Son of Cessna 150 Fuel System
Kevin P. Kleinfelter
tank.]
Well I spent all yesterday fiddling with and flying the 150. (It's a tough
job, but SOMEBODY has to do it. :-) ). Here's the current situation as I
see it:
1. Fuel crossfeeds nicely on the ground. Crossfeed rate is about 8 gal/hr.
(From right to left.) From this I conclude that there is no blockage
in the right fuel feed.
2. Both caps are vented. One is a slightly older design, and vents better.
This one was on the left. When I moved it to the right and then flew
around for a while, fuel actually fed from the LEFT to the RIGHT. Fuel
levels were equal prior to flight. From this I conclude that the left
tank is presurizing or the right tank is developing partial vacuum during
flight.
3. I talked with the previous owner (through an intermediary). The right
fuel lines have been replaced in an unsuccessful attempt to correct the
problem. New caps have been tried in an unsuccessful attempt to correct
the problem. (The plane still has the new fuel lines, but the new caps
were returned.) The previous owner reiterates: "Just ignore the problem,
it'll feed fine when the left tank is empty." From this I conclude that
the previous owner would like to see me practice emergency landings! :-)
4. The fuel system depicted in my '67 owner's manual does NOT depict a
cross vent between the left and right tanks. The fuel system in a
1970 owner's manual DOES depict a cross vent between the left and right
tanks. However, my '67 owner's manual does NOT depict the overflow tube,
and the 1970 owners manual DOES depict the cross vent. Either the
diagram in my '67 manual is wrong, or there is no cross vent in older
C150s. Anyone know for sure? If there is a cross vent in my plane,
it must be plugged, and the left tank is probably getting pressurized
from the overflow tube sitting in accelerated air. (Otherwise, I cannot
imagine a way to drive fuel from the left tank to the right tank in
coordinated flight.)
If there is no cross vent, and the overflow tube looks to be centered behind
the strut, what else can I do?
Many thanks to those whose suggestions have helped me get this far. Please
note that any email responses seem to need to go to the address in the
signature at the bottom, and not the followup address.
--
Kevin Kleinfelter @ DBS, Inc (404) 239-2347 ...gatech!nanovx!msa3b!kevin
Dun&Bradstreet Software, 3445 Peachtree Rd, NE, Atlanta GA 30326-1276
WARNING: I have been advised that email to ke...@msa3b.UUCP may bounce.
It looks like email will have to go via 'gatech' because that is well-known.
Ron Wanttaja
>
> Well I spent all yesterday fiddling with and flying the 150. (It's a tough
> job, but SOMEBODY has to do it. :-) ).
>
Reminds me of a CFI friend who occasionally works with Ben Ellison of
Ellison Fuel Systems; a company which has developed throttle-body fuel
injection for aircraft engines. They'd been selling them for homebuilts
for quite a while when they decided to get an STC. They installed one on a
150.
The FAA, of course, wanted a full test series and ample evidence of proper
operation over time. As part of that, they wanted four hundred hours of
flight time on the test setup.
So Ellison called Ross in, handed him a gas credit card, and said, "Go
put 400 hours on that 150."
Not *quite* as good a deal as you might imagine. Y'see, the Cessna had to
operate in the Experimental Category, with similar restrictions as a new
homebuilt: Stay within 25 miles of the airport, no passengers. Bring your
Game Boy :-).
But Ellison makes it up to him in other ways... seems he's too busy to fly
his Long-EZ or Starduster II often enough, so Ross gets to keep the oil
loose and the dust clear...
Ron Wanttaja
prang@ssc-bee
...rutgers!uw-beaver!ssc-bee!prang
pr...@ssc-bee.boeing.com
Steven Philipson
> [Summary of previous postings: My '67 Cessna 150 won't feed from the right
> tank.]
I've experienced the same problem on a '77 C-150. It happened
intermittently and defied all attempts to fix it. I too was told by
some "old hands" that there was nothing to worry about in that the
right tank would start to feed when the left tank reached empty. This
didn't reassure me, either.
The "normal" behavior of many 150s is that the left tank fuel level
will run slightly lower than that of the right. This isn't a problem.
I did consider it a problem when the left tank would run to empty while
the right tank was still indicating "full". This wasn't an indicator
failure -- I made a precautionary landing when I saw this indication,
immediately got out and visually checked the tanks. The right tank was
full to the top and the left showed bare aluminum. As I watched, fuel
transferred from the right to left through the common feed line that
connects both tanks.
The '77 150 has an underwing vent on the left tank, a cross-vent that
connects the tops of the two tanks, and vented caps on both tanks. The
underwing vent has a check valve which is supposed to let air in but
not let fuel out. This setup was common on earlier 150's as well, but
I'm not sure if it applies to the '67 model.
The cap's valve is very simple -- it's a rubber mushroom-shaped
diaphragm that closes against vent holes in the tank cap. Low pressure
in the tank should cause the diaphragm to flex downward thus allowing
air to enter. High pressure in the tank (from fuel or air) pushes up
on the diaphragm, thus keeping it closed, which prevents fuel from
siphoning overboard. (The tank cap is in a low pressure area on the
top of the wing, so fuel would tend to flow out if there were no cap
or no check valve.)
We checked the cross vent by pressurizing the left tank (with a hose
and a hefty pair of lungs), and listening for air entering the right
tank through the cross vent. Kevin, you might try this and see if your
vent is working. We also checked caps this way -- pressure applied to
the underwing vent did not appear to leak out, thus the caps were
sealing under positive pressure. A negative pressure test would have
shown us whether the caps open in that condition, but we couldn't do
that with our simple tools as the check valve would close, thus
preventing application of vacuum.
I have a hypothesis about what's happening here. In short, I think
that a hydraulic/pressure lock develops. I suspect that that the cross
vent becomes partially blocked by air/fuel bubbles in that line. If the
tanks are fueled to the top, the cross vent will be submerged in fuel.
In flight, the left tank is pressurized by air entering the under-wing,
forward facing vent. This pressure is transferred to the right tank
hydraulically by fuel pressure through the fuel feed line, and thus
works against both tank caps, effectively sealing them. If the right
tank is full and the cross vent partially blocked, the left tank will
feed first (the only outlet for the pressure is the engine fuel line).
Eventually the pressure head of the fuel in the right tank overcomes the
pressurization of the left tank, or the cross vent opens up, thus
allowing pressurized air to enter the top of the right tank, allowing
that tank to feed. (Note: I can't prove this. Creative "debunking" of
this theory is welcome.)
I did find a method of getting fuel to flow from the right tank when
this situation occurred. I was trying to get the fuel to move around in
the tanks, and applied full rudder in each direction. This produced
uncoordinated flight that drove the slip-skid ball to the ends of its
tube. (Steep, coordinated banks will NOT work.) I conjecture that this
moved fuel away from the cross vent tube (and perhaps the fuel pickups
on the left tank), thus allowing air to move into the top of the right
tank and equalizing pressure on the fuel surface of both tanks. This
resulted in immediate flow from the right to left tank (if the air
pressure is the same, the higher level of fuel in the right tank would
cause the fuel to transfer -- a liquid seeks its own level). Fuel flow
is normal after this. I've had this happen several times, and the
technique has always worked.
Kevin noted lots of inconsistencies between diagrams in various
handbooks for the C-150. I suggest that if you want to know exactly
what the configuration is, that you consult the appropriate technical
diagrams. Your mechanic probably has them on microfiche. Also, you
open up the wing root fairings and look. (Would that be cheating? :-) )
Best of luck.
Steve
(the certified flying fanatic)
ste...@decwrl.dec.com