nuckolls.bob(at)aeroelect
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 Posted: Mon Oct 04, 2021 8:17 am Post subject: Estimates of Component Failure Probabilities |
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| Quote: | | Please help by giving your estimates of in-flight failure probabilities and any other thoughts you might have on detection and failure modes on the following components: |
Suppose I gave you a library of numbers on each of these
components/systems . . . now what?
If I were handed such a list along with their predicted
failure rates and tasked with crafting a failure tolerant
system . . . my first act would be to pitch the data
into a 'round file'.
MTBF, MTBO, service difficulty reports, dark-n-stormy
night stories ALL go to make us afraid of the unknown/
unexpected.
Know this about 'failure rates': They go much more to
cost of ownership than to system reliability. The
gold standard for rate studies tests a LARGE constellation
of test articles to failure and then plots numbers
of failures against time.
Time to failure in simple systems will often plot
in a nice bell-shaped curve that will tell you
where the majority of parts will fail. But wait!
A few of the parts can fail right out of the box.
A few will last a very long time. YOUR part will
fall somewhere on that curve.
So you got some numbers . . . now what? Of what
value is it to pick a part with more 'comfortable'
numbers when in fact, the thing can still crap
out about any time.
Those MTBF numbers can be quite useful to the
maintenance logistics manager of a large fleet . . .
but for the guy sitting in the left seat . . .
not so much.
I could wax long and sorrowfully on the time
I think I wasted doing MTBF studies for
the boss . . . but the customer wanted them
and paid for them.
But now YOU are the customer . . . and your
constellation of reasons for going OBAM
aircraft are your own. No doubt SYSTEM
reliability is high on the list along with
cost of ownership, time to first flight,
etc . . . where reliability speaks to
probability of any single failure getting
you or your airplane bent . . . or worse.
This is really a rather simple task. What
items in your ship's construct are things
you cannot LIVE without? Obviously, wings
need to stay on, propellers need to stay
attached and it's nice if the engine runs.
But as you run down the criticality list
from greatest to least, you'll find that
many things on the list do not create
hazards to flight where failures are simply
maintenance items. If things didn't break
on airplanes, FBOs would be out of business.
What you need to focus on are those items
that add value to your system reliability
such that you can get back to the FBO and
haggle with him over the warranty.
So once you've identified those can't-live-
without systems then consider what features
of those systems are under YOUR control.
All you can do personally is install, operate
and maintain. If something breaks inside,
you're SOL. Solution? Plan-B. Have a back-up
for every such appliance.
But when it comes to owner/operator prophylactics
against failure, careful attention to INSTALLATION
(safety wire, bolt torque, lock nut, chaffing
protection, gas tight joints etc), OPERATION (is
that fuel selector valve getting stiff, new noise
during starter engagement, new 'stumble' in one
cylinder after startup, intermittent battery
contactor, etc), and MAINTENANCE (oil changes, battery
cap checks, propeller nicks, slop in belcrank
hinges, etc).
Note that not one of these potential failures
is related to any MTBF study.
Most engine stoppages are 'cause it ran out of
gas. Most unplanned contacts with the earth
terminate after a chain of poor decisions often
combined with a lack of understanding as to how
the ship's systems work. Consider the studies
we've done here on the List for the Dark-n-Stormy
Night stories so beloved by the journals.
Few if any of those studies went to designing
the probability of repeating the failure OUT
of the airplane. None of the went offered any
discovery follow-up by the mechanic that fixed
anything. Virtually all went to crafting
a mind set in the pilot (YOU) for dealing with
a similar failure in the future. Virtually every
electrical issue I've worked over the years
had a foundation in human frailties . . . NOT
in the failure of a components to function within
its design goals and limits.
Hogwash. We either design the failure OUT -or-
make the failure INSIGNIFICANT with respect
to comfortable termination of flight. Trust
me. MTBF studies may be intellectually satisfying
but they have nothing to do with your risks
for a bad day in the cockpit.
Bob . . .
Un impeachable logic: George Carlin asked, "If black boxes
survive crashes, why don't they make the whole airplane
out of that stuff?"
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