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.
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.