Cessna 172 fuel flow problem - any ideas?
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Cessna 172 fuel flow problem - any ideas?
A 1968 Cessna Skyhawk, Continental O-300-D, in flight, doesn't feed from port wing tank, with fuel selector on Both.
Running on the ground, tank selector to port, no problem. Timed static fuel flow from both tanks is identical.
Any advice and/or clues would be appreciated.
Running on the ground, tank selector to port, no problem. Timed static fuel flow from both tanks is identical.
Any advice and/or clues would be appreciated.
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Have you checked the position of the tank breather? It should be just behind the strut out if the direct air flow. When mine got bent the fuel tank pressurised and would not feed from the left tank.
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"A 1968 Cessna Skyhawk, Continental O-300-D, in flight, doesn't feed from port wing tank, with fuel selector on Both. "
How long did you run the test for? My old 172 used to feed from one side for around 45 min then switch itself to the other. I decided never to use "both" so I could keep track of how much was in each tank. Engineering opinion (M3) at the time was that this was normal.
Rod1
How long did you run the test for? My old 172 used to feed from one side for around 45 min then switch itself to the other. I decided never to use "both" so I could keep track of how much was in each tank. Engineering opinion (M3) at the time was that this was normal.
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Had the same problem not too long ago, Rented a 1968 C172 in LAX and flew it to SBA. During flight I noticed the left wing tank staying full and the right wing tank decreasing with fuel selector in both. After landing a fuel dip check confirmed that the indicators where accurate. Filled only the right tank and on the way back, same happened. Let the rental place know about it. As most of the flight took place at low altitudes with few places to land I didn't' try and trouble shoot with the fuel selector in different positions!
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The position of the ball in the slip indicator can also be an indicator of fuel flow in flight 
.
If the plane will happily feed fuel for full power from either tank in flight, and the "both" selection shows up imbalance, then as suggested, the position of the tent tube is the first place to look. The Maintenance Manual provides diagrams as to the correct position. Also fuel caps for a tight fit, and if they are the venting type, the function of their vent(s).
If none of those characteristics are the source of the problem, it's maintenance shop time.
. If the plane will happily feed fuel for full power from either tank in flight, and the "both" selection shows up imbalance, then as suggested, the position of the tent tube is the first place to look. The Maintenance Manual provides diagrams as to the correct position. Also fuel caps for a tight fit, and if they are the venting type, the function of their vent(s).
If none of those characteristics are the source of the problem, it's maintenance shop time.
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Now that's really interesting about the slip indicator. I'm new to flying this aircraft, and I've noticed that the ball is routinely out to the left (full width of the ball), and in the cruise I need a bootful of left rudder to keep her trimmed. There's no cockpit rudder trim on this model.
I can't work out the sense, though - ball well out to the left = reduced/negligible flow from port side tank?
Both fuel caps are new - so that needs looking at as well.
re: How long did you run the test for?
Static test on the ground was around 30secs and 1/2 US gal. identical total flow from both tanks. In the air, around 45 mins, showing 2 US gal. (7 litre?) difference.
I can't work out the sense, though - ball well out to the left = reduced/negligible flow from port side tank?
Both fuel caps are new - so that needs looking at as well.
re: How long did you run the test for?
Static test on the ground was around 30secs and 1/2 US gal. identical total flow from both tanks. In the air, around 45 mins, showing 2 US gal. (7 litre?) difference.
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That's a common 'problem' with older Cessnas, the 182 seem to be more prone to it than the 172s. The reason is the venting. No big deal, it's the way the plumbing works. Usually stabilizes after about 45-60 minutes. If the difference is really drastic, then as Bose-X says, check the position of the vent line, very often this is slightly out of whack on older a/c.
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Before you spend too much time investigating a non-issue: A little imbalance is normal. Especially on older aircraft you may have partially blocked filters, maybe a pipe here or there that got dented or bent. This means that the fuel will flow unevenly, even with the slip ball bang in the middle. However, at some point in time the fuel imbalance will be such that the effects of the restriction are canceled out, and the fuel flow will be equal across both sides.
