Those of you with dual wing tanks, lets talk plumbing to the header?

[C5Engineer 1170]

Mine run pretty evenly. I have had one tank not feed before though. It's isually the left one. Flying uncoordinated with that wing high for 5-10 min usually balances them back out. Fuel will also transfer wing to wing thru the header on the ground. I discovered this the hard way on a cross country to Idaho after a refuel. I premix so I put a known quantity in the tank whenever I refuel so I can do the math for the oil. I put 10 gallons in one day in one wing and then pushed my plane out of the way for the next guy. We got to shooting the breeze for 45 mins or so. When I stuck my head back in the plane to grab my oil to mix I noticed that wing had less than 10 gallons in it! While sitting there it was flowing into the other side. This caused me to have unmixed gas mixing with already mixed gas and to no longer have a known quantity. I made it work but I installed shutoff valves right after I got home from that trip. Makes me feel better when the plane is parked too. Got a flat in the hangar one time and got a call from the hangar mate saying my plane was pissing gas. Fuel had all transferred to the low side and overflowed the tank and made a mess of everything.

[EDMO 1078]

Have you checked strainers, filters, air-locks, kinked hoses, plugged cap tube?

Sometimes we overlook the most obvious....
EdMO

Edited 24 Dec 2014 by Ed In Missouri

[Boat 10]

I shut mine off when I park it. I got a flat once in the hangar and all the fuel transferred to one side and overflowed the other tank and made a heck of a mess in the hangar. I also shut mine off if I am fueling from a pump so I know exactly what went into each tank so I can pour the right amount of oil in. I only have a shutoff on the left tank.

My only fuel system vents are the caps on the wing tanks.  I've always worried that perhaps if I shut the two wing tank valves off and it got too hot in the hanger that the collector tank and plumbing to the engine might heat up enough to expand someplace and cause a leak.

[Av8r3400 1179]

Doesn't your header tank vent back to the right side wing tank? It's supposed to...

[EDMO 1078]

Doesn't your header tank vent back to the right side wing tank? It's supposed to...

That's a Kitfox design - the Avids don't come that way - need to be modified like KF.

EdMO

[akflyer 1427]

I always leave one fuel valve open.... I originally installed the candy cane vent per the avid service bulletin.  The day the IA was coming over to sign it off after the rebuild I brought it in the garage from the nice chilly -10 temps outside.  I closed both tank valves and you guessed it, the damn header tank warmed up and puked gas all over the skylight (brand new just installed the day before)... I ripped the candy cane off for a vent and just use the two tanks to vent each other now as I described above.. and did a real quick replacement of the skylight before the IA got to the house.  It was a pisser for sure to wake up and go to the garage and see the skylight turned white and half of it laying  the cockpit.

 

[IFMT 36]

My system has no vent line from the header. Each wing tank flows directly down to the header (spar tube) tank. From the lower fitting on the header it goes up through a stainless strainer(glass type) and up to the main fuel shutoff valve. From there the line runs through two electric pumps mounted in series. Only one is on at any time, switchable on the dash. The fuel caps are vented but not with a pressure line pointed into the wind. I have had fuel used out of one tank quicker than the other but adjusting my flying has corrected this. I have never ran one tank completely empty. I like to see a little fuel in the sight window of each tank. I don't think it would suck air if one tank went completely empty as the other wold still flow down. Maybe I should be isolating the tanks with valves? Any thoughts?

[herman pahls 9]

This is a necessary subject since many Avid/Fox "fans" have stopped due to fuel flow issues with plenty of fuel on board,  including mine.

What is causing the fuel flow issues if the fuel lines and filters are not clogged or pinched and the fuel pump is working.

 

Seems like only  "air locks", venting or asking fuel to flow up hill would cause the engine outs.

 

I wish some engineer would chime in and give us the bottom line.

So what can we learn from high wing "factory airplanes" that seem to seldom have fuel flow issues except due to operator error?

 

As far as I know factory airplanes do not have header tanks, so why do our Avids/Fox's?

