170 vs 172 fuel Tanks

[bagarre]

Does anyone know if there were any differences in fuel tanks from the 170B to the 1956 172?

To reduce down time, I was going to look for an extra set of tanks to re-work vs taking mine out and grounding the airplane.

Can I use 172 tanks as direct replacements or would I need some type of approval (I'm pretty sure 1956 didn't have the wing vent yet)

-David

[n2582d]

When you say "re-work" are you talking about getting the long range tank mod that Del-Air has the STC for or does the tank get modified as part of the XP-mods STC? If getting the long range tank mod have you compared prices on just buying long range tanks rather than getting standard tanks modified since you plan on getting an extra set anyway? The '56 172 had the wing vent. It wasn't until the '59 model that Cessna used the electric fuel quantity transmitter.

[bagarre]

Rework as in have the fitting welded in for the wing vent.
The 360 STC doesn't require long range tanks but does require the vent in both tanks. The old center vent is removed and that line becomes the fuel return.

I'm not sure if cessna relocated drain fittings or other things when going to a tricycle great.
Would be great of the tanks are interchangeable in the FAAs eyes as well.

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

[blueldr]

David,
The cross cabin vent line does not become the fuel return in the XP Mods STC. The return fuel from the injection system pump goes only back to the header tank and thence back to the engine. There is no fuel returned to either tank. The header tank is VENTED to the cross cabin vent line and will only contain fuel when the vent line floods from full tanks and the aiplane is in a nose down attitude, just as it does on a stock airplane.

[bagarre]

I was reading a copy of the STC last night and it stated that the center vent was removed from the system and the header tank was Tee'd into the vent line as a return to the fuel tanks.
I'll re-read the instructions but it seemed pretty clear.

Need to compare part numbers but since the 172 is on a different type certificate they may be identical parts with different numbers

[blueldr]

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..

[bagarre]

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..

The old above cabin fuel vent, Cessna 170, A, B is eliminated and the line is used for fuel return. New venting is now required. Remove both fuel tanks and alter fuel tanks (...) per appendix A-2, A-3, A-4

They even detail a cover plate for the removed center vent in the cabin top

Appendix A-3 details the welding of the fitting to the tank for venting

There may be other ways to do it but that's the method detailed in the copy of the STC that I have.

[Ryan Smith]

Is the sump in the same location for the 172? I would think that the low point of the tank in a tricycle airplane would be different than that of a taildragger; much the same as the door hinges for the 172 being different from the 170.

[blueldr]

David,
Does your copy of the STC show the fuel retrn going to the old cross cabin vent line? I don't remember that at all. The trouble with that system is that you will never know where the fuel is going and I'm sure you know how unreliable it is to try to depend on fuel quantity indicators. With the fuel return going to the header tank, no fuel will ever go back to the wing tanks.

[bagarre]

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.

[DaveF]

I'd guess the line to the vent tee is a vapor return line.

In the T210, fuel is only pumped back from the fuel control unit or fuel pump to the header tank. But there is also a vent line from each header tank to its main tank. The vent purges vapor that's returned from the engine and allows the fuel to flow smoothly down from the main tank. The vent line may fill with fuel, but that's not its real job. I mention the 210 because that's the more or less standard Cessna setup, and I'd guess the mod copies it (??).

Edit: Looks like I just repeated what BL said.

I don't understand the XP Mods description. Does the fuel selector in the XP Mods IO360 system have ports for the fuel return, or is the return line from the fuel pump sent directly up to the overhead line (formerly the vent)? Is there a diagram you could post?

[blueldr]

With only gravity flow filling the header tank and the engine consuming fuel, there's no way youre going to have fuel going back up the vent line to the wing tanks.

[bagarre]

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.

[n2582d]

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

185 Fuel System.pdf

[n3833v]

This section of the forum makes for very interesting reading about venting and flow.
http://www.homebuiltairplanes.com/forum ... nting.html

John