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Exactly! OR - the pressure gage gets "stuck" - or - pressure is ok,
but gyro bearing is going bad (causing sufficient friction for it
not to spin true) - or - or - or...
Do your best to know everything in your airplane; know how to detect
failures; and be prepared for the time you get surprised by
something you never expected...
Safest cross-checks I know of to detect "everything else is broken"
are my FAR required TSO'd altimeter vented to cockpit
(non-pressurized airplanes only) or some other
internal-but-protected-from-the-elements-location which gives you a
quick confirmation that altitude is at least holding steady; plus
the turn needle centered and the Whiskey compass holding steady
heading.
For the alternate static cross-check, I added a simple twist valve
to switch from external static to alternate static without having to
break any glass. My "turn needle" is on my EFIS, but I always
cross-check the Whiskey compass in case it has developed a "creative
failure mode" I didn't expect.
Fly safe!
Bill
On 01/-10/-28163 02:59 PM, Sky2high@aol.com wrote:
Bill,
Choose wisely indeed!
We are all a product of our experiences
regardless of any scientific bias. Those of us that are much
older seemed to use much of that experience to attain elder
status, albeit a bit wrinkly and gnarly.
Vacuum systems fail slowly? Well the FAA
approved Skymaster I flew had a "pressure differential" AI (If
I remember correctly, the pump was only on one engine) but,
more to the point, the pressure differential gauge was down by
my left knee - not exactly in the normal instrument scan.
Thus, in my wee Lancair, the vacuum gauge was located
immediately above the spinning mass AI so they both are seen
as one instrument. So there......... If the needle ain't in
the green, it's gonna be a mean lean.
Now, about those wonderfully designed and
throughly tested electrical devices - Take the STEC 50 with
the electric turn coordinator, please. If the pitch control
board senses sufficient rpm of the spinning tilted gyro, it
puts out +10 VDC on a pin so that both the Oops flag on the TC
is retracted AND the AP system can become ready. The board
was messed with by that tingly electric cloud I mentioned in
an earlier email in such a way that caused the 10 VDC to
always be present at the pin. "How did I find that out?" A
curious mind might enquire. Wellll, some on-ground panel
testing had the TC breaker pulled and it not reset (Brain MK I
goof) for a subsequent test flight. Without the TC
gyro spinning the AP was available in the "ready" state and,
when engaged, resulted in a gentle declining left turn from
bad gyro data. Hmmmmmmm. Resetting the breaker soon brought
stabilization as the gyro actually spun up..
I can hear it now, "So what?" Well, in spite of
the sophisticated testing, be prepared for odd failures not
supported by all the data gathered from instruments relying on
different input and power sources. Do your own integration.
Use your own creative imaging. Don't completely rely on
nutt'n.
Grayhawk
PS Oh, it is really good to look out the window
every now and then. Traffic alerters notwithstanding.
A
point that has not been brought up by the proponents of
spinning mass is the fact that their degradation modes are
often far more insidious / undetectable than power failure
or EFIS failure.
Whether powered by vacuum or by electrons, spinning gyros
can become "insidiously inaccurate" for a number of
reasons. Off flags meant to warn of such problems fail.
Gyros may retain some spin, but gently wobble (or
be simply wrong). Stopped gyros still display some
attitude, even if wrong.
Each of these modes of failure are common, and have
resulted in deaths.
I will not even attempt to argue which is "more likely" -
EFIS or gyro failure - simply pointing out that both have
failure modes which render them inaccurate (or worse, appear
to be accurate).
As someone else pointed out, these days often the most
likely piece of equipment to become unreliable in flight
is the Mk I brain.
Thanks to your tax contributions, I received more
comprehensive flight training than most of you on this
forum - and it instilled in me a strong sense of
responsibility to know everything about every system on my
plane, including possible degraded modes and insidious
failure modes. Along with other situations I've
mentioned, I've also had the displeasure of flying a
Phantom home in the soup over the North Atlantic with only
my turn and bank and a ground controller calling my
turns. Of all the arguments for a spinning gyro, I think
I'd buy one for an electric T&B over an electric AI,
because it is only attempting to tell you relative
rate of rotation around your vertical axis and will
continue to work in degraded modes. As for spinning gyro
AIs, I've simply seen too many of them telling people to
"go the wrong way" to consider them "better." Not
"worse," just not "better."
If you want to be prepared for any possible situation in
life, there is nothing wrong with having one of every
piece of equipment ever dreamed up on your airplane. Odds
will definitely be better that way, with the dual costs
imposed both to your wallet and to your obligation as a
responsible pilot to completely know, and to be able to
recognize and respond appropriately to, all of the failure
modes of every piece of equipment. If, however, you are
willing to accept the 1:1,000,000 risk associated with
"only" having 3 or 4 working pieces of equipment in the
aircraft, then my recommendation is simply: Choose wisely.
Fly Safe!
