QUESTION:
What instrument do you use to keep on course during the flight?

      
ANSWER fromTerry Rager on November 6, 1995:
The actual instrument the pilot looks at to tell where the airplane is in
relation to the course, is called a Course Deviation Indicator, or a CDI. It
has a compass card and in the middle of the compass is a "road that can slide
to the right or to the left. The pilot chooses the desired course which lines
the road up on the compass. Then the  pilot flys toward the road and when the
CDI is in the center of the instrument, the airplane is on course.
 
 There are two main ways the CDI  knows where the center of the road is when
we are flying: 
1. over the ground we have navigational radios, and 
2. over water we have navigational computers.
          
The radios on land send a signal up to a radio on the airplane and
one of our instruments will point toward the radio and tell us how far 
away it is.
          
The navigational computers are a bit more complicated. There are two
different types: INS and GPS. INS stands for Inertial Navigational
System and once it is running does not need any outside inputs. On
the ground we tell it where it is and through gyros and accelerometers
it can take us any where in the world. However, after a couple hours these 
computers "drift" and the accuracy can be off as much as five miles.
          
GPS is the new Global Positioning Satellites. These are satellites which 
actually tell us where we are by sending a signal to the airplane.
          
The best of both worlds is a Flight Management System which allows
all these different navigational systems to talk to each other by
computer and give the best positional information (as accurate as 3 ft.).
          
Thanks for the question!!