Fuel Flow Discussion, Moved from Float Mounting

[Gerhard Rieger]

I have decided to use left and right and fit a two-gallon header tank, I have 3/8 fuel lines and 1/2 back from the header tank to the mains. IO540 C4B5. electrical and mechanical fuelpumps.

[Nev]

I have decided to use left and right and fit a two-gallon header tank, I have 3/8 fuel lines and 1/2 back from the header tank to the mains. IO540 C4B5. electrical and mechanical fuelpumps.

It makes intuitive sense to select the fuller fuel tank when balancing fuel, which works nearly all the time. But when a system malfunctions things often stop making sense, and in this case it may be that the fullest tank has plenty of fuel, but it’s not accessible.

I wonder if a header tank will actually mask this issue rather than solve it, by providing an additional 2 gallons before the issue makes itself known.

For example, with BOTH selected, if one tank is failing to feed properly due to tank vent blockage, pressure differential, fuel line vapor lock etc, the other tank will continue feeding the header. If the selector is later switched to the fuller tank in an effort balance, the fuel feed problems from that tank will then become known shortly after changing tanks, or 2 gallons later with a header tank. It may also create a delay in getting fuel to the engine to restart, once the unobstructed tank is selected.

[Battson]

I am not an expert, so I am happy to be corrected....

Personally I see little benefit in a header tank with a Bendix injection system or carb system in a Bearhawk. There is plenty of risk with not having a both position on your fuel selector. There is plenty of risk with having a tank of fuel near the exhaust or in the cabin.

[zkelley2]

I also see no benefit of a header tank.

[svyolo]

With Bendix FI or a carb, I wouldn't use a header tank either. With EFI, I wouldn't use them if I used 1/2" fuel lines. If I remember right Whee made the same choice with TCM FI.

[Gerhard Rieger]

The Cessna 210 has L&R and a header tank, it was working for Cessna in this advanced aircraft for more than 40 years. For the Bendix system you need large vent tubes by double venting the header tank making sure it will always fill in no time. Having left and right only allows you to isolate a problem, what I did is venting my aux tanks to the main tanks, the header tank is vented to the left and right main tanks by 1/2 inch lines, they are on the upper right and lower left of the header tank both on the upper limit of the header tank. This will also vent out any air from the tank no matter what the attitude of the aircraft is. If the main tank vent block, it will vent from the aux tank. There are one risk to manage, the fuel selector, to make sure that its either on left or right. Taking off with the fuel selector in the off position will allow you to be air born and running the header tank dry, exactly why its important to use checklists and have operating procedures in place, even on a light aircraft like the Bearhawk. The main issue is to have a functional and proven fuel system. We all test our fuel systems to determine flow to the engin, many believe fuel under the floor is a huge risk, no more a risk than your gascolator ripped off in a crash and fuel pouring out by the gallons.

[schu]

There is plenty of risk with not having a both position on your fuel selector.

What is this risk you speak of? Are you talking about having less fuel flow?

Seems to me you are either risking your tanks staying balanced and having similar head pressure on the both position or, if I'm picking up what you are laying down, less fuel flow by removing one tank.

If it were me, I'd rather risk less fuel flow on one tank. Why?

1. The system must be designed to have enough fuel flow on either tank. If you can't operate at full power with minimal fuel in one tank, then you probably need a placard to declare minimal fuel volumes and the requirement for both tanks on take off. In my mind that is a hack. If the fuel system can't flow enough fuel on one tank, then it needs to be fixed, not placarded.

