Fuel Measurement

[Jim Meade]

A friend helped me take some fuel measurements of the fuel tanks on my 2007 CTSW.

We put jacks under two studs into the main gear assembly and leveled the plane by measuring the dihedral which varied between 1.5 and 1.6 between the two sides. As close as we can get it, I think we were within 0.05 degrees of identical.

We drained the tanks. When nearly done, I noted the left tank was empty per the dipstick and sight gauge and the right tank had 3 gallons per the dipstick (but more on that later). I have no idea why that happened or if it is typical for the left to empty faster. Of course, this was a gravity drain and not when the engine was running in operation so I am not making any assumptions..

We pinched both fuel lines where they enter the engine compartment from the pillar.

We refilled the plane one gallon a side at a time using a 64 oz pitcher X2 and checked the dipsticks and sight gauge after each gallon. After seven gallons a side, we filled it with a measured 5 gallons a side. We added 16.5 measured gallons a side, which supports the specifications of 17 gallon tanks and 0.5 gallons unusable fuel per side.

 

I marked the sight gauge itself using a Sharpie at usable fuel levels of 1.0, 2.0, 3.0, 5.0 and 7.0 Higher markings could have been made but I chose not to because the parallax was getting larger. I figured that I was willing to go off the totalizer and time/fuel flow at quantities higher than 7.0 gallons per side. There was 0.5 gallons unusable a side in the tank, but I marked it as one gallon when I added one gallon to the tank rather than include the unusable fuel. So, the sight gauge is only usable fuel.

 

The FD decals that reflected the sight gauge fuel quantity were in the wrong place and they also did not reflect the quantity according to our measured fill levels. I reinstalled them so their zero is my zero but do not plan on using them for fuel planning.

 

I'm not sure what I'll do when I pull the wings and change the sight gauges.

 

The dipsticks were both inaccurate. After each gallon fill, we filled the second tank before going back to stick the first one. It would be nice to be sure that the baffles allowed the gallon of fuel to equalize but I guess we'll have to guess that giving it a few minutes made it happen. I did all the dipstick measurements from a step ladder trying to be consistent.

 

The left dipstick generally read 0.5 gallon high (it said there was more fuel in the tank than there really was) at lower readings. At higher readings, it read 1 gallon high. These markings are based on total fuel in the tank, usable plus unusable. If the dipstick said the tank would take Y gallons and I put Y gallons in, there was still room for more.

 

The right tank dipstick markings were 1 gallon low at the lower readings and became 0.5 gallon low at the higher levels. This is consistent with my experience. I would stick the tank and it showed that it would take X gallons but if I put X gallons in all at once it would overflow.

 

I am not convinced the totalizer is exactly correct, but now that I know the accurate dipstick markings I can calibrate the totalizer.

 

I thought you might find the exercise interesting.

 

Based on current dipstick markings, if I stuck both tanks when level and it said I had 6 gallons a side, I would actually have 7.5 in the right and 5.5 in the left, which would total 12 gallons (11 usable). If the dipstick said 3 gallons a side, I would have 4 on the right and 3 on the left for a total of 7, 6 usable. 2 1/2 gallons of fuel a side (4 gallons total usable) would show on the dipstick as 1.5 gallons on the right and 2 on the left, for a total indicated of 3.5 gallons.

 

Based on this, the dipstick is conservative at low quantities. One has more fuel than it says. Of course, one is unlikely to be sticking the tank at these low fuel levels. If one sticks the tank at 12 gallons a side, the right will actually have 12.5 and the left will have 11 gallons for a total of 23.5 actual (22.5 usable) compared with 24 indicated on the dipstick. At higher levels the dipstick is too optimistic.

 

Of course, if the fuel was level I wold not get identical readings. Actual 6.5 on each side would read 5.5 gallons on the right and 7 gallons on the left.

