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nuckolls.bob(at)cox.net
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 Posted: Fri Oct 12, 2007 5:54 am Post subject: Z-19 and considerations of SYSTEM reliability |
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At 08:16 PM 10/11/2007 -0500, you wrote:
| Quote: |
By the way, Bob, both Z19 and Z19RB (rear battery) have 2 fuel pumps
depicted (the Eggenfellner Subs. have 2 pumps).
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| Quote: | While Z19 only has a switch to control Pump #2, I don't see any problem with
adding a pressure fail-over switch to automatically turn on #2 in the event
#1 quits pumping.
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Oh yeah, THAT #2 pump switch. Having a sr. moment here.
"Automatic" features are not necessarily "problems",
but they add to parts count which drives up probability
of a SYSTEM failure. For example:
I did what I believe was the first speed controlled
pitch trim system on the Lear 55 and ultimately the
30 series fleet about 1980. The speed control board
was pretty simple and fit on about 3 x 3" of copper-clad.
Then the systems guys began to worry about failures
of the speed control system and levied some requirements
for monitoring and warning. Over the past 25 years,
the preponderance of failures have been in the warning
system . . . and the failures in the speed control system
that produced a too-fast motor were less than 1% of all
failures. The monitor board was about twice the parts
count and accounts for most of the cost-of-ownership
for the system. It catches only 1% of all failures which
turned out to be so benign that the monitor board wasn't
really necessary after all.
My cautionary comments go to the fact that the more
complex your airplane becomes as a complete flight system,
the more it costs you to maintain it and the more likely
it is to present an in-flight conundrum to be pondered
and reacted to. A part that is not present is not going
to be the part that generates an in-flight distraction
or and on-the-ground maintenance event.
Your task as a system integrator is deducing
the return on investment for adding ANY components
to your system. What is the value of that device in
reducing the probability of breaking a sweat while
airborne? My second car was a '57 Chevy BelAir with
all the goodies on it. My third car was a '59 Chevy
six-cylinder, stick shift with no goodies. Subsequent
cars were similarly lacking in fluff. I learned very
quickly that my best return on investment was the simplest
combination of hardware that provided comfortable,
reliable, lowest cost, easiest to maintain transport
from point A to point B.
Our low-wing, TC aircraft have been fitted with
boost pumps since day one. 95+ percent of all engine
falters in flight have to do with fuel issues.
We were taught to hit the pump switch first. Perception,
interpretation and reaction took perhaps two seconds.
. . . and the pump is already ON for takeoff and
landing. So, what is the return on investment of
adding an automatic "failover" device in those aircraft?
Now, if these engines tend to quit and be hard to
get restarted then perhaps some form of automatic,
fast reaction system is called for. But if it just
falters, windmills and recovers immediately upon
return of fuel flow, then perhaps the automatic
system is less than elegant. THAT decision is
entirely yours . . . I'm only suggesting that
you ponder the decision with good data and a goal
of arriving at the elegant solution.
Bob . . .
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Allen Fulmer
Joined: 10 Jan 2006
Posts: 79
Location: Alexander City, AL
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| Posted: Fri Oct 12, 2007 7:20 am Post subject: Z-19 and considerations of SYSTEM reliability |
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Thanks Bob,
I can't seem to find the email but I think Eggenfellner has decided that a
simple switch on Pump#2 is fine. They were having too much trouble with the
pressure switch for automatic fail-over.
As a computer geek I find it so hard to resist "automatic techno gadgets"!
Thanks for the detailed exhortation.
Allen Fulmer
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_________________
RV7 QB Fuselage
Eggenfellner H6 on orde
N808AF reserved
Alexander City, AL |
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longg(at)pjm.com
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| Posted: Fri Oct 12, 2007 9:04 am Post subject: Z-19 and considerations of SYSTEM reliability |
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Et al,
Thanks for all the input. I will go the 2 switch route and decide
whether it is more elegant to hang them off each bus as shown by #2 in
the diag, or incorporate them behind the diodes for primary and
secondary power. Either option should provide the same level of
redundancy. I am very excited to lay this out for real.