2. We have heard from two different bearhawk builders that have had engine stoppage by using 'BOTH'.

3. Other builders have reported enough fuel using a single tank. (Whee obviously tested fuel flow with one tank given his lacking 'BOTH')

Here is a post:

https://bearhawkforums.com/forum/bearhawk-general-discussion-news/1326-precertification-fuel-flow-testing-results?p=1340#post1340

Results:
Brake Specific Fuel Consumption rate of .55 lbs/hr was used to determine BSFC for the 195 hp I0-375 engine installed in aircraft.
195 hp x .55 =107 lbs/hr
107/6(lbs/gal)=17.8 gal/hr max flow rate.
17.8 x 150% = 26.7 gal/hr
To measure in oz/min:
First we need gals/min= 26.7/60=.45 gal/min
Second we need ozs/min= .45x128 (oz in a gal)=57.6 oz/min at 150% flow rate.
Therefore: 26.7 gal/hr = 57.6 oz/min

Gravity feed test:
Right tank = 88 oz/min = 41.25 gal/hr
Left tank = 72 oz/min = 33.75 gal/hr
Both = 108 oz/min = 50.63 gal/hr

High pressure fuel pump test:
Both = 136 oz/min = 63.6 gal/hr

It shows that both tanks by themselves flow enough to feed an o-540 + 50%.

What I find interesting is the variation between tanks in this test. That leads me to believe that fuel line routing greatly affects fuel flow, which could account for the seemly large differences between airplanes and fuel flow numbers. This is strong evidence that using fleet statistics to determine if there is a problem is dangerous. Bearhawks are all on-off airplanes when compared to a cessna.

My $.02 on this subject is pretty simple. If people have noted engine stoppage when selecting 'BOTH', we see unbalanced fuel flow in many airplanes (which indicates different head pressures), and the FAA suggests cross venting to ensure that the top AND bottom of the tanks act as one, then I'll be putting in a cross vent. I've already taken the time to retro in a skylight, float fittings, 4-pt harnesses, floor anchors, extended baggage, and seaplane doors, what's another 2 hours to cross vent.

schu

[Sir Newton]

:-) hey man, I just found this thread today.

[Battson]

What is this risk you speak of? Are you talking about having less fuel flow?

I am talking about the risk of running out of fuel in the tank you're using.

The reason the "BOTH" position was introduced, was pilots falling out of the sky with fuel left in the other tank.

Some of the most experienced pilots I know have run out of fuel in one tank, and forgotten to check fuel quantities during the limited time they had to respond. Then been forced into landing somewhere undesirable. Some ran out of fuel on short finals and fell short of the runway, just seconds to take action... Most modern POH address this by requiring the use of either fullest tank or both tanks prior to any critical flight phase. But people are not good with checklists and procedures. The safest option is leaving the selector on both at all times when in the air.

People are innately forgetful and prone to tunnel vision. If the aircraft fuel system design sets the pilot up to fail, sets a trap so to speak, then it's only a matter of time.

[schu]

Makes sense. Both does make it easier to manage fuel, as long as it works.

Didn't someone test how long it took to get power back if you run a tank try? I think it was Bobby, and only a few seconds.

[Gerd Mannsperger]

The reason the both position was introduce was because some pilots are very bad at doing coordinated turns and say a Cessna 180 (without header tanks)
will simply have the engine stop if you do that (cross control a turn). I demonstrate this to any 180 guy I check out.
They learn to fly coordinated much easier after
You are correct People are not good with Checklists and procedure that is where annual training comes in. Do all the emergency training you can think of once a year. After a few years it is super easy and will stick with you so when you need it it comes naturally.
If you live and fly long enough you probably will have it happen to you so be prepared.( Cessna fuel systems are infamous for venting issues) The only person who can save the day is the pilot in command. Personally I have no issue with just having a right and left position. None of our aircraft have a both position. What ever you fly know your system and learn to use it safely. Extra venting and Balance tubes can help.
At the end of the day it is you as pilot in command that is between you and your beautiful homebuilt rolled up into a ball at some undesirable location.

[whee]

I like the perspective Battson shared here. Recent experience has taught me how true it is that people are forgetful and prone to tunnel vision. Those two things can lead to very costly mistakes. A proficient pilot probably has sufficient excess brain power to easily manage fuel. A pilot that is minimally proficient might get task saturated just flying the airplane and can't handle managing fuel.

Adding a cross vent with a check valve like a Cessna is so easy I'm starting to think maybe I'll add one to my plane.

