Text in force at CS25-28: from CS25-0, effective 17 Oct 2003.
(See AMC 25.1327)
(a) Each magnetic direction indicator must be installed so that its accuracy is not excessively affected by the aeroplane’s vibration or magnetic fields.
(b) The magnetic direction indicator required by CS 25.1303(a)(3) may not have a deviation, after compensation, in normal level flight, greater than 10 degrees on any heading.
(c) Direction indicators required by CS 25.1303(b)(6) must have an accuracy adequate for the safe operation of the aeroplane.
ED Decision 2003/002/RM · Source