On Sat, 23 Sep 2006 17:07:17 +0200, Mxsmanic
wrote:

Peter Duniho writes:

What you SEE on the instruments is just as subjective as what you SEE out
the window. Furthermore, the instruments are mechanical devices subject to
failure. They are not infalliable, and they are significantly less
infalliable than the view out the window.

They are far less prone to failure than human perception. That's why
they are there.

Instruments are 100% affected by human perception, unless you're
talking about coupled autopilot operations.

Instruments are as much a part of the perceptual environment as the
whoosh of the wind, the sound of the engine, the smells, the view out
the windows, the pressure of the seat against ones butt, the pull of
the seatbelt, the force of the hand against the controls, the
vibrations, the loose objects flying around the cabin. The pilot
integrates all of these indications to get a more accurate picture of
the state of the airplane. In instrument training, a common admonition
from the instructor is, "Look at everything, stare at nothing." The
advice doesn't apply only to the instrument panel.

They are an indirect and highly suspect
means of determining reality when compared to using one's own eyes to
directly observe reality.

Highly suspect? How frequently do instruments fail in flight?

Far too often, unfortunately. I've had vacuum pumps fail, all in VFR
conditions fortunately. I've had an electric turn coordinator fail.
I've had an altimeter fail. In that case I ended up rejecting the
altimeter reading and relying on my eyes or I'd have made a mark on
the landscape. Everything on the airplane is subject to Murphy's law,
it can fail, and that includes instruments.

There's a good reason why every time you fly with a CFII he/she will
bring along some suction cups or Post-it notes to stick over
instruments.

And of course when a gyro fails, you may need to disable the autopilot
so it won't be misled by erroneous indications.

RK Henry