"Tanks on both" checklist item

Dave Butler wrote in message ...

If you're paying attention and know your fuel burn you can predict the moment of
running out within plus/minus a few minutes. When the fuel pressure gauge drops
to zero, you still have some time while the engine continues to run smoothly.
Switch tanks when the fuel pressure drops, the pax will never know.

Of course, you've run some on the other tank previously, so you know the fuel in
the other tank is sweet.

Bingo! Dave's right on the money. My plane's flight manual
cautions against running a tank dry (I believe there's an outside
possibility of fuel pump cavitation). Using fuel pressure method
works great. I do this on long cross-country flights where maximizing
my range is important. In my Cherokee, the fuel pressure will begin
to drop with about 1.5 gallons usable in the tank (close enough for
me). At this point, assuming no unusual fuel burn, I have 1.5 hrs of
fuel left in the other tank, which in the real world means I have 1/2
hr. to be on the ground.


This old pilot thinks everyone should do this at least once so that they know
how their plane behaves in this situation. It also provides a chance to make a
direct measurement of exactly what your tank capacity is.

Agreed. I do this on every flight where I'll be flying at maximum
range. By running a fuel tank almost dry, I know if my fuel burn is
normal and can complete the flight as planned. If the tank runs out
before the clock says it should, I still have a bunch of gas to make
it to an alternate (assuming I'm not flying in some of the more remote
parts of the Northern Territories of Canada :-).

By using this method, I can usually guess the fuel burn on a long
cross-country flight to within 1 gallon. This kind of knowledge makes
flying in sparsely populated areas a lot more comfortable and allows
me to get better utilization of the amount of fuel my plane is able to
carry.

Note : Do not take the above to mean that I fly my tanks to
dangerously low levels. My personal limit is to land with a minimum
of 1 hr. in the tanks. In 16 yrs. I've never landed with less, but by
gauging my fuel burn precisely, I can often reach my destination with
1.2 hrs. remaining, thereby maximizing my range, yet minimizing my
risks.

John Galban=====N4BQ (PA28-180)

"Corky Scott" wrote in message
...

snip


The designer implied that this is a standard aviation design (the
different locations for the fuel outlets).


Dunno about others but in the few I've had occasion to help build-

The PA-12 has outlets only at the aft end of the tank. This worked great
for our use, banner towing, as most our flying was at high AoA (though,
since the gauge was at the FORWARD end of the tank they didn't work after
30-45 minutes).

The PA-18 has pickups at both ends, then feeds into a Y fitting, that then
goes to a header tank (small 1 gal) then to the selector valve. Worked
even better.

Tony Cox wrote:

"Frank" wrote in message
...
Tony Cox wrote:

Some models of 172 *require* you to select a single tank
above 5000', due (supposedly) to vapor lock problems.


No 'supposedly' about it. Ours locked at 7500' on a humid day because one
of
my partners didn't believe it could happen and didn't follow the
procedure.
Fortunately an airport was within glide range. Strangely enough, by the
time he landed the engine was running fine again....
--
Frank....H

Its happened to me too. I said 'supposedly' because I've not
heard a plausible explanation as to why selecting one
particular tank rather than both should affect the chance of
vapor lock. Is the fuel selector close to a heat source?


The explanation I got had to do with pressure (or lack of it) above 4500'.
This problem is exacerbated by high humidity. It seems that the plumbing
allows a condition to develop whereby there is not enough pressure from
gravity to push the fuel through the plumbing without forming "bubbles". It
has to do with the venting. Closing off one tank (ie selecting left or
right) solves this.

This admittedly vague explanation is only intended to point out that
pressure differences are the culprit. No one has ever shown me in great
detail just how it happens. Nonetheless, I'm a believer.

--
Frank....H

"Frank" wrote in message
...
Tony Cox wrote:

Its happened to me too. I said 'supposedly' because I've not
heard a plausible explanation as to why selecting one
particular tank rather than both should affect the chance of
vapor lock. Is the fuel selector close to a heat source?