 

There is no objective or point to posting this exercise. Your plane is quite possibly different than mine so you should not take my figures as representative of your airplane. You might conclude that my dipstick is "close enough" and I would not argue with that. I may recalibrate it anyway to be right on. I'm very glad that I accurately marked the sight gauge. One does not intend to fly that low on fuel, but if something happens it is good to have confidence in the accuracy of the sight gauge.

[Runtoeat]

Jim. Nice report. Does your dipstick have unique markings for right and left fuel tanks? My dipstick does have different fuel levels for R and L sides and the difference should agree with your findings. I'll check this out. Not totally sure but think that the tanks are molded up to the wing spar on the back side and leading edge on the front. Someone might verify this? Due to the offset of the spars, the tanks assume different shapes and thus show different levels for the respective gallons inside. FWIW, I ignore the factory fuel level sticker inside and rely on my empirically developed fuel levels (3 gal. and 5 gal. remaining) on each sight tube.

[FlyingMonkey]

Thanks Jim, I'm about to go through a similar exercise once my tanks are empty enough to make draining them easy. Did you think about re-marking the factory dipstick as you went along to make it accurate for your airplane? It's just soft metal so you should be able to scratch/etch/grind marks in it pretty easily to indicate "actual" numbers for your airplane instead of the generic factory numbers. Now that you have calibrated numbers on your tubes, you could still go back and do this by getting the fuel to a particular line on the tube and then checking the stick and marking it.

[FlyingMonkey]

I agree Dick, those factory stickers are a bit of a joke. It would almost be safer if they were not there. I'd say mine are accurate to within +/-3 gallons per side at best.

[Jim Meade]

The dipstick does show "left" and "right" markings that are not the same. I was careful to be sure to use the correct side for each tank.

I will likely remark the dipstick or even make a new dipstick. Or, maybe I'll be a generic fuel hawk type tube and mark that. I haven't got that far yet.

The tanks are long and flat and the one diagram I have seen, which I think I posted from a U.K. AD, showed the baffle to be closer to the wing root end than the wing tip end of the tank. I don't have any idea how large the holes in the baffles are.

My sight gauge stickers are both for "RIGHT", so I have to assume it was lunch break time in the Ukraine or the applier didn't know right from left. . Sadly, they're both wrong.

 

[FlyingMonkey]

The big lesson on fuel is not to trust the sight guages And assume dipsticking is off too because the plane may not be level enough to get a consistent reading.

 

If you mean that you can't ascribe a particular gallon value to a particular place on the tubes, you are correct unless you calibrate as Jim has discussed. however if you mean somehow that you can have fuel in your tanks and it not show in the sight tube, or fuel in the tube but not in the tank, that is not true.

 

I would think that carefully calibrated sight tubes would be at least as accurate as the Dynon fuel sensor, since the latter relies on knowing exactly how much is in the tanks when you start. That is tricky unless you first drain the tanks, put in a known quantity and program the Dynon, then thereafter are VERY fastidious about putting in the *exact* amount of the fuel you add each time. Otherwise the accuracy of the Dynon will "drift" over time until you do the drain and refill exercise again.

[WmInce]

. . . "Using the sight tubes I underestimated the fuel in the tanks, and WXBRIEF did not predict the headwind. So we ended up on the header tank on the last 50 miles in with the big red low fuel light on reducing the throttle to 3800 and using the altitude to make the last five miles. . . . If there was no header tank, we would have run out of fuel (about 1.2 useable)." . . .

. . . "The big lesson on fuel is not to trust the sight guages" . . .

 

Interesting.

Seems to me there is a much bigger lesson to be learned.

[Ed Cesnalis]

... if you mean somehow that you can have fuel in your tanks and it not show in the sight tube, or fuel in the tube but not in the tank, that is not true...

 

Andy,

 

You can realize fuel starvation in your CT with fuel remaining in a wing tank. If that occurs you will not see the fuel because it will be unported and sloshed outboard.

