"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/