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Ben2k9

Fuel starvation

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Ok I’ve been warned about possible fuel starvation should you let fuel get low but shift more into one wing tank and let the other get empty.  But I’m curious how that would happen given the below design. Let’s say you’re flying one wing a little high, and notice in one of the fuel tubes that it’s empty, but have say 7 gallons in the other. 

Wouldnt the fuel from one wing still drain down into the gascolator?  How would fuel starvation be possible as long as there is fuel in one side?

thanks

 

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It is less of an issue with the CTLS because of the baffle and flapper valve. It tends to keep more fuel inboard than with the CTSW. The issue is the tanks are long and thin. The best way to visualize it is to take a bottle of water that is a little less than half full. Lay it on its side. Imagine the fuel pick up being located in the bottom corner of the bottle. Now tip the bottle towards the cap. See how the water runs away from the imaginary fuel pick up? When comparing the ratio of the tank to the bottle on it side the bottle 3/1, the tank is closer to 9/1. The bigger the ratio the easier it is to move the fuel away from the pick up. 

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If you try to keep your tanks roughly even when flying, this will never be an issue (unless of course you run them both dry!).  The problem is when one is empty, the other low, and then you go into an (uncoordinated) turn.  Now the fuel sloshes away from the pickup, and you have two dry pickups which equals bad things.

With some fuel in both tanks, fuel sloshing away from a pickup on one side will slosh toward the pickup on the other side, and the problem goes away.  Fuel management is so easy on these airplanes, I don't understand why anybody would let either tank run dry.  Unless just by unfamiliarity with the setup and how to manage it, as unfortunately has happened. 

 

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2 hours ago, Ben2k9 said:

How would fuel starvation be possible as long as there is fuel in one side?

Fuel starvation isn't possible as long as there is fuel in one side, if that fuel is inboard at the pickup.  As Ed has wisely stated, "if you can see fuel, so can the engine."

There are several potential problems:

1) Any fuel outboard of the pickup (fuel you can't see) is unusable, unless you take steps to move it inboard.  Obviously, range is reduced.

2) The forces acting on the fuel and the resulting visible indications may change with a configuration change, like what you do slowing to pattern speed, or abeam the numbers pulling throttle.  Suddenly, that fuel inboard may shift outboard (can no longer see the fuel the tube), and if the other tank is empty, you have starvation.

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There are quite a few threads on this topic.  If you have not already done so, it would be worth your time to read them.  In addition to "if you can see fuel, so can the engine," the other very valuable mantra of Ed's is: "fuel follows the ball."

Related to that, something I have learned the hard way is to NOT fly with "half a ball" on the left with full tanks.  Like most, that configuration results in fairly balanced fuel flow, but evidently not because that leads to wings most level, at least in my plane.  If I fly with half a ball with full tanks, I land with ugly fuel streaks from the left vent.  Most veteran pilots are probably saying, well duh.  Fly with centered ball until several gallons are gone, then balance flow.

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8 minutes ago, JLang said:

2) The forces acting on the fuel and the resulting visible indications may change with a configuration change, like what you do slowing to pattern speed, or abeam the numbers pulling throttle.  Suddenly, that fuel inboard may shift outboard (can no longer see the fuel the tube), and if the other tank is empty, you have starvation.

That can certainly happen, but won't if you use the rudder to stay coordinated when making throttle changes.

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10 minutes ago, FlyingMonkey said:

That can certainly happen, but won't if you use the rudder to stay coordinated when making throttle changes.

Completely agree, the point I was trying to make is that it takes active effort, and without that effort starvation can occur.  I know that when I transitioned from legacy aircraft as a student pilot, I was unprepared for the amount of left rudder required when pulling throttle, and it took some time before coordination was second nature in the pattern.  Had I flown in the scenario I described early in my time with my CTSW, I would have definitely been at risk of starvation.

