Fuel dump switch in homebuilt

Ron Wanttaja writes:

On Mon, 01 Dec 2003 20:09:35 GMT, Kyler Laird
wrote:

Ron Wanttaja writes:

Well... as far as homebuilts are concerned, fuel exhaustion (defined as the
pilot running the airplane out of fuel) plays only a minor role in the
overall accident rate. During 1998-2000, only 4.5% of all homebuilt
accidents involved fuel exhaustion (including some accidents that occurred
during precautionary landings due to a low fuel state).

How many of "all homebuilt accidents" involved an "emergency landing"?
(I don't think we mean "landing" to include "falling to earth in pieces".)

About 20% of the homebuilt accidents in that period involved a loss of
power due to mechanical failure of the engine or fuel system (vs. pilot
mismanagement of fuel or power system). About 15% engine related, about 5%
fuel-system related.

So are you saying that there's a 1:1 relationship between losing power and
making an emergency landing? No one loses power and performs a stall/spin
return to the ground?

I'm just trying to get back to the point about fuel probably being already
exhausted when an emergency landing is executed. Are you saying that three
of every four emergency landings are made with significant quantities of
fuel on board? (Another complication is that it's fairly common to still
have gobs of fuel on board even when fuel starvation is the cause of the
accident.)

--kyler

Ed Wischmeyer writes:

BTW, do any homebuilts use easily-sheared tip-tanks (like some Cessnas and
Lears do)?

Easily sheared Cessna tip tanks, like on a twin? Don't think the
accidents show that those shear... what's your source?

No source. I just heard that that's the way they were designed. I assume
they shear a lot more easily than the tanks in my wings do.

--kyler

On Tue, 02 Dec 2003 17:09:41 GMT, Kyler Laird
wrote:

Ron Wanttaja writes:

Well... as far as homebuilts are concerned, fuel exhaustion (defined as the
pilot running the airplane out of fuel) plays only a minor role in the
overall accident rate. During 1998-2000, only 4.5% of all homebuilt
accidents involved fuel exhaustion (including some accidents that occurred
during precautionary landings due to a low fuel state).

How many of "all homebuilt accidents" involved an "emergency landing"?
(I don't think we mean "landing" to include "falling to earth in pieces".)

About 20% of the homebuilt accidents in that period involved a loss of
power due to mechanical failure of the engine or fuel system (vs. pilot
mismanagement of fuel or power system). About 15% engine related, about 5%
fuel-system related.

So are you saying that there's a 1:1 relationship between losing power and
making an emergency landing? No one loses power and performs a stall/spin
return to the ground?

Certainly, to a depressing extent, especially when the engine failure
occurs on takeoff. I apparently didn't make my point clear: Another
poster commented that "If you are making an emergency landing in a small
plane, chances are that you don't have any fuel left on board to dump." My
response was meant to counter this, in that fuel-exhaustion-related
accidents were a relative minority.

I'm just trying to get back to the point about fuel probably being already
exhausted when an emergency landing is executed. Are you saying that three
of every four emergency landings are made with significant quantities of
fuel on board?

No. All I said on my original posting was that 4.2% (I posted "about
4.5%") of all homebuilt accidents in the January 1998 to December 2000 time
period (29/692) involved fuel exhaustion. I make no claim about how much
fuel remained in the remaining 660-odd accidents that year, other than to
guess that the NTSB investigator apparently considered there was sufficient
fuel on board to rule out lack of fuel as a reason for a loss of power
situation.

(Another complication is that it's fairly common to still
have gobs of fuel on board even when fuel starvation is the cause of the
accident.)

Out of the 29 cases of fuel exhaustion, two cases involved a crash during a
precautionary landing due to a low fuel state, one involved the fuel tank
unporting when the pilot maneuvered at low attitude with a low fuel state,
one had a bad header tank, and one pilot failed to select his reserve tank
when the main tank ran dry. Three cases of extremely low fuel, two of some
amount of fuel. In the remaining 24 cases, the engines quit because the
pilot used up all his or her usable fuel (one case included a leaking fuel
tank). That would still leave several gallons aboard each aircraft.

In ~660 accidents, then, the airplane crashed with sufficient fuel on board
to have enabled continued flight. I personally don't think a fuel-dump
valve would have helped in most of these cases. But I suspect it's like
the ballistic parachute argument; if you need one, then you REALLY need
one. :-)

Ron Wanttaja

Thanks, Bob K, or your insightful contribution to the discussion.

Thats good data to have about the fuel pump requirements for the pump
that feed the engine. Have recently done some looking into of pumps
for a diesel to veg oil conversion project and I've discovered that
you really design pumps differently depending on what your goal is.
For a fuel pump, you want to deliver small volumes at high pressure.
In contrast for dumping fuel you want to move large amounts to a
negative pressure. Have you seen any of the pumps they use in washing
machines?

