Flying with an empty wing

[NC Bill]

The SC accident (N78BZ) prompts me to post this.

 

I flew my first multiple fuel stop x-cty this week. On the second leg I became concerned because after 12 gals used I showed 6 in the left tank per the tube sights. With my destination 12 more gals to go I decided to make a cautionary fuel stop. Sure enough the right tank was almost full :unsure:

 

I arrived at my destination without incident and commenced my annual. Fuel flow from both tanks checked normal as did everything fuel related (except for a leaking fuel pump - more on that elsewhere). On the return trip the next day the same fuel imbalance was noted. This time I took the plane off autopilot when the left wing showed 5 gals remaining and flew uncoordinated until refueling.

 

Apparently the plane is flying right wing low.

 

2 questions I'd like to put up for discussion.

1 - How do you confirm you're flying a CT straight & level? My DYNON Skyview and 696 Panel view and my visual view of wings to horizons led me to think "OTTO" was flying S&L.

 

2 - If the plane is that small amount out of coordinated flight then will any of the 17 gals in the other tank flow thru the system or will the engine starve with a full (17 gals) opposite tank? If not, then with how much fuel in that tank will the engine starve? The 7 1/2 gals the SC had? Less?

 

P.S. Uncoordinated hand flying for an hour isn't the fun I thought it might be.

[Roger Lee]

Hi Bill,

 

This is not an uncommon occurrence in aircraft, other aircraft do the same thing. Your plane's sensors and the mushroom may not be perfectly straight. The best way to check on your coordinated flight is to take a piece of yarn about 8" long and tape it on the white area between the center cowl fastener and the windshield. Tie a knot in the ends of the yarn you it doesn't fray. Now go fly and you'll most likely see you really aren't in a perfect trim with the instruments. If you run out of fuel in one wing and the other is full then you should be okay. If you get really low in one wing and empty on another you could be in trouble just like the other guys in SC. This discussion has been around since 2005 with the CT's and comes up with new CT pilots all the time. FD addressed this on their website about 3 years ago. When I fly I usually fly with the ball out to the left 1/2 ball. This makes both wings drain equally. This is a function of having a flat fuel tank design. Like I said it is not just CT's, but other aircraft including GA aircraft have the same issues. It really isn't a big deal because you can trim the rudder to fly that 1/2 ball out to the left. My plane will fly hands off for quite a long time that way. We are only talking a slight trim change. I have run out of fuel on one wing before in another CT and it did not starve.

This is nothing more than fuel management and no different than having a fuel selector valve to manage fuel or when you approach a field to land in managing your speed and decent. Once you know about it it becomes second nature.

[josjonkers]

Another technique to try is to transfer fuel in flight from the full tank to low fuel tank by deliberatly flying uncoordinated.

 

As an example if your L tank is quite full and your R tank low fly with the ball off to the R by using L rudder and the centrifugal force will transfer fuel from the L tank to the R tank. Do that for about 15 minutes and check the results in the fuel tubes and continue until you have things acceptable again.

[Jacques]

and...it's been said before : " if you see fuel, .you have fuel.."

[Jim Meade]

I have never been taught or heard of deliberately flying uncoordinated to improve the balance of fuel flow. The CTSW has a 2' dihedral - isn't that enough to bring fuel to the port? I can't find the FD site citation for this phenomenon. Can someone give me the URL, please?

[coppercity]

I have taken long trips in my CT's and they do tend to draw a little more fuel from the left wing then the right in cruise. A lot of high wing plans do this, but since we don't have a fuel selector to switch to an individual tank we have to get creative to keep things in balance. When I see the fuel in the site tubes is getting a few gallons out of balance I simply trim the rudder to the high fuel side a half a ball out for 10 minutes or so just as Roger mentioned, then back to normal trim. It's really easy with the autopilot on because it keeps you on track even when out of trim.