Bill
On 01/-10/-28163 02:59 PM, Gary Casey wrote:
I think there might be a difference
of understanding regarding failure modes. I don’t
think anyone is proposing that we abandon all
electronic devices and go back to vacuum-powered
gyros. What is being proposed by Brent (and me, I
suppose) is that a vacuum-power spinning gyro is
perhaps the most reliable backup system available.
How can the “least reliable” system be the “most
reliable” backup? Two reasons: First, the spinning
gyro is not susceptible to catastrophic electrical
failures (like a lightning strike) and will keep
running as long as the engine is turning. Second,
it is logical to limit one’s thinking to “single
failure modes”. In other words the ONLY
requirement of the gyro system is that it keep
working from the time of the electrical failure to
the earliest possible landing. What is the
likelihood that the gyro will fail in those 15
minutes? What is the likelihood that the
electrical system would quit and then the engine
stop turning? All this is dependent on the vacuum
system being “required equipment” for IMC
operation (if the vacuum system fails on the
ground, it is a no-go. If it fails in the air
under IMC, it is a “land immediately” condition).
The poorest choice for a primary
system then becomes the best choice as a backup.
Logical, I think.
Gary Casey
Sent:
Tuesday, August 09, 2011 5:55 AM
Subject:
Re: Re: Re-doing my panel - carefully
thinking through failures
Brent,
I suppose I should have been more specific - I
assume that pilots of experimental aircraft will
exercise sufficient judgment not be flying into
thunderstorms, and thus the likelihood of
in-flight lightning strike is nearly nil. Not
absolutely nil, naturally, but approaching or
below the likelihood of vacuum failure, which is
fairly common. Of course, perhaps I should not
make such an assumption given that a very famous
pilot died just last year flying his Bonanza into
thunderstorms.
Absent thunderstorms, we will simply have to agree
to disagree. Vacuum pumps and vacuum-operated
artificial horizons are notoriously fallible, and
a poor vacuum can give insidious symptoms causing
catastrophic results. While there have been some
experimental EFIS units (notably one you mention,
also the original pioneer Blue Mountain) which had
early individual failure rates much higher than
vacuum equipment, still the likelihood of two or
more going down simultaneously is rather rare. In
my personal experience my TSO Garmins failed just
as often as my experimental equipment - anecdotal
evidence, to be sure, but 3 TSO failures in 600
hrs not counting vacuum pump failure and attitude
indicator partial failure ("lazy" attitude, "sort
of" working).
None of this absolves individuals contemplating
use of experimental equipment from the burden of
research to draw their own conclusions about
reliability.
Your statement that TSO is required for legal
flight is simply untrue. If you wish to dispute
this, please feel free to point out the section of
the FARs that you believe says otherwise (it does
not exist, but knock yourself out). I don't expect
to convince you of that; it seems that there are
some folks who have made up their minds and aren't
interested in anyone else's opinion. That's fine,
you are entitled to yours. I, like many, have
reviewed the pertinent FARs along with (among
others) my mechanic who was a chief avionics
safety inspector for a major airline. For the type
of flying for which Experimental aircraft are
authorized there is no such rule stating that each
piece of equipment must be certified to pass TSO.
The altimeter must, or pass the test for
equivalent accuracy (performed during the annual
pitot-static check). Doesn't mean it's a bad
thing, only unnecessary for legal flight. Just as
you admonish people who are not engineers (I too
was a software engineer by trade) to form
unwarranted opinions about avionics, you too
should not consider yourself an expert on FARs
simply because you build avionics. Glass houses,
etc.
Blocked pitot or static tubes are no longer a
killer for correct attitude indication on any of
the three leading experimental EFIS units (GRT,
Dynon, MGL). May also not be on others, haven't
kept up. Of course, you will not get accurate
airspeed with either blocked and will not get
accurate barometric altitude with static blocked,
but that would happen irrespective of the type of
avionics you use. However, you will still have
accurate horizon, and GPS can provide altitude and
ground speed which, combined with a pilot's
knowledge of their aircraft power settings, etc.
should enable you to fly safely to landing.
At the end of the day, you are putting your own
life on the line. If you feel more comfortable
with spinning gyros, by all means load up.
However, if you feel you have done your research
and would rather replace that vacuum pump for a
second alternator to prevent power-out and ditch
the gyro for a small self-contained backup EFIS,
then your odds of total failure will ultimately be
about the same - just different causes.
Fly safe!
Bill
On 01/-10/-28163 02:59 PM, Brent Regan wrote:
Bill speculates:
<<Given that two EFIS units with battery
backup are more reliable than a single vacuum
pump, your argument that people must
have "TSO'd" equipment is logically ridiculous -
especially if they also have as part of
their panel an independent 2-axis
autopilot.>>
The primary assumption here is false. It is not
"given" that "two EFIS units with battery backup
are more reliable than a single vacuum pump".
Analysis and data show the opposite is true.
--
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