At what point this happens differs from aircraft to aircraft, and only pilots who fly this particular aircraft regularly will know what is normal and what is not. But a few gallons imbalance that stays constant over time is nothing to worry about in general.
IMHO it only becomes a serious problem if it leads to controllability issues, or if one of the tanks don't feed at all, even when the other tank is empty.
If the slip ball is not in the middle with the aircraft in cruise configuration (and, most importantly, at cruise power), then you need to get an engineer to adjust the fixed tab on the rudder. You should be able to cruise with your feet off the pedals.
(Although honestly it also depends slightly on the load distribution - if you're flying solo the aircraft will tend to roll to the left. If you compensate for that with the ailerons to the right, you might need a little rudder to counter the adverse yaw. In contrast, if the aircraft was properly trimmed with just a pilot on board, and you are now flying with passengers, the effect will be reversed.)
But yes, if the slip ball is not in the middle, then you're flying out of balance. You don't only feel this in your bum - which wants to slide sideways - but is also felt by the fuel. I know pilots who are deliberately flying out of balance in C172s, with the wing containing the fullest tank slightly high, to balance out the fuel tanks.
Anyway, talk to the owner(s) and regular pilots of this particular aircraft. Tell them what you observed, get their opinion, and see if this is normal for this particular aircraft.
At what point this happens differs from aircraft to aircraft, and only pilots who fly this particular aircraft regularly will know what is normal and what is not. But a few gallons imbalance that stays constant over time is nothing to worry about in general.
IMHO it only becomes a serious problem if it leads to controllability issues, or if one of the tanks don't feed at all, even when the other tank is empty.
If the slip ball is not in the middle with the aircraft in cruise configuration (and, most importantly, at cruise power), then you need to get an engineer to adjust the fixed tab on the rudder. You should be able to cruise with your feet off the pedals.
(Although honestly it also depends slightly on the load distribution - if you're flying solo the aircraft will tend to roll to the left. If you compensate for that with the ailerons to the right, you might need a little rudder to counter the adverse yaw. In contrast, if the aircraft was properly trimmed with just a pilot on board, and you are now flying with passengers, the effect will be reversed.)
But yes, if the slip ball is not in the middle, then you're flying out of balance. You don't only feel this in your bum - which wants to slide sideways - but is also felt by the fuel. I know pilots who are deliberately flying out of balance in C172s, with the wing containing the fullest tank slightly high, to balance out the fuel tanks.
Anyway, talk to the owner(s) and regular pilots of this particular aircraft. Tell them what you observed, get their opinion, and see if this is normal for this particular aircraft.
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If the slip indicator is one ball out, rudder trim may be a factor, but adjustment of the wing cams may also be required. The angle of incidence of each wing is adjusted until the plane flies well coordinated hands and feet off. When doing this, it is nice to have the two wings adjusted so they are either both "forward" or both "aft". One of each will result in an aircraft which remains a little more difficult to trim.
Sustained flight from full tanks with one ball out is certainly going to affect your fuel flow balance, and your butt too, I would think!
Sustained flight from full tanks with one ball out is certainly going to affect your fuel flow balance, and your butt too, I would think!
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.
Just a few home truths about C172 fuel systems...
The gauges only need to be accurate at Zero Fuel, so not a really reliable indication in flight.
If using 'Both' the two tanks are inter-connected, so look at the slip-ball to ensure the levels stay the same. If one tank is 4 inches higher than the other, then the fuel levels will ultimately be 4 inches different when landing.
You can even use the slip-ball as a spirit level when parking on uneven grass. If you park with one wheel in a rabbit hole the lowest tank will fill from the higher tank, and could even over-flow whilst you have your tea and scones at the café.
.
Just a few home truths about C172 fuel systems...
The gauges only need to be accurate at Zero Fuel, so not a really reliable indication in flight.
If using 'Both' the two tanks are inter-connected, so look at the slip-ball to ensure the levels stay the same. If one tank is 4 inches higher than the other, then the fuel levels will ultimately be 4 inches different when landing.