 

Also, many high wing factory airplanes depend solely on gravity (no fuel pumps) since the carbs are the lowest point on the engine.

Seems like on all high wing airplanes, when that tank gets to a certain level air will enter the fuel lines depending on the angle of banking.

 

Why do factory airplanes seem to not have air lock problems?

 

If they do not have header tanks, I assume they do not have dedicated vent lines that we have discussed here.

On the Pacer I flew for years, you were not to take off and land on the right tank if the fuel level in that tank was below a certain level.

This  was due to the right side fuel lines  having to go up hill to reach the left side fuel valve as I understood the issue.

Most rear header tank Avids/Fox's have the Pacer right side fuel flow scenario on both sides, where the fuel has to flow up hill before it gets to the fuel pump.

 

Apparently some Avids and Fox's have front  header tanks which makes more sense to me than a rear header if the the fuel lines are routed forward from the tanks to possibly gravity feed the fuel pumps on 582/912's 

The problem I see with small forward header tanks is that these airplanes could possibly climb at an angle that would starve the header.

 

Do Cessna's risk fuel starvation from long steep climbs and they do not have header tanks?

 

A forward header tank may cause packaging problems but my model 2 Kitfox has an 8 gallon header tank that the wing tank feeds.

Since I am not smart enough to know one way or another, I have no choice but to follow the Kitfox factory recommendations of venting the rear located header tank to the top of the right side tank since that tank comes tapped for a fitting.

As was mentioned in an earlier post, the key to preventing "air locks" seems to be fuel lines that travel continually upwards from the header to the wing tanks to prevent air from getting trapped and stopping flow.

 

Herman 

[Av8r3400 1179]

Hi Herman.

 

Header tanks are needed because Avids and Kitfoxes only have one fuel pickup per wing tank and it is located in the rear of the tank.  This is to allow the plumbing to flex during the wing fold.  Pipers (maybe Cessnas?) have a pickup in the front and rear of the tank.  The Avid/Fox pickup can become unported during decent with low amounts of fuel in the wing tank, therefore needing the header to supply fuel during the unporting event.

 

Air locking often happened in the early Avids when the (very tiny) header tank was not vented.  Proper venting of the header is imperative to keeping the fuel flowing.

 

Wing folding is also a common source of fuel flow stoppage.  The flexing and kinking of the hoses between the tank and fuselage cause damage on the inside of the hose causing flow to be blocked.  Especially when you complicate the compromised hoses with the crap they pass off for automotive gasoline these days...

 

Also as you said, crazy hose routing causing bubble loops in the hose don't help matters much.  Remember, these are experimental planes.  The builder can basically do as they please for plumbing.  Certificated planes have very stringent rules for plumbing.  Not to mention most of the plumbing is hard lined in certificated planes.

 

The cowl tank in your early Kitfox is not what I would consider to be a true "header" tank.  It was the only tank in the very early planes and is just refilled by gravity from the auxiliary wing tank(s).  A header tank (IMO) would be meant to be always filling from the wing tanks, automatically.

[Av8r_Sed 82]

A lot of early Kitfoxes did have a small (1 gallon or so) aluminum header tank mounted behind the dash to go with wing tanks.  It did have a vent connection, but only used with a manual valve to purge air from the system.  It didn't work so well and resulted in the factory coming up with the poly behind seat header.  I'm pretty sure there was a service bulletin put out pretty much mandating replacement.

[skelly 12]

This is all very interesting as I am currently planning my fuel system.

Question.

Should I use 1/4" (6MM) bore hose from the wing tanks to the header tank or 5/16" (8MM) bore hose ?

Does one size flow better or has better anti air lock properties ?

A mate of mine suggests I use the 6MM bore hose as its lighter and has less tendancy to kink. This does sound sensible.

Any thoughts ?

Cheers.

Skel.

[wypaul 448]

I’ve been running 1/4” on my avid for the last 10 yrs feeding 100 hp. Engine without issues.  I would suggest tapping a third hole in the top of the header for the fuel return if you are planning on a 912 Rotax, not sure if a return is also mandatory on other Rotax engines to help reduce vapor lock