Tachometer Maintenance Magnetic Compass Maintenance and  Compensation 1.  The compass indicator should be checked for correct  readings on various cardinal headings and re- compensated if necessary. 2.  Moving parts of the compass should work easily. 3.  The compass bowl should be correctly suspended on  an antivibration device and should not touch any part  of the metal container. 4.  The compass bowl should be filled with liquid. The  liquid should not contain any bubbles or have any  discoloration. 5.  The scale should be readable and be well lit. should be handled carefully during inspection. The following  items are usually included in an inspection: the aircraft’s altimeter with the calibrated test unit’s altimeter  while performing the static system check. However, this  does not meet the requirements of 14 CFR section 91.411  for altimeter tests.  Upon completion of the leak test, be sure that the system is  returned to the normal flight configuration. If it is necessary  to block off various portions of a system, check to be sure  that all blanking plugs, adaptors, or pieces of adhesive tape  have been removed. Tachometer indicators should be checked for loose glass,  chipped scale markings, or loose pointers. The difference in  indications between readings taken before and after lightly  tapping the instrument should not exceed approximately  15 rpm. This value may vary, depending on the tolerance  established by the indicator manufacturer. Both tachometer  generator and indicator should be inspected for tightness of  mechanical and electrical connections, security of mounting,  and general condition. For detailed maintenance procedures,  the manufacturer’s instructions should always be consulted. Compass magnetic deviation is caused by electromagnetic  interference from ferrous materials and operating electrical  components in the cockpit. Deviation can be reduced by  swinging the compass and adjusting its compensating  magnets. An example of how to perform this calibration  process is given below. The results are recorded on a compass  correction card which is placed near the compass in the  cockpit.  [Figure 10-139] There are various ways to swing a compass. The following  is meant as a representative method. Follow the aircraft  manufacturer’s instructions for method and frequency of  When an engine equipped with an electrical tachometer  is running at idle rpm, the tachometer indicator pointers  may fluctuate and read low. This is an indication that the  synchronous motor is not synchronized with the generator  output. As the engine speed is increased, the motor should  synchronize and register the rpm correctly. The rpm at  which synchronization occurs varies with the design of the  tachometer system. If the instrument pointer(s) oscillate(s)  at speeds above the synchronizing value, determine that the  total oscillation does not exceed the allowable tolerance. Pointer oscillation can also occur with a mechanical  indication system if the flexible drive is permitted to whip.  The drive shaft should be secured at frequent intervals  to prevent it from whipping. When installing mechanical  type indicators, be sure that the flexible drive has adequate  clearance behind the panel. Any bends necessary to route  the drive should not cause strain on the instrument when it  is secured to the panel. Avoid sharp bends in the drive. An  improperly installed drive can cause the indicator to fail to  read or to read incorrectly. The magnetic compass is a simple instrument that does  not require setting or a source of power. A minimum of  maintenance is necessary, but the instrument is delicate and  10-85 Figure 10-139.  A magnetic compass with a deviation correction  card attached, on which the results of swinging the compass should  be recorded. 