[svyolo]

Lots of great info and experience. I got a lot out of this thread,

I only flew light GA for 80 hours in the early 80's. Almost all low wing. L or R only. I ran a tank dry once in 80 hours, nothing bad happened, luckily. I flew for a living for 31 years, and only had to manage fuel for 1 of those years as a flight engineer. 500 ish hours. In 500 hours I screwed up fuel management twice, which was about average. Again, nothing bad happened other than a bruised ego. The other 30 years it was all automatic. Never had a problem.

Apparently I am not smart enough to manage fuel. I prefer BOTH.

[Collin Campbell]

This has been a long thread and I have been watching it with interest. Wow! There are certainly a lot of things to consider! I am thinking of a situation that has not been brought up. (or at least if someone did, I missed it) Here goes:

Suppose you are flying on a long trip, and coming in to land with low fuel. You have a fuel selector with only left/right positions. You have selected the LEFT tank...let's say there are only 5 gals in that tank. (pushing your luck...probably not recommended) As you are making your turn to final you notice that you are way too high with full flaps already extended. So you go to a full pedal forward slip to lose altitude...it's how it's done, right? Right rudder, left aileron...it's fun, right? You notice that the ball is way over to the left...what might not be realized is now the fuel in that LEFT tank is doing the same thing as the ball on your slip/skid indicator. it is moving to the far left side of the tank. How long before both of the fuel pickups in that left tank are unported and you start picking up air?? Who knows. Maybe you won't have to hold that slip for long and everything will be fine. Maybe not. This is one reason (a good one I believe) for having a fuel selector with BOTH positions. I think the uncoordination thing is a big deal.

[whee]

I've been in that scenario many times, same as thousands of other pilots. It's up to the PIC to use their brain and not cause an engine out. When slipping to a landing always make sure to select the uphill tank. Or don't slip. Luscombes have no both position, no flaps, and glide forever; slips are a regular part of landings. With a measly 25gal of fuel it is common to land with little fuel on board while on a cross country.

However, with a carbureted engine at idle you have quite a while before the bowl is dry and and engine quits. If your on a 2 mile final in a hard slip to get down to the runway you might need a little refresher training😉

[zkelley2]

whee, if you're on a 2 mile final and need to slip, you don't even need the engine at that point! haha.

For left/right say a 40 gal tank burning 10 per hour, you'd takeoff on the Best tank(might on might not be a thing depending on aircraft) lets call that the left. At a safe altitude, switch to the right, burn an hour, back to left, burn an hour, back to right, burn until the engine sputters, then land on the right with a full hours of gas and more fuel in a tank then if you split that 10 gallons into 2 tanks.

[Gerhard Rieger]

If you do not have both, then a header tank makes sense. the exact reason I am using a two Gallon header tank. I will not have a problem slipping my aircraft and un porting will not influence it. Having larger vent lines will make sure that the header tank stays filled at all times. put an opened beer can to your mouth upside down. then stab a hole in the bottom which is now on top. hope you can swallow fast. it will be down your throat before you know it. Make sure you vent the header tank well, I have two 1/2 inch vent lines, one to each wing tank. One will always be higher than the other because one is in the front left corner and the other rear right corner. any bubbles from air entering the fuel system because of un-porting the main tank will go up and out the highest vent line and breath back to the main tank, it will also make sure that the header tank stay full as you resume normal flight, if you crash and your aircraft's belly is ripped open ripping of your fuel selector, you will have your two main tanks spilling their fuel out at an alarming rate below the floor and have exactly the same problem I have using the header tank. this is a very interesting read, lots to consider.

[MattS]

Can someone give a list of planes that have;
1) A fuel system that allows for "BOTH"
AND
2) Does NOT have interconnecting tanks?
AND
3) Requires a fuel pump for normal operations.

I'm sure I could do the research, but without a floatplane I am traveling by speedboat which cuts into my free time!

https://www.amazonsaltandlight.org

[zkelley2]

Such an airplane would not meet the old FAR 23(or any of the CAR's) and I doubt it'd meet the new one either. So I would venture one does not exist.

[schu]

Can someone give a list of planes that have;
1) A fuel system that allows for "BOTH"
AND
2) Does NOT have interconnecting tanks?
AND
3) Requires a fuel pump for normal operations.

