 1/2 ,....that both tanks appear to be feeding. In such a case, ...not only is it not broke...but also, the tank that appears to be doing the feeding ...ain't necessarily even the one that's really feeding the engine. It might be the one that appears full! (Because it's re-filling itself by siphoning fuel from the opposite tank, which is indicating no longer full.)
As Mike points out, the advice is "if it's not broke ...don't fix it"... while the questions was...."is it broke?"
In the situation of the Cessna 170A/B airplanes, it's not broke if one tank appears to feed faster than another, as long as after one tank is down to 1/2 ,....that both tanks appear to be feeding. In such a case, ...not only is it not broke...but also, the tank that appears to be doing the feeding ...ain't necessarily even the one that's really feeding the engine. It might be the one that appears full! (Because it's re-filling itself by siphoning fuel from the opposite tank, which is indicating no longer full.)
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Mike Smith
- Posts: 175
- Joined: Mon Jun 02, 2003 2:53 pm
George,
Good point on the "apples to apples". I also agree it's not a bad question. I just wanted to add my "ten cents worth" that it seems that no matter the aircraft and no matter what the engineer says, they typically burn a little uneven for a variety of reasons - not all of which mean it needs fixin'. I'm just a caveman pilot tryin' to keep things simple ... they tend to get all complicated without me even tryin'.
Good point on the "apples to apples". I also agree it's not a bad question. I just wanted to add my "ten cents worth" that it seems that no matter the aircraft and no matter what the engineer says, they typically burn a little uneven for a variety of reasons - not all of which mean it needs fixin'. I'm just a caveman pilot tryin' to keep things simple ... they tend to get all complicated without me even tryin'.
Mike Smith
1950 C-170A
1950 C-170A
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zero.one.victor
- Posts: 2271
- Joined: Tue Apr 23, 2002 12:11 am
So, is it an accurate assumption that if one tank becomes empty because of uneven fuel flow and the fuel switch is on both the engine will continue to receive fuel from the tank with fuel? Is there an advantage to having the switch on both or pointed to the tank with fuel?
Joe
51 C170A
Grand Prairie, TX
51 C170A
Grand Prairie, TX
The tank should continue to deliver fuel, and it should do so before the opposite becomes empty. Other than for balancing purposes, there's no advantage to selecting one tank or the other. I run mine on both, continuously. My right tank indicates slightly more than 3/4 when the left tank begins to drop from the full mark.
But if half the quantity indication (i.e. 1/8 )is the closest we can expect for accuracy on these gauges, you tell me,....which gauge is telling the truth?
I'd have to fly until my right indicated a 3/4, then land and dipstick both tanks to see which is really lower. I suspect they're within a gallon or two anyway, based upon re-fueling records.
But if half the quantity indication (i.e. 1/8 )is the closest we can expect for accuracy on these gauges, you tell me,....which gauge is telling the truth?
I'd have to fly until my right indicated a 3/4, then land and dipstick both tanks to see which is really lower. I suspect they're within a gallon or two anyway, based upon re-fueling records.
Last edited by GAHorn on Tue Sep 09, 2003 12:52 am, edited 1 time in total.
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Harold Holiman
- Posts: 579
- Joined: Tue Apr 23, 2002 1:54 pm
My tanks have been converted to the late style Cessna vent under the left wing. However my right tank still feeds more than my left tank. Like George's tank, my right tank goes down about 1/4 tank ahead of my left tank if I leave the selector on both. It would seem like the left tank on mine would be most likely to feed down first since it is first vented but apparantly this does not affect the flow.
Harold H
Mbr #893
Harold H
Mbr #893