 

If you remain in a prolonged uncoordinated condition fuel can transfer from the leading wing to the trailing wing until there is no fuel remaining in the leading wing. As the trailing wing becomes exhausted due to consumption and venting the remaining fuel could eventually unport, and slosh outboard where you can't see it.

 

In a fuel emergency a good rule is that if you can see fuel in a sight tube then it is available to your engine. Yawing left and right allows you to move the fuel level in the sight tubes up and down and figure out where the middle is.

[FlyingMonkey]

Andy,

 

You can realize fuel starvation in your CT with fuel remaining in a wing tank. If that occurs you will not see the fuel because it will be unported and sloshed outboard.

 

If you remain in a prolonged uncoordinated condition fuel can transfer from the leading wing to the trailing wing until there is no fuel remaining in the leading wing. As the trailing wing becomes exhausted due to consumption and venting the remaining fuel could eventually unport, and slosh outboard where you can't see it.

 

In a fuel emergency a good rule is that if you can see fuel in a sight tube then it is available to your engine. Yawing left and right allows you to move the fuel level in the sight tubes up and down and figure out where the middle is.

 

I agree with that, but when your fuel is that low in both tanks, I would say that there is not much fuel to see anyway...

[Ed Cesnalis]

Interesting.

Seems to me there is a much bigger lesson to be learned.

 

Let me try, if your thinking is "If there was no header tank, we would have run out of fuel (about 1.2 useable)" then the bigger lesson to learn is Monitor your fuel condition in flight and land and re-fuel rather than landing with less than legal reserves or realizing a fuel related emergency.

 

Hundreds of us don't have a header tank and the lack of a header tank has never caused fuel starvation not even once. It isn't the size of fuel tanks that cause starvation, it isn't even poor planning, in a plane with sight gauges it is pilot error, don't proceed without adequate fuel!

[Roger Lee]

Hi Ed,

 

In some CT's there is a little one way door in the baffle that keeps fuel from going all the way out board. The early CT's didn't have this. If you get so low you have to worry about 2 gals. you have already done something terribly wrong. When I do a hose change I mark the tubes at 2.5 and 5 gals. anything past 5 gals on the plus side isn't worth doing. The tubes show full until you get about 10+/- in each wing. When you see 2.5 in each wing you need to be on the ground and I'm usually on the ground way before that so I never worry. I don't believe in shorting myself of fuel.

[Ed Cesnalis]

Roger,

 

Baffles are problematic, for instance sloshing isn't the only way to get gas outboard. The original filling of the tank got a lot of fuel outboard of the baffle and a prolonged slip can simply help keep it there even after the other wing is dry and the wing at issue is dry at the sight tube.

 

I've never seen an aircraft that will happily slip along and provide few clues to the pilot like a CT can.

 

Its true we should never get fuel critical but we should know how to manage our fuel if the event occurs. If you are down below 30 minutes remaining fuel more management is required. A good example would be a forward slip on final if fuel critical should be done where at least 1 wing is showing fuel in the tube. If you do such a slip the wrong way you could encourage fuel starvation with fuel on board. I'm not advocating that you fly low fuel I'm advocating that you understand the management issues.

 

 

CTLSi,

 

Have another look, your sight tubes are essential and they actually work pretty well with the limitation that they are not precise.

 

If I have more than 10 gallons in a wing I know that much at a glance. I usually move a wingtip up and down as I approach my cockpit to more easily see the levels.

 

Any thing under 10 gallons I can see, I don't think in terms of gallons but hours or half hours.

 

In flight I watch the usage, if it is not balanced I use rudder trim to correct.

[FlyingMonkey]

I mean fuel will show in the sight tubes, if 100LL its blue, if Mogas its clear, but you cant be sure HOW MUCH is in the tanks. The calibration numbers are wrong, the relationship of the tube to whats in the tanks is wrong. The only thing the tubes do as far as I can tell is tell you there is still a little fuel in the tanks if you see some in the tubes at all.

