Thank you for answering, Philip! Although, you seem to have misunderstood my problem, or I'm bad in explaining my problem The HSI CDI is fine. I tune the CDI to the navaid and radial I want to follow and I'm aware of how to set the CDI correctly. However, the Heading of the HSI does not match the GPS. This is made in zero-wind conditions where the track and heading are supposed to be identical. At the beginning of the flight, the heading and track match (zero-wind) correctly. After I have been flying for some time, the HSI seems to wander, i.e. the heading does not match the track of the GPS (still zero-wind). The difference can be quite large, which makes instrument flying difficult. Let's say I want to follow a track of 090° in zero-wind conditions - I need to use heading of 080° or maybe even 070° in order to maintain the track - the difference is 10-20° off, which is not supposed to be. Best regards, David