On Sun, 30 Nov 2003 14:49:37 -0800, Jay wrote:

After reading some of the transcripts from the "Last Words" website, where
the flight engineers were dumping fuel when it became evident that an
emergency landing was a certainty, it dawned on me that there might be some
benefit for a small plane as well. The less energy you carry into a crash
landing the better off you're going to be. And since KE is mv^2, you get
a proportional benfit from dumping the weight of the fuel which might be
20% the weight of the airplane, and the lower weight allows for a slower
stall speed which cuts the V factor, and since thats squared, it counts
for a lot. And then of course you may have a larger glide radius with
that reduction in weight in addition to the reduced fire potential upon
landing and breakup

Maybe a fuel selector switch that ports to a low pressure area near the
tail would act as a light weight solution to draw out the fuel from the
tanks. A safety wire that would have to be broken would be a good idea so
it isn't accentally selected. Like the WEP setting on the WW2 fighters
with water injection.

Interesting idea, but I think there are good reasons why people haven't
done this on light aircraft:

1. If you have a major problem and need to do an off-airport landing (e.g.
engine failure, fire, etc), you usually don't have much time to play with,
so you probably wouldn't be able to get rid of enough fuel to make much of
a difference. Anything you do in the design to increase the dump rate
only makes the consequences of an uncommanded fuel dump even worse (see
item 3).

2. If you only have a minor problem (i.e. you have to land, but you are
not in any big rush), then you should have time to get to a suitable
airport where you can safely land at the current gross weight.

3. If you design in the ability to dump fuel, you have added a failure
mode where fuel gets dumped when you don't want it to. This could cause
an off-airport landing. Sure, this failure mode wouldn't happen too
often, but it only has to happen once to really ruin your day.

All things considered, this system would probably decrease the overall
level of safety, not increase it.

Fuel dump systems are on some large jets because of the large difference
between max approved take-off weight and max approved landing weight.
There are also some failures that make it attractive to be able to greatly
reduce the gross weight. E.g. failures of flaps and slats mean up to a 60
kt increase in approach speed on some aircraft. They may also have
partial brake failures to contend with, etc.

Note 1: even though big jets may have a big difference between max
approved take-off weight and max approved landing weight, that does not
mean that it is unsafe to land right after take-off and max approved
take-off weight. You just need to do a smooth landing, and you need a
long runway. Landings at max approved take-off weight are done routinely
during the take-off performance flight testing, as this type of testing
consists of multiple take-offs at max weight, followed a few minutes later
by a landing so the test can be repeated yet again.

Note 2: the glide ratio does not vary with gross weight. I won't try to
explain the math or physics as there are lots of references available on
the web for your googling pleasure.

--
Kevin Horton RV-8 (finishing kit)
Ottawa, Canada
http://go.phpwebhosting.com/~khorton/rv8/
e-mail: khorton02(_at_)rogers(_dot_)com