Earlier, (Jay) wrote:

...The solution I suggested adds
another port on the fuel selector valve,
and a piece of plastic tubing to the
low pressure port.

Well, even with a bit of help from a low-pressure area, you're going
to need additional pumping capacity to get the fuel overboard in a
reasonable amount of time.

According to _Firewall Forward_, FAR part 23 guidelines specify that
pumped fuel system be capable of delivering 125% of the takeoff power
fuel flow, and gravity flow systems 150%. That's open-port flow with
zero backpressure.

For example:

Say you're using a 180 h.p. or so motor that draws 18 gallons per hour
at max takeoff power (let's say). The most you can expect the
appropriately-sized fuel pump to move is about 25 gallons per hour.

But, more likely, you haven't got an hour. The vast majority of
concievable small aircraft emergencies are likely to be over in 15
minutes or less. That means you only have time to send 25/4 or about
6.25 gallons (37.5 lbs) of fuel overboard.

And that's if the pump runs wide open at the outlet. With any
substantial backpressure, the flow rates will be much lower. And you
might happen to need some of that pressure to apply fuel pressure to
the engine fuel system inlet. I suppose that you could add a rate
restrictor to the overboard port, so that fuel pressure is maintained
even while dumping. But that's going to cut the dump rate
substantially.

Of course, both of these points can be easily addressed. You can add a
separate dumping pump just to pump fuel overboard, and you can size it
to achieve the desired rates. You can also give the pump its own fuel
supply and overboard plumbing, again sized for the desired dump rate.
You can even plumb the dump system with a standpipe so you can't
inadvertantly run the tanks completely dry with it.

However, that pump weighs something, and the fuel, electrical, and
mechanical connections that service it also weigh something. And when
you add up all that weight, it is substantial, and it will have a
measurably deleterious effect on takeoff, cruise, and landing
performance. It also adds many points of potential failure, both
mechanical and human. And for those accidents caused by fuel
exhaustion (a big slice of the pie), it means that the airplane hits
the ground with more weight and energy, not less.

What you end up with is a compromise that balances constant and
measurable penalties (weight and complexity) against hypothetical
gains (safety).

Thanks, and best regards to all

Bob K.