But since its a negative pressure system, technically speaking, you
don't need a pump at all to drain the tank. The only question, is how
fast do you need to let it go, that will dictate how big a pipe do you
need from the tank to the valve to the low pressure port on the
surface of the fuselage. Of course the bigger the pressure
differential you can find the better.


(Bob Kuykendall) wrote in message . com...
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.

Ron Wanttaja wrote:

No. All I said on my original posting was that 4.2% (I posted "about
4.5%") of all homebuilt accidents in the January 1998 to December 2000 time
period (29/692) involved fuel exhaustion. I make no claim about how much
fuel remained in the remaining 660-odd accidents that year, other than to
guess that the NTSB investigator apparently considered there was sufficient
fuel on board to rule out lack of fuel as a reason for a loss of power
situation.

(Another complication is that it's fairly common to still
have gobs of fuel on board even when fuel starvation is the cause of the
accident.)

Out of the 29 cases of fuel exhaustion, two cases involved a crash during a
precautionary landing due to a low fuel state, one involved the fuel tank
unporting when the pilot maneuvered at low attitude with a low fuel state,
one had a bad header tank, and one pilot failed to select his reserve tank
when the main tank ran dry. Three cases of extremely low fuel, two of some
amount of fuel. In the remaining 24 cases, the engines quit because the
pilot used up all his or her usable fuel (one case included a leaking fuel
tank). That would still leave several gallons aboard each aircraft.

In ~660 accidents, then, the airplane crashed with sufficient fuel on board
to have enabled continued flight. I personally don't think a fuel-dump
valve would have helped in most of these cases. But I suspect it's like
the ballistic parachute argument; if you need one, then you REALLY need
one. :-)

Let's not rule out the one advantage of the fuel dump we haven't
discussed - that anyone who had to deadstick in due to fuel starvation
could then claim he had dumped the fuel to prepare for the deadstick
landing. Sure, he'd have some 'splainin' to do when the beast fires
right up when resupplied with go-juice, but he could always pull a
Unka BOb and claim it was just one of those evil intermittent auto
engine systems.

Mark Hickey

Let's not rule out the one advantage of the fuel dump we haven't
discussed - that anyone who had to deadstick in due to fuel starvation
could then claim he had dumped the fuel to prepare for the deadstick
landing. Sure, he'd have some 'splainin' to do when the beast fires
right up when resupplied with go-juice, but he could always pull a
Unka BOb and claim it was just one of those evil intermittent auto
engine systems.

Mark Hickey
++++++++++++++++++++++++++++++

All machinery has the capacity to be evil.
*Certified* is just less evil in competent hands.


Unka BOb --

On Wed, 03 Dec 2003 10:59:02 -0700, Mark Hickey wrote:

Let's not rule out the one advantage of the fuel dump we haven't
discussed - that anyone who had to deadstick in due to fuel starvation
could then claim he had dumped the fuel to prepare for the deadstick
landing. Sure, he'd have some 'splainin' to do when the beast fires
right up when resupplied with go-juice, but he could always pull a
Unka BOb and claim it was just one of those evil intermittent auto
engine systems.

Actually, he might not have to explain too much. This sort of scenario
does happen (where the engine runs OK afterwards), and the NTSB usually
just chalks it up to "Engine failure for undetermined reasons."

Ron Wanttaja

or one could say it was certifiably evil.....
Sean


RR Urban wrote:
Let's not rule out the one advantage of the fuel dump we haven't
discussed - that anyone who had to deadstick in due to fuel starvation
could then claim he had dumped the fuel to prepare for the deadstick
landing. Sure, he'd have some 'splainin' to do when the beast fires
right up when resupplied with go-juice, but he could always pull a
Unka BOb and claim it was just one of those evil intermittent auto
engine systems.

Mark Hickey

++++++++++++++++++++++++++++++

All machinery has the capacity to be evil.
*Certified* is just less evil in competent hands.


Unka BOb --

or one could say it was certifiably evil.....
Sean

Mark Hickey wrote:

Let's not rule out the one advantage of the fuel dump we haven't
discussed - that anyone who had to deadstick in due to fuel starvation
could then claim he had dumped the fuel to prepare for the deadstick
landing.

I am reminded of the following accident report which I found quite
humorous. This is a partial quote from an NTSB accident report of a
C-150 forced landing near Four Corners Regional Airport, Farmington,
New Mexico circa October 2003,

"According to the tower operator at FMN, as the airplane was entering
a left base for runway 07, a expletive was heard over the tower
frequency and shortly thereafter an emergency locator transmitter
(ELT) signal was heard. According to emergency personnel, when they
arrived on scene, they found the pilot had already pulled himself out
of the inverted airplane and was sitting on the wing. The pilot told
them "don't worry about a fire, there's no fuel."

David O -- http://www.AirplaneZone.com