--
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bakerocb
Joined: 15 Jan 2006
Posts: 727
Location: FAIRFAX VA
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| Posted: Sat Oct 13, 2007 4:49 am Post subject: Z-19 and considerations of SYSTEM reliability |
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10/13/2007
Hello Bob, You wrote: " . . . and the pump is already ON for takeoff and
landing."
Just a reminder that in some aircraft the electrically driven fuel boost
pump is not turned on for takeoff or landing.
The problem in those aircraft is that with the engine driven fuel pump and
the electrically driven fuel boost pump both operating the engine can be
drowned in fuel and not operate properly or at all.
The function of the electrically driven fuel boost pump in those aircraft is
to prime the engine prior to starting the engine and to be turned on in
flight in case of failure of the engine driven fuel pump.
'OC' Says: "The best investment we can make is the effort to gather and
understand knowledge."
--------------------------------------
Time: 06:54:48 AM PST US
From: "Robert L. Nuckolls, III" <nuckolls.bob(at)cox.net>
Subject: RE: Z-19 and considerations of SYSTEM
reliability
.... snip .............
Our low-wing, TC aircraft have been fitted with
boost pumps since day one. 95+ percent of all engine
falters in flight have to do with fuel issues.
We were taught to hit the pump switch first. Perception,
interpretation and reaction took perhaps two seconds.
. . . and the pump is already ON for takeoff and
landing. So, what is the return on investment of
adding an automatic "failover" device in those aircraft?
Now, if these engines tend to quit and be hard to
get restarted then perhaps some form of automatic,
fast reaction system is called for. But if it just
falters, windmills and recovers immediately upon
return of fuel flow, then perhaps the automatic
system is less than elegant. THAT decision is
entirely yours . . . I'm only suggesting that
you ponder the decision with good data and a goal
of arriving at the elegant solution.
Bob . . .
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glcasey
Joined: 01 Jul 2007
Posts: 23
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| Posted: Sun Oct 14, 2007 5:54 am Post subject: Z-19 and considerations of SYSTEM reliability |
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I missed the start of this thread so pardon me if my post misses the
mark. Regarding an automatic way to turn on the backup pump, as I
understand it either or both Columbia and Cirrus have this feature.
I put it in my Lancair, but it has a Lycoming engine in which fuel
pressure does not affect fuel flow - it doesn't matter whether it has
one or both pumps working. Some engines (Continental fuel injection
systems) are affected by inlet fuel pressure and there have been
problems with over-fueling these engines, according to what I have
read. Both Cirrus and Columbia use Continental engines, so that is
contradictory. My system consists of a pressure switch (normally
on), a latching relay and a warning light. The manual switch has
Off, Auto and On positions. I turn it to Auto to prime the engine,
after which I turn it Off and then back to Auto to verify there are
no fuel leaks (the pump should stay off with the mixture pulled and
the engine not running). Before takeoff I verify the pump is in the
Auto position. The problem is that I have the pressure switch set at
the minimum operating pressure of about 11 psi the pump will
occasionally turn on. Maybe I should set the switch at a pressure
significantly below operating pressure. If I run a tank dry the pump
will come on BEFORE the engine quits, although it doesn't prevent it
from surging for a few seconds until all the air is purged.
Botttom line - I think this feature is a good thing for engines that
aren't affected by having the backup pump come on even though the
primary pump is working well. In my mind, an "active" safety system
in an airplane that has the pilot in the loop is not a good thing.
Sometime unavoidable, but still not a good thing. I like the idea
that the backup pump will turn on instantly when needed without me
having to do it.
Gary Casey
| Quote: |
Time: 05:49:23 AM PST US
From: <bakerocb(at)cox.net>
Subject: RE: Z-19 and considerations of SYSTEM
reliability
10/13/2007
Hello Bob, You wrote: " . . . and the pump is already ON for
takeoff and
landing."
Just a reminder that in some aircraft the electrically driven fuel
boost
pump is not turned on for takeoff or landing.
The problem in those aircraft is that with the engine driven fuel
pump and
the electrically driven fuel boost pump both operating the engine
can be
drowned in fuel and not operate properly or at all.
|
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_________________
Gary Casey |
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