Wouldnt it be great to have a simple, yet comprehensive visual display of your aircraft's heading and position in relation to a desired course? Well, you can with a compass system. Or should I say a slaved compass system; or, more often called a heading system; or, better yet, a slaved heading system. Whatever you call it, it makes instrument flight much easier.
There are several manufacturers of slaved compass systems. Allied Signal make the KCS 55A System, S-Tec makes the ST-180, and Century makes the NSD360A and the NSD1000. Some of you may even have a Collins PN101. There are also several variations of what could be considered a heading system. The characteristics of all of these units are essentially the same, the significant difference being some have a remote mounted gyro while others have the gyro built into the indicator. All slaved heading systems contain four main system components.
The Indicator, or HSI (Horizontal Situation Indicator), is the component the pilot will be most familiar with in this system. This unit is the heading indicator and the CDI with a glideslope needle. It is mounted in the pilot instrument panel in clear view. Not only will this indicator display the horizontal navigation situation; it will also provide manual controls for course and heading selections. A typical HSI will include a compass card, heading flag, autopilot heading bug, slaving meter, lubber line, course arrow, nav flag, miniature airplane, course reciprocal pointer, and left-right deviation bar. All of this will help the pilot simplify navigation.
A slaved heading system will also have a magnetic azimuth transmitter; better known as a magnetic slaving transmitter; better know as a flux gate; better known as a flux detector; better known as a flux value; or, better known as a magnetic flux sensor. Whatever you choose to call it, it senses the direction of the magnetic field of the earth and continuously gives that information to the gyro. The gyro will take that information and correct itself when it tries to precess or drift.
The next component of the heading system is the slaving control unit. This could also be called a slaving panel, and probably half a dozen other names. This unit is usually mounted in the instrument panel, and will let the pilot select either a slaved gyro or a free gyro. When the gyro is slaved the system is working together to give the pilot an enormous amount of navigational information. When everything is not working properly, the pilot can choose to have a free gyro. Though the navigational information is not as grand in this mode, at least the pilot still has a directional gyro.
Last, but definitely not least, is the heart of the system: the gyro. Most manufacturers have a remote mounted gyro, though some mount a gyro inside the HSI. You can think of this as a remote mounted directional gyro giving the basic heading reference. This provides gyro-stabilization for the system. Just remember part of Newton's First Law, "A body in motion tends to move in a constant speed and direction unless disturbed by some external force." This is the basic theory behind the gyro.
It may begin to seem that nothing is standard in the world of avionics. Though the components do the same thing, each manufacturer titles their components a little differently. Whether this is a marketing ploy, or simply that the engineers working for these manufacturers truly disagree on what to call them, is beyond me. However, if you have a heading system or are thinking about getting one, most avionics shops should know enough of the lingo to understand what it is that you want.