 

Dipsticking is a little more accurate, but not exact either. If the plane is not on level ground, both dipsticking and sight tubes are even more error prone.

 

The best and most sure way to know how much fuel you have is to put the plane on as level ground as possible, and shine a flashlight on the fuel after filling it to as close to the top as possible.

 

In the CTLSi, there is a 1.5 gallon header tank in the baggage cmpt. It also has a sight tube, but its useless too. And only 1.2 gallons are said to be useable in that tank. The header tank does increase the fuel capacity of the CT, but it is not fuel one should count on unless there is an emergency and you run out in the main tanks. Then you are glad you have it, but it only provides a short distance of travel at lowered RPM, but its better than the older models that have the wing tanks only.

 

If you calibrate the way Jim mentioned, the relationship to what's in the tubes and what's in the tanks will be correct. Then you can be pretty confident of how much is in the tanks, if you check while straight and level in coordinated flight. I agree that if you just look at the tubes without calibrating them to your airframe, you don't get a great idea of how much fuel remains.

 

Your method of putting the plane on level ground might work...but only on the ground. Fuel when you are parked is not critical, fuel in the air is critical. How would you solve the problem of knowing how much fuel you have when in the air, without looking at the sight tubes? Even if you completely get your Dynon fuel flow 100% correct, that will not tell you if you have a leak and are siphoning off fuel, only the sight tubes will tell you that.

 

Which leads to another question...why has FD not put a fuel level sender in its newer aircraft? That adds another layer of fuel information that would give some useful information.

[FlyingMonkey]

 

If I have more than 10 gallons in a wing I know that much at a glance. I usually move a wingtip up and down as I approach my cockpit to more easily see the levels.

 

Any thing under 10 gallons I can see, I don't think in terms of gallons but hours or half hours.

 

In flight I watch the usage, if it is not balanced I use rudder trim to correct.

 

So how much time do you consider the tubes to provide? Two hours per full tube? An hour and a half? Less, more? Having just the sight tubes in my airplane, I have not really gotten a good handle yet on what my actual usage is. I generally fly with way more than I need so not an issue, but still it would be nice to be able to throw out a SWAG on how much time remains in the air before the fan stops at any given time.

[Ed Cesnalis]

The lesson, repeated once more, is NOT TO TRUST THE SIGHT TUBES OR EVEN DIPSTICKING if going on a long trip. If you dont know how much fuel you had to start with you cannot monitor how much is left during flight.

 

The problems associated with a lack of a reliable way to tell QUANTITY in the tanks is the problem. So, the only real solution as I have offered, is to fill the plane all the way to the top and verify that visually at the filler cap. Even then you will not know the EXACT number of gallons in the plane.

 

There are many places to fly where fuel is not available in nice neat distance increments. Nevada is one of those places. Between Hawthorne, NV and Las Vegas, NV for example.

 

Factors such as headwinds, and throttle play a big role too. But if you are off by several gallons in the estimate, you will chance a fuel emergency unless you start with full tanks, and have conservative waypoints along the way planned according to that known quantity of fuel.

 

The sight tubes are irrelevant in preparing for a long trip because they show a level only after I have used 14 gallons. Quantity on departure isn't worth staking your life on, how much fuel do you lose from your wing vent when you turn? What if you get a leak? What if the gas fil comes off?

 

The sight tubes are the answer, you might not be able to peg it to plus or minus 5 minutes but at a glance you can see how many hours of fuel that you have reaming as your fly towards your destination. With your big headwind and minimal fuel you certainly should have been aware of your situation and re-fueled in Tonapah. When the fuel emergency arrived and you were 50 miles out you should have fueled in Mercury. Your answer that the highway was your alternate as you proceeded without safe or legal fuel is really poor judgement.

 

Your post is full of excuses, all you have to do is glance at your sight tubes once an hour while en-route and such needless emergencies can be avoided 100% of the time.