You can even use the slip-ball as a spirit level when parking on uneven grass. If you park with one wheel in a rabbit hole the lowest tank will fill from the higher tank, and could even over-flow whilst you have your tea and scones at the café.
.
It is impossible to get totally even fuel flow from the Cessna SEP's however if the airplane is flying in continually uncoordinated flight than you definitely will get uneven fuel flow.
The airplane should fly hands off wings level with the ball in the middle at 2300 RPM. If it does not it is broken and needs to be fixed.
The best way to evaluate the rig is to start with both tanks full and 2 people in the front and a still air day. In level flight set cruise power let everything settle down and note where the balls is and whether the airplane wants to roll.
Now use the rudder to put the ball in the centre and again note whether the aircraft wants to roll. If it does not than an adjustment of the rudder tab is required. If it still wants to roll, the usual case than the best place to start is the ailerons. The rod ends for the ailerons can be adjusted easily and this is usually where the problem is. Test fly again and note if, with the ball in the centre, the airplane still rolls. If it does not then it is time to adjust the rudder tab.
If small adjustments of the ailerons are not working than as Step, Turn suggested it is time to look at the wing root cams.
Unfortunately the usual thing when a pilot complains about the aircraft rig is to just gronch the rudder tab. I have seen just stupid amounts of tab deflection on some airplanes
. If the wing is not flying level on its own than no amount of rudder tab tweeking is going to make the airplane fly straight.
The airplane should fly hands off wings level with the ball in the middle at 2300 RPM. If it does not it is broken and needs to be fixed.
The best way to evaluate the rig is to start with both tanks full and 2 people in the front and a still air day. In level flight set cruise power let everything settle down and note where the balls is and whether the airplane wants to roll.
Now use the rudder to put the ball in the centre and again note whether the aircraft wants to roll. If it does not than an adjustment of the rudder tab is required. If it still wants to roll, the usual case than the best place to start is the ailerons. The rod ends for the ailerons can be adjusted easily and this is usually where the problem is. Test fly again and note if, with the ball in the centre, the airplane still rolls. If it does not then it is time to adjust the rudder tab.
If small adjustments of the ailerons are not working than as Step, Turn suggested it is time to look at the wing root cams.
Unfortunately the usual thing when a pilot complains about the aircraft rig is to just gronch the rudder tab. I have seen just stupid amounts of tab deflection on some airplanes
. If the wing is not flying level on its own than no amount of rudder tab tweeking is going to make the airplane fly straight.
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A couple of things.
- a bit uneven flow from tanks is quite normal at 172's
- trim is crucial, if the ball under normal cruise condition is not about middle correct the static trim
- check the overflow vent under the left wing if positioned correctly (see service manual, i guess it was somewhere in the 13 section, at mine it is 13-13, where alle the measurements should-be's are.
- check the filler caps! At this age the 172 was delivered with two different caps, one with vent, the other one without. I think it was left without and right with. If they are interchanged, you have quite uneven flow. If you look for replacement, get two filler caps with vents, as this is what is now used also at the old planes. Do not, repeat, do not, use third party filler caps without vents! (90+% of oddities I came across where these shiny OEM caps sealing the tanks ...
- check the air vent connection under the wing (service manual 13-9). It is not uncommon that the above connection is a bit rotten and if you have a hole in that line, you also get uneven flow.
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One thing that I recall, from C172 days, is that:
See the diagram here: C172 Fuel System
OC619
- One tank is vented via the "Vent"
- Other tank is vented via fuel cap
See the diagram here: C172 Fuel System
OC619
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Yes, just checked the outyellowed records - in the old days left tank had the breather vent underneath and a closed filler cap, right tank had a vented filler cap and right was vented through overhead balance tubes to left. As this was over thought, Cessna changed to two vented filler caps.
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The 177 Cardinal can suffer from this problem to, which is due to fuel in one of the tank vent lines. Which can be cleared, I believe by side slipping with the non feeding tank the high wing. It clears with a spray of fuel from the low wingtip vent.