[FredG]

As mentioned by Roger, many airplanes do not show a dead center ball when the wings are truly level. So, what is indicated as coordinated flight (ball centered) is actually flight with one wing a little above the other. In flight or on the ground, fuel will accumulate in the lower tank. To end my problem with unequal fuel fuel levels, my mechanic leveled the airplane and then leveled the Dynon (ball centered). Now, when the ball is centered, the fuel levels remain equal in the tanks. Dihedral won't fix a ball that is off center.

[Roger Lee]

Hi Jim,

 

Here you go. It has been removed off the old FD website, but I have it here for you.

 

un-even fuel tank emptying answer from FD

March 17, 2007

 

*

 

There have been several on-going discussions on the issue of un-even fuel tank emptying with the CT fuel system.

 

We at Flight Design USA are well aware of this, and in an effort to make the fuel system of the CT better understood and to promote safety, will publish a series of articles to illustrate the design philosophy behind the fuel system and correct fuel management procedures.

 

As a first step, below is an excerpt from a letter discussing this topic written by Oliver Reinhardt, Technical Director Flight Design GmbH and addressed Tom Peghiny, President of Flight Design USA. (The content has been edited for clarity and is for illustration purposes only. JNC)

 

 

 

“Tom,

 

Before going into any technical aspect, it has to be stated clearly, that correct fuel management is in the sole duty of the pilot in command. It is his/her clear responsibility to ensure proper filling of the tanks and to monitor fuel usage throughout the flight. Aside from any technical reason, fuel exhaustion or starvation would be the direct result of pilot carelessness. Running both tanks dry in-flight, taking off or continuing flight with a fuel configuration known to cause fuel starvation is reckless and endangers the safety of the occupants as well as persons on the ground.

 

The CT is a high wing aircraft with gravity feed fuel system. The fuel tanks are in the wings. It is a commonly known fact that the fuel tanks of this type of aircraft, due to their extremely flat shape, are subject to sloshing effects and uneven emptying. The CT fuel tanks are of a high aspect ratio design. They are located in front of the spar for design simplicity and are long to give larger tank volume. Because of their shape the high aspect ratio tanks have sloshing effects. Baffle ribs are incorporated to minimise these effects. But the effect can not be reduced to zero. Therefore, as with any other aircraft with fuel tanks inside the wings, it is in the responsibility of the pilot to make sure that an empty wing tank is not kept high under prolonged slipping conditions.

 

There are different philosophies when it comes to reducing the un-even emptying of fuel tanks:

 

One is to provide the pilot with a fuel selector valve allowing him/her to manage fuel usage. However, history has shown that fuel mismanagement is a leading cause of aviation accidents and may be a contributing factor in as much as 10% of all accidents. Early CTs were no exception. In Europe, the planes delivered up until the end of 1999 were equipped with fuel selector valves and were equally prone to fuel mismanagement by pilot. Since changing to a simple coupled shutoff valve, there have been no reported fuel mismanagement incidents in Europe. This alone clearly shows that the safety gain achieved by the current system is significant.

 

Analysis and tests have shown that uneven air pressure inside the fuel tanks can indeed cause uneven fuel usage. But it can be easily shown that this cannot lead to fuel starvation, as the achievable pressure differences are, by far, not sufficient to push air to the engine, while one tank still has some fuel. The pressure difference that would be needed to do this would be close to .1 bar (1.4 psi). For comparison: The dynamic pressure in cruise flight is .02 bar, being only 20% of this value.

 

Other philosophies involving features such as common fuel tank vents, header tanks, and tank connection lines were not chosen for the CT due to clear reasons. Any design feature on an aircraft is a compromise. Unfortunately, in technology as in life, there is no such thing as a “free lunch”. Looking at these variants shows the following:

 

 

 

1. A common fuel vent line may have several issues:

 

Fuel spillage with the aircraft sitting on ground due to unchecked fuel cross flow is one of the well known things. The common vent opening is often lower than the fuel level in at least one of the full tanks, when the aircraft is sitting on uneven ground.

