14 C.F.R. § 29.1327 Magnetic direction indicator.
Title 14 - Aeronautics and Space
(a) Each magnetic direction indicator must be installed so that its accuracy is not excessively affected by the rotorcraft's vibration or magnetic fields. (b) The compensated installation may not have a deviation, in level flight, greater than 10 degrees on any heading.
Title 14: Aeronautics and Space
PART 29—AIRWORTHINESS STANDARDS: TRANSPORT CATEGORY ROTORCRAFT
Subpart F—Equipment
Instruments: Installation
§ 29.1327 Magnetic direction indicator.