Had an interesting incident in flight today

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fudge80

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fudge80
I am pretty low time private pilot with my own Cessna 172N. I just completed my 3rd longer(to me anyway, it was only 300nm) cross country flight to attend a wedding.

On my way back earlier today my left tank was going empty alot faster than the right tank so about 90 miles from home I had my left tank between 1/4 and empty and my right tank was still half full. When I reached down to turn the tank selector to the right tank it took about 3 seconds before my engine died. Scared the crap out of me but I quickly put the tank selector back in "both", crammed the mixture back into full rich and the engine just popped back to life.

I know it wasn't a true emergency the engine dying but still shook up this 90hr private pilot. I am in ND so its not like I don't have options but the thought of having to dead stick it onto a gravel road or wheat field is not exactly on my bucket list.

Talking to the CFI friend that trained me he told me most likely my right fuel cap is not venting and that is what was causing the uneven fuel draining and that engine dying when switched to that tank.

Just thought I'd share.
 
Or the fuel vent tube. Had one w/ a bug in it and engine started running rough around 5-6000'. Descended and it went away around 4000'. Mechanic found it partially stop up with a bug.
 
Sounds like you did the right thing and continued to aviate.
 
Glad you're ok


mscard88 said:
Or the fuel vent tube. Had one w/ a bug in it and engine started running rough around 5-6000'. Descended and it went away around 4000'. Mechanic found it partially stop up with a bug.
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That was the first thing that came to mind.


Another thing you might want to look into, do some backcountry off airport training with your plane, it'll change the way you view off field landing options, what will work, what won't, risks, and kill the "boogie man" factor of landing off a airport.
 
All pilots get concerned whenever their single engine loses power, even if temporarily. It's not a good feeling!
 
I had this happen to me at 38hours total time. I know what you felt lol.

Now you know ;)
 
Buy yourself something nice. . .you done real good; something wasn't right, you recognized it, took action, and then got it all back in order.
 
You done good! Our 182 has the same issue with fuel burning over twice as fast from the left side, but it runs fine on the right tank alone.
 
I have heard of a few fuel selectors on older 172 models just wearing out and not actually being in the position indicated by the selector lever. Have your mechanic check it out.
 
Lachlan said:
I have heard of a few fuel selectors on older 172 models just wearing out and not actually being in the position indicated by the selector lever. Have your mechanic check it out.
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Thats the plan, got an annual coming up in the next month or so, I'll see what my mechanic thinks. In the mean time I'll check the right cap.
 
Good job. Various things cause uneven fuel burn in Cessnas but they're definitely not supposed to quit when something other than "Both" is selected. Get 'er checked out.
 
Did you check your fuel when you land? What was the true fuel consumption for your trip?
 
denverpilot said:
Good job. Various things cause uneven fuel burn in Cessnas but they're definitely not supposed to quit when something other than "Both" is selected. Get 'er checked out.
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This. If it turns out not to be something obvious like fuel cap not venting, don't wait until annual, have it looked into and figure out what caused it. Then get it fixed.
 
Uneven fuel flow in the 172 is hardly unusual. Most of the 172 models the left tank will drain faster than the right. Amusingly, this is independent of which tank(s) are selected on the fuel selector. Note closely how the 172 fuel system is constructed. You've got two wing tanks. There is a vent tube on the left wing by the wing strut. The right tank has no external vent (what about the cap you say, ... hold your horses, I'll get to that). There is a vent line that runs from the left tank over the top of the fuselage (behind the headliner) to the right which vents that. Both tanks have fuel lines that gravity feed down to the fuel valve (via the door posts), from there it runs to the engine.

So what is happening in most cases is that until you get the fuel level in the left tank below the level of that tank-to-tank vent line, you're feeding fuel to the right tank. Once you burn the left tank down to a point, the fuel flow usually evens out. The Cessna Pilot's Association has a little information sheet on how to bend the external vent tube to adjust the feed pressure to alleviate this.

Now about the fuel cap. On most 172s you should have ONE cap with a vent on the right side. That vent is actually normally closed. It's purely there so that in case a venting problem occurs that it provides a backup to let air into the right tank.

The other interesting effect (though I never encountered in in 25+ years of 172 flying), is that there is a known vapor locking condition that can occur at high altitudes and warmer temperatures. As a result you should have a placard telling you to run either LEFT or RIGHT (but not BOTH) above 5000'. This increases the flow rate through the line (one line flowing 6gpm rather than two at 3gpm) which keeps things cool enough to avoid the problem.
 
flyingron said:
The other interesting effect (though I never encountered in in 25+ years of 172 flying), is that there is a known vapor locking condition that can occur at high altitudes and warmer temperatures. As a result you should have a placard telling you to run either LEFT or RIGHT (but not BOTH) above 5000'. This increases the flow rate through the line (one line flowing 6gpm rather than two at 3gpm) which keeps things cool enough to avoid the problem.
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Never seen the placard, never seen the problem, and haven't flown many days in 20 years below 5000' MSL. What even could get hot enough prior to the fuel selector to vaporize fuel anyway? The only place those lines are separate is from the tanks through the wing roots and down the pillar into the floor. Doesn't make sense.
 
denverpilot said:
Never seen the placard, never seen the problem, and haven't flown many days in 20 years below 5000' MSL. What even could get hot enough prior to the fuel selector to vaporize fuel anyway? The only place those lines are separate is from the tanks through the wing roots and down the pillar into the floor. Doesn't make sense.
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Totally agree. No such placard either. The only placards are fro a few for fuel grade and capacity, and one for takeoff on both tanks and L & R tanks in level flight. I always flew with the selector on both and the tanks drained fairly evenly. One can use L & R tanks in cruise flight, and must use BOTH for takeoffs and landings. And I've been flying 172s for 40+ years, so I got that going for me.
 
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My data is right out of the M, N, and P models POHs.
 
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Does un-coordinated flying have much effect on the C172? In the CT line of planes, it can have a dramatic affect on the L/R feeding and consumption of fuel.
 
CT4ME said:
Does un-coordinated flying have much effect on the C172? In the CT line of planes, it can have a dramatic affect on the L/R feeding and consumption of fuel.
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Not if you use BOTH position. The POH specifically mentions that.
 
Yours is also a 182 and not a 172. The 182 fuel system is different. The 182 has external vents for both tanks. And the caps should be vented as well. The fuel lines are also necessarily larger (you burn more fuel with the O-470).
 
flyingron said:
Yours is also a 182 and not a 172. The 182 fuel system is different. The 182 has external vents for both tanks. And the caps should be vented as well. The fuel lines are also necessarily larger (you burn more fuel with the O-470).
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Still doesn't make any sense. Where are the lines getting that much heat from?

The 182 does not have two vents, btw. Just the left side. There's a crossover vent about halfway up the bladders to handle the right and if either the main left vent or the crossover vent line get plugged that's when the cap vent sucks open. It's really common to see caps that haven't been replaced in forever that the little diaphragm won't move at all and has turned crunchy. There's even a kit to just replace the squishy parts.
 
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