 

In (uncoordinated) flight, unchecked fuel siphoning overboard may occur when the vent is lower than the fuel level and fuel starvation when the vent is higher than the fuel level and tank-to-engine fuel line.

 

Lack of redundancy: One single vent block would stop both tanks. This is a safety aspect that favours dual tank vents.

 

For the above reasons, we believe the disadvantages outweigh any gains.

 

 

 

2. A header tank was not chosen, so as to have as little fuel as possible stored in the cabin or within the engine compartment in case of a serious accident. This can easily make the difference between surviving or not if trapped in a wreck after a crash. Nevertheless, a sort of header tank functionality is provided as the metal fuel drop lines located in “A” pillars have been intentionally chosen with a significant diameter, allowing for .3l (about 10 fluid oz.) of fuel to be stored in each to feed the engine. This does not include the volume in the lines, the fuel valve or within the gascolator. This system is by far not unprecedented. The popular Citabria and Scout Part 23 certified airplanes have a very similar fuel system.

 

 

 

3. Direct tank-to-tank connection lines were tested with the CT. The result unhindered flow between the tanks which resulted in much greater imbalance between the tanks and significant fuel shifting effect. Therefore, the current solution of limited fuel flow through the Tee-fitting at the firewall has been determined as safe and suitable compromise.

 

FD has, with good reason, established fuel flow check procedures for regular maintenance inspections, and for fuel flow tests to be completed after each airplane assembly. It must be clearly pointed out that performance of these checks is the responsibility of the owner/operator of the aircraft and has to be done, to be on the safe side of the aircraft, as designed.

 

Oliver”

 

 

 

Flight Design USA understands that Light Sport Aircraft, while outwardly similar in appearance to General Aviation aircraft, are in many ways different in design philosophy. This is part of what makes the new category fresh and exciting. However, this means than we, as pilots, must not take anything for granted and must open our minds to different possibilities and as with any aircraft we fly become thoroughly familiar with the systems.

 

 

Jonathan N. Carter

 

Director of Safety and Training

 

Flight Design USA.

[Runtoeat]

Regarding Fred G's comments about his mechanic leveling the ball. I am suspect of the accuracy of my ball and wondering what the best method might be to find "level" in the side-to-side direction so that the ball can be calibrated. I'm thinking about using a straight piece of metal channel that spans from one fuel filler over to the other fuel filler and laying a level on this? Comments welcome.

[Tad Olmsted]

I wouldn't trust the ball, the mushroom always comes from Europe out of level. We adjust them as best we can, but in my mind it is only a reference at best.

 

I recommend the "yarn approach" as Roger stated earlier for verifying your coordination ball.

[Jim Meade]

Thanks for the FD letter. It seems the key point is: "Therefore,...., it is in the responsibility of the pilot to make sure that an empty wing tank is not kept high under prolonged slipping conditions."

 

What I take from the letter is that an owner/operator like many of us should ensure that the airplane is properly rigged and instrumentation calibrated to accurately reflect the plane's attitude. If this is done and the plane is flown in a coordinated manner, normal fuel management should be sufficient. The amount of fuel reserves a person chooses to carry on board should not need to be exaggerated out of fear of unporting a fuel tank if the plane is correctly assembled and flown.

 

If there develops a sense in the industry that FD requires more than normal fuel management due to design aspects, we can expect some lawyer or bureaucrat to do our decision making for us. From what I read in the letter, that should not be necessary. I'm going to look into rigging and calibration and appreciate the heads-up.

 

 

 

 

 

[opticsguy]

It's counter-intuitive, but if you are flying un-coordinated the higher wing will SHOW more fuel even though it has less. It took two years before I got my Dynon level enough to trust the ball.

[ozairangel]

In my ctsw the control panel housing had sagged, resulting in an inclined efis. this was proven by the dynon agents here in Australia. they elongated the panel mounting holes and leveled the dynon with the wings. This was discovered early in the life of the aircraft. Alternatively the Dynon may not have been mounted level with the wings in some cases.

