Both VFR and IFR pilots must be familiar with these instruments, know how to correct the errors associated with each, and understand how they practically relate to each other.

The magnetic compass

The compass points to the magnetic north pole, but that is not the same as the geographic north pole. So, you will need to correct for the variation this difference creates between your true and magnetic course headings. Review the sectional chart to find the number of degrees to subtract for easterly variation or add for westerly variation.

Another magnetic compass error is caused by aircraft magnetic fields, creating the need to fly specific headings a few degrees off the direction indicated. A compass correction card located next to the compass indicates which heading to fly to adjust for that deviation. For example, navigate using a heading of 176 degrees instead of 180 degrees. To determine magnetic heading from true heading, adjust for variation and then add or subtract heading deviation as indicated on the compass card.

The heading indicator

The heading indicator is not affected by the factors that cause errors in the magnetic compass. It operates with an internal gyro that spins horizontally, and the aircraft rotates around it. A heading card indicates the aircraft's direction of travel. While it is not affected by magnetic variation or deviation, there are errors associated with the heading indicator. First, friction causes the heading to drift from where you set it. This is called “precession.” Second, the rotation of the Earth at a rate of 15 degrees per hour affects the gyro and causes the heading indicator to stray from its accurate heading. Adjusting the heading indicator to match the compass is part of your pre-takeoff checklist, but you also must manually correct the instrument regularly in flight (approximately every 15 minutes). Otherwise, the heading indication will stray from the actual heading at a rate of 15 degrees per hour.

Depending on the length of your flight, these errors—if left unadjusted—could cause you to stray far enough off heading to become disoriented when you try to visually identify landmark references. Many GA pilots have the advantage of additional heading references on electronic instrumentation, but that doesn’t take away the need to understand the basics as well.