Before the days of GPS and MFD moving map displays that can pinpoint our exact location to within a meter, the instrument pilot had to mentally visualize his/her position with reference to ground based navigation systems. For most instrument students, one of the most confusing aspects of learning to fly SPIFR (single pilot IFR) is interpretation of these instruments while in the clouds. In the “soup”, and under high workloads, it was not uncommon for some pilots to lose positional awareness, at least momentarily. (Though this has personally never happened to me :-)).
One valuable (and expensive) flight instrument usually reserved for higher end general aviation and commercial aircraft is the HSI, or Horizontal Situation Indicator. Most small aircraft have only a Directional Gyro (DG) and VOR/ILS indicator. The HSI combines both of these separate indicators into one single flight instrument, greatly simplifying pilot workload and positional awareness. Essentially, the HSI overlays the VOR/ILS Course Deviation Indicator (CDI) on top of the directional gyro.
Most common are mechanical HSI units, like the King KCS-55A in my Bonanza. One feature of most upper-end mechanical HSI units is that they are slaved to a remote gyro, eliminating instrument precession that usually accompanies vacuum or pressure gyros.
There are also electronic units like the Sandel, which use powerful LEDs for their display. Avionics manufacturers nowadays integrate fully electronic HSIs directly into the new glass cockpit navigation units like the Garmin G1000. Some EHSIs allow you to overlay weather and traffic onto the display itself.
So how does an HSI work? Like a wet compass on a ship, the white, or sometimes orange, hash mark at the top of the HSI is known as the “lubber line”. This indicates your current heading and is fixed. The compass card, or compass rose, is the 360 degree circle around the HSI, and each tick mark represents 5 degrees. When the pilot turns the aircraft, the compass card rotates to indicate the current heading as it aligns with the lubber line.
The Course Select Knob is the yellow arrow on the lower left side of the unit. The pilot turns to select the desired course line, which could be either to the left or right of your current position. The Course Deviation Indicator Bar, or CDI, deflection represents the direction you need to fly in order to intercept that desired course line. If the bar is to the right, the pilot flys right to intercept it. The CDI will move in towards the center of the course selector as you get closer to your desired course. Once there is no deflection on the CDI it means that you now are on course.
So now the questions is, how far does the pilot turn towards the CDI for the best angle of intercept? Here is a trick I learned to simplify this process from one of my instrument instructors.
In this example, the pilot in the illustration below is NW of a VOR and is told by ATC to intercept the 360 degree radial (North) of the VOR and track it outbound. In the illustration, the pilot has turned right to about 045 degrees until the bottom of the lubber line touches the top of the CDI. Doing so gives the pilot the perfect intercept angle for the desired course of 360. On the left side of the illustration, you can visually see that the aircraft is pointed in the correct heading to make the intercept.
The great thing about using this technique of turning the aircraft until the bottom of the lubber line touches the top of the CDI deviation bar is simplicity and consistency. It works whether you are just a degree or two off the desired course while inbound on an ILS, or 30 miles away from a VOR intercepting a Victor airway, or tracking a course you set up in your GPS navigator using the OBS function. The optimal angle of intercept will vary according to how far off course you might be.
Even though I have been flying for over 15 years, I still love learning new stuff about being a better, more professional pilot.