[Tad Olmsted]

In my ctsw the control panel housing had sagged, resulting in an inclined efis. this was proven by the dynon agents here in Australia. they elongated the panel mounting holes and leveled the dynon with the wings. This was discovered early in the life of the aircraft. Alternatively the Dynon may not have been mounted level with the wings in some cases.

 

OZ, you nailed it... the panel must be level with the wings!!!

 

I have adjusted mushrooms that were adjusted by techs that are not "CT guys". The one thing that throws the SW out of level is the aluminum gear legs getting bent and throwing the plane out of level on the ground.

 

I had a customer with a 3" out of level SW and swore he never had a hard landing. I explained it would not take a hard landing to bend the gear legs. He asked " then how the heck did they get bent?"

 

I asked him if he ever made a cross wind landing? " of course " he said.

 

I then explained that all the weight of the plane was on one leg until the other leg touched down.

He said "ohhhhh, makes sense now". The legs just won't take it year after year with out bending a bit.

 

The SW legs will bend slightly under normal wear and tear, don't sweat it, just make sure your mechanic levels the wings before adjusting the intrument panel.

[Runtoeat]

Tad, do you have any advice for leveling the mushroom - maybe where the main attachments are to do this? Also, what method do you use to determine if the wings are level and then how you determine if the mushroom is level to the wings! Thanks.

[Tom Baker]

What I have found to work well for me when flying on autopilot is to look out and use rudder trim to level the wing tips with the horizon. When I do this the ball is off center, but the fuel drain is even. I do this from the start of the flight. I don't wait until the fuel level is off. Tom

[Roger Lee]

Did you notice when that article was written by the factory? We had been discussing it for much longer.

We have all been talking about this very subject since 2006 and it is present in most (not all) CT's regardless of the year. Some have made a few corrections, but most just know that their left wing (for example, some are the right wing) drains faster. So most of us know that after a few flights and when you fly you fly with the ball out on the instrument panel about 1/2 ball to the left. Remember that the fuel will follow the ball. In my plane when I fly with the ball to the left for a long distance trip or even a 1-2 hour hop I usually land with fairly equal fuel in each tank. There are approximately 325 of us CT people in the state and around 1500+ world wide and we don't seem to have any real issue with this. It is just a quirk of the plane, just like other planes have quirks.

The only issue has been people trying to skimp on fuel load and make it to their destination with only 1-2 gals left. That's just foolish in itself. That has caused more aircraft incidents in all aircraft types than any other problem. If you purposefully fly a CT with minimum fuel on board then you set yourself up for failure right off the bat. No one knows exactly what a fuel burn for a given trip may be before the flight as you may encounter other variables that you may have over looked or developed during the flight. Allowing any plane to get down to a bare bones fuel situation is not ever in your best interest. You may make that flight today ok, but one day it will bite you in the butt.

 

 

Be smart, Be safe!

[josjonkers]

It is almost like clock work that every year this discussion comes up.

There are many references in different parts of the forum on this since 2006.

I have flown the ct with one tank empty and then was able to transfer fuel in flight from the full tank.

 

The only drawback with unbalanced fuel is that the "heavy" wing with the extra fuel will tend to drop so you can't fly hands off.

 

I would strongly recommend that every ct pilot experiment with the technique of transferring fuel from the full to the empty tank and become familiar with it. All this may seem scary but poses no safety risk unless you are at minimum fuel.

[Roger Lee]

I agree with Josjonkers,

 

This comes up usually for new CT owners and the rest of us tend to forget about it.

[Tad Olmsted]

Tad, do you have any advice for leveling the mushroom - maybe where the main attachments are to do this? Also, what method do you use to determine if the wings are level and then how you determine if the mushroom is level to the wings! Thanks.