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2 hours ago, Tom Baker said:

It is less of an issue with the CTLS because of the baffle and flapper valve. It tends to keep more fuel inboard than with the CTSW. The issue is the tanks are long and thin. The best way to visualize it is to take a bottle of water that is a little less than half full. Lay it on its side. Imagine the fuel pick up being located in the bottom corner of the bottle. Now tip the bottle towards the cap. See how the water runs away from the imaginary fuel pick up? When comparing the ratio of the tank to the bottle on it side the bottle 3/1, the tank is closer to 9/1. The bigger the ratio the easier it is to move the fuel away from the pick up. 

Ahah. Yes I get it now 

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That is why planes like super cubs had 2 small header tanks ( one for each wing tank) in the fuselage.

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1 hour ago, JLang said:

Completely agree, the point I was trying to make is that it takes active effort, and without that effort starvation can occur.  I know that when I transitioned from legacy aircraft as a student pilot, I was unprepared for the amount of left rudder required when pulling throttle, and it took some time before coordination was second nature in the pattern.  Had I flown in the scenario I described early in my time with my CTSW, I would have definitely been at risk of starvation.

Yeah, the CT yaws pretty hard if you are going along at a high throttle setting and pull it to idle too.

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regulations only require 30 minutes reserve fuel beyond destination. So in a CT this is let’s say 3 gallons. From a fuel starvation standpoint in the CT, is there a different minimum one might not want to go below period?

 

 

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26 minutes ago, Ben2k9 said:

regulations only require 30 minutes reserve fuel beyond destination. So in a CT this is let’s say 3 gallons. From a fuel starvation standpoint in the CT, is there a different minimum one might not want to go below period?

30 minutes is a broad regulatory minimum.  Of course, the correct answer beyond that is, "whatever you are confident in managing, knowing the limitations of your aircraft and yourself."  I believe there are good, experienced pilots on here who routinely land with not much more than that minimum, all the fuel transferred to one tank.  I've flown my CTSW about 200hrs and I start to get anxious below 10gal.  That's on the conservative side, but I'd advise not going much below that until you have lots of practice transferring and managing fuel and gauging the true fuel level in flight.

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Just my personal minimums, but when my site tubes are one third from the bottom, it's time for me to land and refuel.

That withstanding, my butt has a shorter range than my CTSW.

It is a great and simple little airplane, with excellent capability.

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I think the lowest I have landed with was 5-6 gallons, and that was in the CTLS. I had fuel evenly distributed between both tanks. With the CTLS, because of the inner tank baffles and flapper valve I think you could run it lower if you keep a close eye on making sure there is fuel in both tanks. That being said for a new pilot in the CT I would want an hour, maybe 2 for reserve.

The correct answer for the checkride is, the regulations say I need 30 minutes, but in this airplane I would at least double that.

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30 minutes is scary low in a CT.  You probably could not see fuel in either sight tube unless all the fuel was in one tank.  Since we have such generous fuel capacity, there is really no reason to let your fuel get that low.  Is it legal?  Sure.  But it’s not enough safety margine for these airplanes IMO, considering the fuel system design. If the tanks were sloped to force fuel to the pickups, it might be, but not as it.

My minimum takeoff fuel is ten gallons. I won’t leave the ground with less, unless I know I am just going to stay in the pattern, in which case I might take off with 7-8.  I have landed with 6 gallons after a long leg when I knew I could make it easily, but in general I want to be on the ground or looking to land for fuel at around ten gallons.  

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Lots of comments on the Forum about this.  Myself and other Forum members mark our sight tubes during the 2 year wing pull and sight tube replacement.  Since the wings have been drained to carry out the wing pull/inspection, some of us put a known amount of fuel back into each wing and mark the sight tubes with the fuel level.  Personally, I am looking for a place to land for fuel when my sight tubes show me at my 6 gallon mark.  If I'm in unknown surroundings, I'll plan ahead and search for a airport that has fuel when my sight tubes show the fuel level is approaching the marks.  We just had a aircraft crash a few miles from the outer boundary of Detroit City Airport.  The pilot reported landing gear problems and was circling to check this out.  He had taken off from another state, I don't recall which.  He planned on having enough fuel to just make it to Detroit but didn't plan on circling with gear problems.  2 dead and one is on life support that was pulled from the wreckage.  Fuel starvation incidents occur over, and over and over.  Sad.

