Since the attitude indicator is capable of showing precise  pitch and bank angles, the only time that the attitude indicator  is a primary instrument is when attempting to fly at a specific  bank angle or pitch angle. Other times, the attitude instrument  can be thought of as a control instrument.  The horizontal situation indicator (HSI) is a rotating 360°  compass card that indicates magnetic heading. The HSI is the  only instrument that is capable of showing exact headings. The  magnetic compass can be used as a backup instrument in case  of an HSI failure; however, due to erratic, unstable movements,  it is more likely to be used a supporting instrument.  In order for the pilot to achieve the desired rate of change,  it is important for him or her to understand the relationship  between the rate at which the HSI changes heading displays  and the amount of bank angle required to meet that rate of  change. A very small rate of heading change means the bank  angle is small, and it takes more time to deviate from the  desired straight flightpath. A larger rate of heading change  means a greater bank angle happens at a faster rate. The heading indicator is the large black box with a white  number that indicates the magnetic heading of the aircraft.  [Figure 7-54]  The aircraft heading is displayed to the nearest  degree. When this number begins to change, the pilot should  be aware that straight flight is no longer being achieved. The roll pointer indicates the direction and degree of bank.  [Figure 7-53]  The roll pointer is aligned with the aircraft  symbol. The roll pointer indicates the angle of the lateral axis  of the aircraft compared to the natural horizon. The slip/skid  indicator will show if the longitudinal axis of the aircraft is  aligned with the relative wind, which is coordinated flight.  With the roll index and the slip/skid indicator aligned, any  deflection, either right or left of the roll index causes the  aircraft to turn in that direction. With the small graduations  on the roll scale, it is easy to determine the bank angle within  approximately 1 degree. In coordinated flight, if the roll  index is aligned with the roll pointer, the aircraft is achieving  straight flight.  An advantage of EFDs is the elimination of the precession  error. Precession error in analog gauges is caused by forces  being applied to a spinning gyro. With the new solid state  instruments, precession error has been eliminated. The turn rate indicator gives an indirect indication of bank.  It is a magenta trend indicator capable of displaying half- standard as well as standard rate turns to both the left and  Turn Rate Indicator Horizontal Situation Indicator (HSI) Heading Indicator Roll pointer Roll scale Slip/Skid indicator Roll scale zero Pitch scale Aircraft symbol Horizon line VOR 1 270 °  270 °  Slip/Skid indicator Turn rate indicator T urn rate trend vector Figure 7-54.  Slip/skid and turn rate indicator. Figure 7-53.  Attitude indicator showing a 15° left bank. Figure 5-53. Figure 7-52.  Attitude indicator. 7-38