The explanation I got had to do with pressure (or lack of it) above 4500'.
This problem is exacerbated by high humidity. It seems that the plumbing
allows a condition to develop whereby there is not enough pressure from
gravity to push the fuel through the plumbing without forming "bubbles".
It
has to do with the venting. Closing off one tank (ie selecting left or
right) solves this.

This admittedly vague explanation is only intended to point out that
pressure differences are the culprit. No one has ever shown me in great
detail just how it happens. Nonetheless, I'm a believer.



My 'vapour lock' occurred at 7500' on a cold November day
with low humidity (ceilings were well above us). If anything,
switching to one tank rather than both should actually *lower*
the fuel pressure slightly - faster flow = greater friction loss.

So I'm not really impressed at all by the explanation. Luckily,
engine power was restored when passing through 3500',
shortly (as it happens) after fiddling with the fuel selector as
part of the emergency checklist. Incidentally, that was the last
time I assumed that the POH for one plane was the same as
another.

--
Dr. Tony Cox
Citrus Controls Inc.
e-mail:
http://CitrusControls.com/

Tony Cox wrote:

"Frank" wrote in message
...

snip

The explanation I got had to do with pressure (or lack of it) above
4500'. This problem is exacerbated by high humidity. It seems that the
plumbing allows a condition to develop whereby there is not enough
pressure from gravity to push the fuel through the plumbing without
forming "bubbles".
It has to do with the venting. Closing off one tank (ie selecting left or
right) solves this.

This admittedly vague explanation is only intended to point out that
pressure differences are the culprit. No one has ever shown me in great
detail just how it happens. Nonetheless, I'm a believer.


My 'vapour lock' occurred at 7500' on a cold November day
with low humidity (ceilings were well above us). If anything,
switching to one tank rather than both should actually *lower*
the fuel pressure slightly - faster flow = greater friction loss.


Right, lower fuel pressure means less oomph to get it through the
pipes....Seems consistent with the 'theory'. I don't know enough about it
to say whether it's right or wrong.


So I'm not really impressed at all by the explanation. Luckily,
engine power was restored when passing through 3500',
shortly (as it happens) after fiddling with the fuel selector as
part of the emergency checklist. Incidentally, that was the last
time I assumed that the POH for one plane was the same as
another.


I know the explanation wasn't very satisfying, I'd love to hear it explained
better too. But, as you have apparently also experienced, the phenomenon is
real. So there is something going on and pressure differences due to
venting/plumbing seems to explain it better than temperature related ones.

--
Frank....H

"Frank" wrote in message
...
Tony Cox wrote:

My 'vapour lock' occurred at 7500' on a cold November day
with low humidity (ceilings were well above us). If anything,
switching to one tank rather than both should actually *lower*
the fuel pressure slightly - faster flow = greater friction loss.


Right, lower fuel pressure means less oomph to get it through the
pipes....Seems consistent with the 'theory'. I don't know enough about it
to say whether it's right or wrong.


To get maximum oomph, it would seem that 'both' should be
selected. Which is just what they tell you not to do!


I know the explanation wasn't very satisfying, I'd love to hear it
explained
better too. But, as you have apparently also experienced, the phenomenon
is
real. So there is something going on and pressure differences due to
venting/plumbing seems to explain it better than temperature related ones.


I just draw a blank on this. Unless there is a heat source near
the selector. Then I can rationalize an explanation. If there's some
venting/plumbing explanation, I'd expect some admonition to
"select the fullest tank" or "don't fly above 5000' without more
than 10 gallons in a tank". Makes no sense as is.

tc

Tony Cox wrote:

snip
I just draw a blank on this. Unless there is a heat source near
the selector. Then I can rationalize an explanation. If there's some
venting/plumbing explanation, I'd expect some admonition to
"select the fullest tank" or "don't fly above 5000' without more
than 10 gallons in a tank". Makes no sense as is.

tc

As far as I can tell there is no heat source anywhere near the selector.

One day I'll get the real story, until then....I'll just select L or R above
5000' and chalk it up to one of those mysteries of the universe.
--
Frank....H