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34 minutes ago, Runtoeat said:

Lots of comments on the Forum about this.  Myself and other Forum members mark our sight tubes during the 2 year wing pull and sight tube replacement.  Since the wings have been drained to carry out the wing pull/inspection, some of us put a known amount of fuel back into each wing and mark the sight tubes with the fuel level.  Personally, I am looking for a place to land for fuel when my sight tubes show me at my 6 gallon mark.  If I'm in unknown surroundings, I'll plan ahead and search for a airport that has fuel when my sight tubes show the fuel level is approaching the marks.  We just had a aircraft crash a few miles from the outer boundary of Detroit City Airport.  The pilot reported landing gear problems and was circling to check this out.  He had taken off from another state, I don't recall which.  He planned on having enough fuel to just make it to Detroit but didn't plan on circling with gear problems.  2 dead and one is on life support that was pulled from the wreckage.  Fuel starvation incidents occur over, and over and over.  Sad.

Is that 6 gallon mark on each side, so 12 gal? Or combined...

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Ben, you have a 5 gallon mark for each tank. Half way from that mark to the bottom on each side would give you about 5 gallons total. Personally I would want to be on the ground at that point, or at the very least descending to land. If you only have fuel showing on one side I would plan on being on the ground when it hits that 5 gallon mark.

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1 hour ago, Tom Baker said:

Ben, you have a 5 gallon mark for each tank. Half way from that mark to the bottom on each side would give you about 5 gallons total. Personally I would want to be on the ground at that point, or at the very least descending to land. If you only have fuel showing on one side I would plan on being on the ground when it hits that 5 gallon mark.

Roger that! Thanks Tom

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At 12 gallons I'm looking for a place to get fuel but I can take time to find a convenient place.  I don't like to let the fuel get below 12 gallons but will do so on occasion if conditions are calm.  If it's rough and the fuel is moving making it hard to read but looks like it is trending below the marks, I'll land and get fuel.  I'm with Tom and will be on the ground well before total fuel, in any combination, gets down to 6.

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I ran to starvation recently.  I can climb to 15,000' in under 15 minutes if my TOW is around 900lbs.  The downside is I run out of gas on the way home :)

One upside is I can practice using the last 3 gallons of 'unusable fuel'.  after running the first tank dry if I fly in a slip to keep remaining fuel visible I can burn it all and land with both tanks empty.

Hardest part is exiting the runway with enough speed to taxi all the way to my hangar for fuel.

 

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Ed, I'm wondering if you have a "day job" as a high wire walker that you haven't mentioned?  

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7 hours ago, Ed Cesnalis said:

I ran to starvation recently.  I can climb to 15,000' in under 15 minutes if my TOW is around 900lbs.  The downside is I run out of gas on the way home :)

One upside is I can practice using the last 3 gallons of 'unusable fuel'.  after running the first tank dry if I fly in a slip to keep remaining fuel visible I can burn it all and land with both tanks empty.

Hardest part is exiting the runway with enough speed to taxi all the way to my hangar for fuel.

Read with tongue-in-cheek.

Sounds like a background narrative to a newspaper article.

Be careful out there, Ed.

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Flying for 50 yrs I have seen lots of planes run out of fuel and a lot of people die. With 34 gal capacity in a CT there will never be an excuse for running out of fuel.

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So as long as you can SEE fuel in at least one tube, no prob right?  I was flying yesterday and somehow left side got down very low but right side had a lot, like a full tube.  I got worried and entered into a slip, maybe that was not necessary?

 

 

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