On 06/23/2013 02:36 PM, Karl Heindl wrote:
> 1. What exactly is the reason again for having the tank vent in the
> airstream instead of a static position? Are we doing this because
> everybody else has always been doing it or is there a real safety aspect
> ?
I think it is the first. Especially when you have the tank vent
connected to the cobra, there is no risk of sucking the fuel out of the
tank even if there would be a negative pressure.
> I don't see the need for pressurization, especially since this
> must be miniscule, in a pump driven system.
In our soft tanks pressurization is even harmfull. It may cause the tank
to jam the aileron control, or abrasion of the tank when it repeatedly
expands, or other bad things. Best thing would be a neutral or slightly
negative pressure (the latter helping to keep fuel fumes out of the
cockpit). Like you said, we don't have gravity fuel feed like Cessna's
but we are dependant on a fuel pump anyway.
> 2. Why has no PFA/LAA or other inspector ever pointed out the fact that
> the drain tap arrangement is completely useless ? There would have to be
> an awful lot of water in the tank before any of it would show up in a
> drain cup.
I never got any drop of water out of these drains, and have never heard
of anyone else who got any water out of it.
(If anyone has, I would like to hear about it, and also whether this
water showed up in the gascolator as well).
Instead, I think the drains and associated plumbing are increasing the
risk of leaks, and are a hazard during an accident: if the gear is
ripped off the next thing to contact the ground and be ripped off are
the fuel drains, and together with the unavoidable sparks this will for
sure trigger a fire. And the drains can not be closed with the fuel
valve so there is nothing you can do about it.
The next time I will have maintenance on the fuel plumbing I will take
the drains out. It is just a matter of judging the safety advantages
against the safety disadvantages, and to my judgement the latter puts
more weight in the equation here.
The only things I found them useful for is for draining the entire tank
for maintenance and inspection (I have valves that can be locked in the
open position) and to fill the bottle of my camp stove. But all this can
also be accomplished by other methods.
Avoiding water in the tank can also be accomplished by refueling after
each flight so there is only little air left in the tank. Having said
that, I was surprised after the winter stop in an unheated barn with
only a little fuel in the tanks that despite all the temperature cycles
(and associated "breathing" of the tank) and our wet climate that no
water at all condensed in the tank, not a single drop!
Also, how is this arranged with cars? I have never seen a car with a
fuel drain, but yet water in the fuel seems to be unheard off (obvious
causes of forgetting to put the cap on the tank not counting). What's
the story here?
> The obvious reason is
> the high point of the fuel exits and the high barb fitting on the taps
> themselves.
I have copper tubes inside the tank running to a position lower than the
fuel outlet for the engine. So in theory it could work.
Frans
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