170 vs 172 fuel Tanks

[DaveF]

The gascolator is connected to the header tank directly.
Header tank goes TO a shutoff valve and then onto the fuel pump(s)
The fuel pumps pumps return excess fuel TO the header tank.
Header tank is connected to the overhead vent line.

If gravity fills the header tank, as soon as I turn on my fuel selector, fuel will flow into the header tank.
Since the wing tanks are higher than the header thank, the fuel will fill the header tank and travel up the vent line to equalize with the fuel level in the wing tank. No?

I need to sit down with the drawings to confirm the plumbing but that's the way I was reading it.

That sounds like what I'd expect, except maybe for the gascolator being upstream of the header tank. The engine driven fuel pump sends vapor and excess fuel to the header tank, and the vapor bubbles up the line from the header tank to the overhead vent. The line should mostly be filled with fuel, as you said. Not always, though. In very hot conditions the fuel pump can send significant amounts of vapor back to the header tank. That's been the suspect in several T210 power loss accidents.

[DaveF]

Here are two pages of the 185 fuel system from the 100 series service manual:

185 Fuel System.pdf

That's pretty interesting -- there's no L/R selection, only on/off! I didn't know that about the 185.

[blueldr]

If the fuel tanks were so full that the cross cabin vent line was flooded, then, yes, the fuel will run all the way up to the vent line tee. When the engine is sterted the consumed fuel will reduce the flow to the point that it will pull the level down. At any rate, fuel consumption will be great enough to preclude fuel returning to the wing tanks. That's the advantage in having a header tank, unlike some of the "cheepy" Beachcraft airplanes powered by a TCM IO-346 where the return fuel always goes to a certain tank. You must always deplete the fuel from that tank enough to make room for return fuel from one of the other tanks in order to keep from pumping fuel overboard.

[cessna170bdriver]

...My wings aren't perfect so if I was going for long range tanks, I'd find a set if 175 wings. But who needs four hours of fuel when I have two hours of bladder

There's no rule that says you have to use all of your endurance in one leg. There are plenty of places out on this side of the continent where it's 100 miles or more between airports, and many of those are desirable destinations that don't have fuel available. There are MANY more (for guys at least) places to land where a bladder can be emptied than there are where tanks can be filled...

[minton]

David,
The header tank VENT line is tied into the cross cabin vent line. If you study a schematic if the system you will see that there is no fuel getting into the vent line in normal circumsances.
If I remember correctly, the fuel tank vent valves that I installed were not welded in. They had a threaded body similar to a bulhhead fitting and were were sealed with a gasket. I got them from a junk yard that used to be up in Oregon. They also incorporated a check valve of rubber which have precluded welding..

You're loos'in me on the "Header tank" Could you please provide a illustration or parts book reference for me? I know that some STC's go way overboard Just would like to know???

[blueldr]

minton,
The STC calls for a header tank located on the inside of the left boot cowl adjacent to the pilots left leg. On my installation I removed the gascolator, ran the fuel line to the left across the firewall and back through the firewall to the header tank. Leaving the header tank the fuel went through a firewall shut off valve, to a gascolator, to the electric boost pump, and to the engine fuel injection pump. From the injection pump return through a check valve and back to the header tank. The header tank vent went back and up through the laft forward door post and back into the cross cabin vent line. The Maule header tank that I used had four fittings on it. One for fuel in from the selector valve, one for fuel out through the FWSOV, one for the fuel return from the injection pump, and one for the vent. The tank was about two inches thick and would protrude through the upholstery by the pilots leg, so I ran it through the band saw and cut it to one inch thick, had a new side welded on it and it was all back inside. Sorry I don't have any pix of it.
It is true that if you were able to fill the wing tanks full enough to flood the cross cabin vent line, the vent line to the header tank would flood too. Kind of hard to fill the main tanks that full.
The obvious advantage to having a header tank is that the pump returned fuel just goes back to the header and then back to the engine. Without a header tank, if you route the return fuel to the cross cabin vent line, you dont know where it is going from there.

[minton]

I'm think'in after reading all of the hoops required for your installation that C-175 wings and tanks might be a better option, ??

The STC thats out there for C-175 wings I don't think requires a header tank. The early C-172 and C-175's surely didn't and the field approval I did was signed off without, nor did it require an additional vent in the right hand tank. Sounds like they (FAA) operates like the IRS. Every one wants to reinvent the wheel.

And where is this fuel pump that needs to return bypass fuel Only fuel injection requires that as I recall and some models require the fuel selector to become involved. I do know that if bypass fuel is in the picture it has to be routed to the same feed tank OR be kept downstream of the fuel selector as the mechanical pump does on the 0-360 installation.

What a mess!

OR by my plane! It's for sale and flying.

[bagarre]

Although this thread is about fitting 172 tanks in a170, it's part of an IO-360 upgrade. Thus the header tank, fuel pump and extra wing vents.

I was just looking for tank options to delay taking my plane apart.

Even if I put 175 wings on, I'd still need two vents per the STC for the IO-360. But 175 wings would be a nice upgrade.

[GAHorn]

I'm think'in after reading all of the hoops required for your installation that C-175 wings and tanks might be a better option, ??

The STC thats out there for C-175 wings I don't think requires a header tank. The early C-172 and C-175's surely didn't and the field approval I did was signed off without, nor did it require an additional vent in the right hand tank. Sounds like they (FAA) operates like the IRS. Every one wants to reinvent the wheel.

And where is this fuel pump that needs to return bypass fuel Only fuel injection requires that as I recall and some models require the fuel selector to become involved. I do know that if bypass fuel is in the picture it has to be routed to the same feed tank OR be kept downstream of the fuel selector as the mechanical pump does on the 0-360 installation.

What a mess!

OR by my plane! It's for sale and flying.

Minton, all fuel INJected systems require a header tank. Carb systems do not.

[GAHorn]

It has the fuel return going to the header tank and from the header tank back to the fuel pump as well as up to a tee in the vent line which is now a fuel return line.
I wonder how long the thing would have to run before filling the header tank, up the line and back to the tanks ?

Why not put a tee in between the fuel selector and gascolator for fuel return? It seems much simpler, removes the header tank, allows you to select a tank and eliminates the seconds shutoff valve. The returned fuel to the tee would mix right back into the fuel in the line from the selector.

It seems too simple so I must be missing something.

There must be a header tank in order to vent-off vapors which collect during shut-downs. (These would be the vapors which sometimes create a hot-start problem for those who don't understand how the system works.)

[170C]

So George, are you saying that on some conversions to injected engines on aircraft that previously had a non injected engine that one of the main fuel tanks may "act" as a header tank? I seem to recall seeing at least one, maybe more, 170's that did not have a separate header tank while sporting a Continental IO-360.

[blueldr]

On an engine conversion to a fuel injected engine, about the only place the return fuel can conviently be routed without a header tank is into the cross cabin vent line. if you dump it into this line, where is the fuel going to go? Right or left? You have no control over it. When my fuel tanks get down to the last quarter, I select that tank and run the fuel out until the engine quits. I then know that all my remaining fuel is only in the other tank. I would not want any returning fuel to go to the tank I had just emptied.

[GAHorn]

Frank, header tanks are installed as a nicety to avoid having to plumb a return line all the way back to one of the existing fuel tanks and to avoid that issue which Beech created for so many of their airplanes. (I'm thinking of the need for some airplanes to burn fuel out of a particular tank for a stated period of time in order to create space for the returned fuel. C-400 series does the same thing with tip tanks, which are the main tanks. They must be used first, in order to avoid overfilling them with returned-fuel, in-flight. A header tank solves both issues.)