While not specifically "rotary" in nature, I thought this would be of
interest to at least few on the list.  I suspect it was likely written
either by a technical writer or an engineer.  Enjoy...

-----Original Message-----


====================================

A pilot gave the following explanation when asked, why in this age of
FMS, GPS, RNAV, VNAV, ATC, and Inertial Nav, he got lost:

"The  aircraft knows where it is at all times. It knows this because it
knows  where it isn't. By subtracting where it is from where it isn't,
or
where it  isn't from where it is (whichever is the greater), it obtains
a
difference,  or deviation. The Inertial Guidance System uses deviations
to generate error  signal commands, which instruct the aircraft to move
 from a position where  it is to a position where it isn't, arriving at
a
position where it wasn't,  or now is. Consequently, the position where
it
is, is now the position where  it wasn't, the Inertial Guidance System
has acquired a variation. Variations  are caused by external factors,
the
discussions of which are beyond the  scope of this report. A variation
is
the difference between where the  aircraft is and where the aircraft
wasn't. If the variation is considered to  be a factor of significant
magnitude, a correction may be applied by the use  of the autopilot
system. However, use of this correction requires that the  aircraft now
knows where it was because the variation has modified some of  the
information, which the aircraft has, so it is sure where it isn't.
Nevertheless, the aircraft is sure where it isn't (within reason) and it
knows where it was. It now subtracts where it should be from where it
isn't, where it ought to be from where it wasn't (or vice versa) and
integrates the difference with the product of where it shouldn't be and
where it was; thus obtaining the difference between its deviation and
its variation , which is variable constant called "error"."


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