[Ed Cesnalis]

So how much time do you consider the tubes to provide? Two hours per full tube? An hour and a half? Less, more? Having just the sight tubes in my airplane, I have not really gotten a good handle yet on what my actual usage is. I generally fly with way more than I need so not an issue, but still it would be nice to be able to throw out a SWAG on how much time remains in the air before the fan stops at any given time.

 

I figure each tube is ~2 hours, half is 1 hour, half that is half an hour, etc. The 1st 2 hours can't be seen when flying full.

 

Each tube 1/2 is my reserve when flying anywhere, I'll fly the pattern with less.

[Roger Lee]

I agree with Ed. Those site tubes should be a good guide and help manage fuel in each wing. Another thing I won't do and that is short myself of few to save a few pounds of over weight. Crashing from low fuel is not an option.

[FlyingMonkey]

I figure each tube is ~2 hours, half is 1 hour, half that is half an hour, etc. The 1st 2 hours can't be seen when flying full.

 

Each tube 1/2 is my reserve when flying anywhere, I'll fly the pattern with less.

 

So you basically keep a two hour reserve? That seems like...a lot. Especially if you are going somewhere and need weight for other stuff. I understand it where you live, but I think an hour-ish reserve is enough for my terrain. Of course, I've been landing with 2+ hours fuel on every flight anyway, LOL.

[Ed Cesnalis]

So you basically keep a two hour reserve? That seems like...a lot.

 

My CT is light and my girlfriend is light so weight is a non-issue for me.

 

I have Mogas in my hangar, most of my flights are 2 hours ( 4 hours round trip ) so coming home with 2 hours is the norm when things go right. When things go wrong, unless I'm coming home and Bishop works as an alternate 30 minute reserves don't even come close.

 

Trip to Vegas with 6 hours of fuel, trip home is 2 hours and 2 hours reserves.

 

Depart North Las Vegas and girlfriend gets uncomfortable, she wants to be on course which is north up 95 but there is a broken line of snow showers about 30 miles out. Ok its broken we can go that way but when I get to Mercury the opening I was using closed up and I turned around, tried another opening and turned around again and headed back to Vegas.

 

I crossed the red mountains east of Vegas and was soon over Pahrump with a bumpy ride and a green girlfriend. After a while she needed to be on the ground so I descended to sea level to land at Furnace Creek Death Valley where the crosswind was stronger than my rudder, so I headed for Stove Pipe Wells where there was too much shear.

 

Rats, have to climb back to 10,000' and dodge snow showers and mountains, next stop Bishop, but before I descend to 4,000' I might as well try Mammoth first. Mammoth works but just barely and I land with 40 min reserves even though I had 2 hours planned.

 

I have been low fuel in the Tahoe area and it took me 3 airports before I found one where I could get it on the ground. Its these kind of experiences and the CTs long legs that motivates me to use big reserves. If you have to cross the Sierra Nevada to find a suitable field you will burn almost an hour doing that.

[procharger]

With a fuel range of 700 to 900 miles no one should ever run out just saying??????

[Jim Meade]

With a fuel range of 700 to 900 miles no one should ever run out just saying??????

 

And if you take off at gross weight with 12 gallons on a checkride with the idea that it will be over in an hour but it stretches for another 15 minutes and sudden weather moves in and you have to go 50 miles to an alternate......

 

Of course, if one always takes off with full tanks regardless of gross weight.....

[Roger Lee]

"Of course, if one always takes off with full tanks regardless of gross weight..... "

 

You have my vote

[Farmer]

I never take off over gross

 

2X2=3

 

14+11+4=19

 

Pilot+Wife+some fuel + duds= just right

[Doug G.]

The only thing reliable is to land well before you need to worry about fuel - always!

[Tom Baker]

The sight tubes are not reliable. Anyone not heeding that advise will pay one day. Final word.

 

It takes some time to get to know a new airplane, and how the fuel gage reads related to fuel burn. After you fly your airplane a while you will learn to judge how much fuel you have left by using the sight tube.