 

Dick, to get the plane level I use a digital level. You can get one at home Depot that works great. The best place is the tunnel. Get the a/c level longitude then proceed to do it lateral. You can let the air out of the tires as needed to dial it in.

 

Then elongate the holes on the left side of the mushroom down by your right leg and foot. There is one up by your right toe, so don't forget that one. Depending on how bad it is, you may run out of edge distance. It's ok, just put a large washer on the screw head.

 

BE CAREFUL, IT DOESN'T TAKE MUCH HEAT!!!

 

Next, there are 2 pieces of foam rubber behind the EFIS and EMS. They may need to be adjustested as well or else your mushroom may vibrate and buzz.

 

In some cases the mushroom then doesn't fit down tightly as it once did. simply grab your heat gun and carefully apply some heat to the affected area and hold it in place with a long screwdriver or drift. Then as it cools it will hold it's shape. To speed up the cooling process, have your shop air and a blow gun handy.

[Tad Olmsted]

What I have found to work well for me when flying on autopilot is to look out and use rudder trim to level the wing tips with the horizon. When I do this the ball is off center, but the fuel drain is even. I do this from the start of the flight. I don't wait until the fuel level is off. Tom

Tom, is your ball centered on the ground?

[Tom Baker]

Tom, is your ball centered on the ground?

 

You know, I'm not sure I'll have to check. I know when the airplane feels right in the air the ball is centered. I've used the wings level on autopilot method in several different airplanes with good results. When flying on the autopilot if you have a crosswind the airplane is going to bank into the wind to try and stay on course. Remember heading is controled by a roll servo. By adjusting the rudder trim to keep wings level the fuel level stays even.

[Tad Olmsted]

You know, I'm not sure I'll have to check. I know when the airplane feels right in the air the ball is centered. I've used the wings level on autopilot method in several different airplanes with good results. When flying on the autopilot if you have a crosswind the airplane is going to bank into the wind to try and stay on course. Remember heading is controled by a roll servo. By adjusting the rudder trim to keep wings level the fuel level stays even.

I should mention I have been referring to the ball in the Dynon EFIS, not one fixed to the panel. I bet it's not centered while on the ground. Even after the mushroom is adjusted it seems to sag back to where it was. This is where the heat trick comes in and re-forms the mushroom to fit happily

[FredG]

I'm no mechanic, but, I check my airplane for level with a four foot rigid straight edge laid carefully on the wing from root to filler cap. I make sure to find a line with no odd lumps that might make the angle untrue. Then I use a good digital protractor and measure angle. I do same on the other side and then shim under the wheel on low wing side with whatever it takes to get the wing angles equal. Then I check the ball for center. If not centered, the panel/instrument placement is the problem.

 

My tunnel is close, but not perfectly level when the wings are level. So, I use wings rather than tunnel.

 

In theory, one could use the fuel level in the tanks as the level, given enough time (with airplane on the ground, of course). Just keep putting shims under the wheel of the tank that has more fuel and wait for levels to re-establish stable levels (several hours, probably). Do this until the fuel level is the same in both tanks and stays the same over time. If the panel were then adjusted so that the ball was centered, flight with the ball centered would keep fuel levels equal. Of course, this is no guarantee that the wings are truly level, only that the tanks are level (of course, we'd hope these are close). If one were to use this method, it would be logical to check wing level independently (see above paragraph) to ensure that no discrepancy exists between wings and tanks.

 

Like I said, I'm no mechanic, but I like to confirm that my plane is set up correctly.

[Jim Meade]

Roger,

 

I have to protest against advising people to deliberately fly an airplane in a perpetual slip, that is, uncoordinated, to overcome fuel feed problems that would seem to be better addressed by proper rigging. Flying inefficiently goes against the grain of many of us, and should.

 

I'll look, but maybe someone can tell me the dimensions of the wing tanks. Are they parallel to the wings? Do the wings have dihedral? I'm going to try to figure out how much fuel is where under various conditions.