Directional Gyro interior
U.S. Dept. of Transportation, FAA
2016-01-01
Pilot's Handbook of Aeronautical Knowledge, FAA-H-8083-25B, <a rel="nofollow" class="external free" href="https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/phak/media/10_phak_ch8.pdf">https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/phak/media/10_phak_ch8.pdf</a>