Troubleshooting the Comanche fuel system

[Thomas]

I was flying a friend to an airport some 200 miles away where he’d had
some work done on his Mooney. I have a 1966 PA-24-260 with two main tanks,
two aux tanks and two tip tanks. About 5-10 minutes after I had leveled
off at 5500 feet and switched to the left aux tank, the engine started
acting up. It ran unsmoothely and the power varied up and down. I tested
the magnetoes, put on alt air, varied the power setting and mixture (I was
flying at 75% power, 24”/2350, OAT was 14F at 5500 feet) but it wasn’t
until I switched to the main tank that the engine reverted to normal
operation.

Later on I switched to the other (right) aux tank. A few minutes later,
the same problem appeared. This time I didn’t touch anything. Instead I
observed the fuel flow and the EGT indicator. Fuel flow increased from
15.3gph to 21.7gph, where it stabilized, while the EGT increased by 50F.
Before the increase in EGT I was running at 100F ROP. I let it go on for
maybe a minute or two, then switched back to one of the main tanks and the
problem went away. EGT and fuel flow came back to normal.

So what happened? The change in EGT and fuel flow are inconsistent. The
EGT indicates that the mixture is getting leaner while the fuel flow
indicator says the mixture is getting richer.

My friend’s first hypothesis was that air was entering the system. This
would cause the turbine blades in the fuel flow transducer to turn faster
(thus showing an increase in fuel flow) and give a leaner mixture
(explaining the rise in EGT). Sound plausible, but I don’t see why the
problem happens on both aux tanks but not on any of the main tanks. I have
two fuel selectors, one for the three left tanks (main, aux and tip) and
one for the three right tanks.

Other ideas that came up were clogged fuel lines to the aux tanks or
clogged air vents in the aux tanks.

On the flight home I got up to a higher altitude (12500 feet) and flew at
55% power. OAT was -13F. I was able to reproduce the problem. This time it
occured within a minute of my shifting to the right aux tank. Fuel flow
went up from 10.0gph to 17.2gph. Then it varied erratically between
14.2gph and 17.2gph. Unfortunately I didn’t check the EGT this time. When
I shifted back to the main tank the problem went away again.

I’m going to take the plane up tomorrow for a test flight. There are two
things I’m planning on testing. First, what effect does applying the
electrical fuel pump when the problem appears have? Second, does the
problem appear at a lower altitude, say 2000 feet, where it’s less cold?

Any ideas on what’s causing this problem?

/Thomas

[Ben Jackson]

On 2006-03-24, Thomas > wrote:
> I was flying a friend to an airport some 200 miles away where he?d had
> some work done on his Mooney. I have a 1966 PA-24-260 with two main tanks,
> two aux tanks and two tip tanks.

I have a Comanche with mains and auxes, but no tips. The first thing
I'd check in your setup is the tip tank pumps. My understanding is that
when you have tip tanks, you fly the auxes down, then pump in from the
tip tanks. Perhaps some misoperation of those pumps could affect your
fuel flow.

Also, try operating the fuel drain in the various settings. Do you see
bubbles or excessive flow from the drain when the auxes are selected?

--
Ben Jackson
>
http://www.ben.com/

[kontiki]

Ben Jackson wrote:
> I have a Comanche with mains and auxes, but no tips. The first thing
> I'd check in your setup is the tip tank pumps. My understanding is that
> when you have tip tanks, you fly the auxes down, then pump in from the
> tip tanks. Perhaps some misoperation of those pumps could affect your
> fuel flow.
>
> Also, try operating the fuel drain in the various settings. Do you see
> bubbles or excessive flow from the drain when the auxes are selected?
>
Comanche tip tanks don't use pumps. They can be used anytime while in
cruise (not for t/o or landing). I think his problem is either a clogged
tank vent (one time a mud dauber made a nest in my right tip vent) or
possible fuel line blockage. It could be an air leak in the line as
well, but that would generally cause some leakage that you might notice.

[Orval Fairbairn]

In article >,
kontiki > wrote:

> Ben Jackson wrote:
> > I have a Comanche with mains and auxes, but no tips. The first thing
> > I'd check in your setup is the tip tank pumps. My understanding is that
> > when you have tip tanks, you fly the auxes down, then pump in from the
> > tip tanks. Perhaps some misoperation of those pumps could affect your
> > fuel flow.
> >
> > Also, try operating the fuel drain in the various settings. Do you see
> > bubbles or excessive flow from the drain when the auxes are selected?
> >
> Comanche tip tanks don't use pumps. They can be used anytime while in
> cruise (not for t/o or landing). I think his problem is either a clogged
> tank vent (one time a mud dauber made a nest in my right tip vent) or
> possible fuel line blockage. It could be an air leak in the line as
> well, but that would generally cause some leakage that you might notice.

How about a little water in the sump and ice crystals blocking the
finger screens in the tanks?

