The thorny issues that come up in this issue of aircraft
reliability are amply examined in "the Pilot's Burden".

Basicly, what it's author (an airline pilot) says, is:

You have a part that has to operate or the plane crashes,
say the frobulator.

So you put a sensor on to make sure the frobulator is
working, and ring an alarm if it isn't.

But the sensor might fail, so you make the pilot
test the frobulator sensor before takeoff.

Carry this mentality too far and pretty soon
you've got the pilot so busy testing frobulator testers
he's too busy to put the gear down.

He analyses in detail the history of dealing with a common
pilot decision -

At takeoff, the pilot goes to max power and rolls down the
runway. At some point the gummifrunchy overheat lamp goes on.
Should he:

a) take off anyway?

b) Try to stop

This is a hellishly complicated problem. It depends on an appreciation
of the probability that the plane will actually be able to fly around and
land, and an appreciation of whether the plane can stop before the end of
the runway, and what's beyond the end of the runway.

But the pilot doesn't get realistic training in making this critical decision
in the fraction of a second s/he has to make it. Instead, flight training for
this consists of memorizing complicated theoretical formulas about the situation
.
He gives all the formulas and factors, in thirty mind numbing pages, and at
the end I had less idea how to make such a decision than when I started.

Anyway, for anybody interested in design of life-dependent systems with man in
the loop, I strongly suggest this book.

--
Anniepoo
Need loco motors?
http://www.idiom.com/~anniepoo/depot/motors.html