I'm sure I could do the research, but without a floatplane I am traveling by speedboat which cuts into my free time!

www.amazonsaltandlight.org

I certainly can't. I doubt such an animal exists.

[patrickh99]

No fuel venting system is going to be perfectly symmetrical: different lengths, different bends, bugs, etc. The recommended fuel caps for the Bearhawk have the potential to aggravate the situation if they are not both machined exactly the same AND installed/aligned exactly the same. That is a small airfoil on the top of the cap; turn it slightly and you switch from pressure, to less pressure, to suction. Thank Bernoulli for that. All of the external venting components are affected by the relative wind angle & velocity, so you will never have equal pressure in tanks that do not have an internal cross-vent. Fuel pump presence or absence is irrelevant to this issue.

[Nev]

This is a very good point you’ve made.

[Ray Strickland]

In my first homebuilt I had 1/4” vent tubes with a bulb of screen wire protecting the entrance from insects. No cross vent tube. Ran on both for 400 hrs w/o a problem. However, I have often thought that the small Bearhawk cap vents could be plugged by a direct strike from a fat, juicy bug. While it hasn’t happened in 200 hours (running on both), this discussion has convinced me to install a cross vent tube at the next annual—just to be safe.

[Archer39J]

This is what I've been looking at, might be easier than a full cross vent and would work if both caps get clogged. https://www.mcmaster.com/5492k51

[JJohnston]

What would you do with that (check valve)?

[Archer39J]

Put it near the filler neck. Debating whether to even finish with a tube poking outside, but then this would also be subject to icing. Once I get one in hand and can do some testing I may decide to just have it terminate inside the wing with a weep hole in the skin, of course zero fuel/vapors dumping inside a wing bay would be the requirement.

[Archer39J]

If I don't like how this one works I can just machine my own, just one more thing to design and test though....

[Bcone1381]

I like data and I like declared known best practices.....standards. Without data problems cant be defined. Can we get more data? Thank you those who fly Bearhawks who contribute. Can the silent of the 100+ Bearhawk owner operators help here?

If BOTH is selected then a significant imbalance between the tanks indicates an air pressure imbalance exists. Is this common? Can we collect data in order to discover if a fuel imbalance is common (when using both)? Can, a value be declared or a limitation set to define a significant imbalance? For example, The max imbalance allowable imbalance when BOTH is selected is 7 gallons.

Fact, It is possible to see the fuel tank pressure and how it changes and what factors effect the pressure inside a tank. It seems like folks could put a TEE at the top sight gage bung and hook up a VSI, Altimeter or a manometer.

Like I said, I like data.

Here is an accident report of N22GM. Next week I'll search more of the FAA data base, but I thought this one relevant.

https://app.ntsb.gov/pdfgenerator/ReportGeneratorFile.ashx?EventID=20190327X81141&AKey=1&RType=Prelim&IType=LA

[svyolo]

I read the NTSB report a couple of times before. It doesn't seem like the Feds want to spend much time analyzing EAB accidents. Not much useful information. I am pretty sure their description of the "Emergency Power Switch" is way off.

N22GM probably had a complete dual EFII system. It is very redundant (more than I actually require) Other than an electrical feed or fuel problem, the only single failure that makes the engine go quiet is the Crank trigger (Hall Effect Sensor).

Just not enough info in the NTSB report. Too bad. Thats what those reports are for. So the rest of us can learn something.

[zkelley2]

A lot of efi systems do not get installed correctly leading to loss of power. We can't really say here but it's quite common.

[Archer39J]

Or they follow the installation instructions without an understanding of the design and how different platforms will affect it's performance.

[AKKen07]

Not to keep beating the poor horse but I was just browsing my Bingelis Firewall Forward book when I ran into a paragraph stating that any fuel system with tanks that feed simultaneously MUST have a cross vent to feed equally. I think I’m pretty well convinced at this point.

[Archer39J]

More like their pressures need to be equalized, which you can do in ways other than a cross vent.