[JJS]

>> Ben Jackson wrote:
>> > I have a Comanche with mains and auxes, but no tips. The first thing
>> > I'd check in your setup is the tip tank pumps. My understanding is that
>> > when you have tip tanks, you fly the auxes down, then pump in from the
>> > tip tanks. Perhaps some misoperation of those pumps could affect your
>> > fuel flow.
>> >
>> > Also, try operating the fuel drain in the various settings. Do you see
>> > bubbles or excessive flow from the drain when the auxes are selected?
>> >
>> Comanche tip tanks don't use pumps. They can be used anytime while in
>> cruise (not for t/o or landing). I think his problem is either a clogged
>> tank vent (one time a mud dauber made a nest in my right tip vent) or
>> possible fuel line blockage. It could be an air leak in the line as
>> well, but that would generally cause some leakage that you might notice.
>
Wrong fuel in the tip tanks?



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[Dave]

Same... thinking wrong fuel..

Was fuel added to the the tips?, and ended up with a mix of 100 + jet
A?

Dave



On Fri, 24 Mar 2006 23:52:50 -0600, "JJS" <jschneider@remove socks
cebridge.net> wrote:

>
>>> Ben Jackson wrote:
>>> > I have a Comanche with mains and auxes, but no tips. The first thing
>>> > I'd check in your setup is the tip tank pumps. My understanding is that
>>> > when you have tip tanks, you fly the auxes down, then pump in from the
>>> > tip tanks. Perhaps some misoperation of those pumps could affect your
>>> > fuel flow.
>>> >
>>> > Also, try operating the fuel drain in the various settings. Do you see
>>> > bubbles or excessive flow from the drain when the auxes are selected?
>>> >
>>> Comanche tip tanks don't use pumps. They can be used anytime while in
>>> cruise (not for t/o or landing). I think his problem is either a clogged
>>> tank vent (one time a mud dauber made a nest in my right tip vent) or
>>> possible fuel line blockage. It could be an air leak in the line as
>>> well, but that would generally cause some leakage that you might notice.
>>
>Wrong fuel in the tip tanks?
>
>
>
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[Thomas]

On Sat, 25 Mar 2006 00:11:41 +0100, kontiki >
wrote:
> I think his problem is either a clogged
> tank vent (one time a mud dauber made a nest in my right tip vent) or
> possible fuel line blockage.

I was out at the airport this morning. We blew some compressed air in each
of the tank vents (from below the wing) to see if the air would circulate
up to the fuel cells. With the fuel caps removed, looking down in the tank
we could see air coming up in three of the fuel tanks, but nothing
happened when air was blown in the tank vent on the left aux tank. With
the left aux tank it sounded like air leaked out inside the wing.

This seems to indicate that the tank vent on the left aux tank is clogged.
However, according to my notes from the flights where the problem occured,
the problem happened twice with the fuel selector on right aux and once
with the fuel selector on left aux. My friend thinks I may have gotten
that wrong. Can't rule it out but it seems strange.

I didn't get to make a test flight today to see if I could reproduce the
problem. We'll start by removing the left aux tank some time next week to
see if the tank vent is indeed clogged.

/Thomas

[Ben Jackson]

On 2006-03-24, kontiki > wrote:
> Ben Jackson wrote:
>> I'd check in your setup is the tip tank pumps. My understanding is that
>> when you have tip tanks, you fly the auxes down, then pump in from the
>> tip tanks. Perhaps some misoperation of those pumps could affect your
>> fuel flow.
>
> Comanche tip tanks don't use pumps. They can be used anytime while in
> cruise (not for t/o or landing).

What tip tanks are we talking about? The Twin was available at some
point with factory tip tanks and a fuel selector that included them.
A single with tip tanks would be aftermarket from Brittain/Osborne.

--
Ben Jackson
>
http://www.ben.com/

[Thomas]

On Sun, 26 Mar 2006 23:49:27 +0200, Ben Jackson > wrote:
> On 2006-03-24, kontiki > wrote:
>> Comanche tip tanks don't use pumps. They can be used anytime while in
>> cruise (not for t/o or landing).
>
> What tip tanks are we talking about? The Twin was available at some
> point with factory tip tanks and a fuel selector that included them.
> A single with tip tanks would be aftermarket from Brittain/Osborne.

My tip tanks are from Osborne. There's no pumping involved. You set the
fuel selector on the tip tanks just as you would for the other tanks.

/Thomas

[Thomas]

On Sat, 25 Mar 2006 00:11:41 +0100, kontiki >
wrote:
> I think his problem is either a clogged
> tank vent (one time a mud dauber made a nest in my right tip vent) or
> possible fuel line blockage.

We discovered that the vent hose on the left aux tank was off. That may
account for the problem running on the left aux tank, but I still thought
the same problem had occured while running on the right aux tank.

We then checked the fuel flow from the electric fuel pump. With the left
aux tank selected we got about 55 gph. With the right aux tank selected
the fuel flow was significantly less, maybe half, and the fuel pump
sounded like it had to work harder.

We drained the right aux tank and had a look inside with a flashlight and
a mirror. It all looked fine, no obstructions. We tried to get to the hose
attaching the fuel tank to the fuel line by just removing the inspection
plate under the wing, but we couldn't remove the hose. We'll have to take
the tank out completely to inspect it thourougly.

/Thomas