What should you do if the CDI needle drifts when trying to maintain the radial?

When trying to maintain the radial and the Course Deviation Indicator (CDI) needle drifts, the best practice is to return to the last centered heading and then make adjustments accordingly based on any changes in the needle position. This method ensures that you are referencing the most accurate direction that was indicated prior to the drift. By flying back to the last known centered heading, you regain control of your course over the radial.

Using this approach allows you to establish a baseline from which to work. Once you are back on that centered heading, you can assess how much and in what direction you need to adjust your heading to maintain an accurate course. This method also reduces the risk of overcorrecting or becoming disoriented.

Maintaining the same heading for an extended period, constantly changing your heading, or solely relying on communication with air traffic control do not effectively resolve the issue of needle drift and may lead to difficulties in navigation. They lack the precision necessary for fine-tuning your approach to stay on the desired radial, which is critical in instrument flying.