Airworthiness Advisory Circulars - Part 6-64 - General advice
Part 6 - General advice
Aircraft Alternator Maintenance
A series of Major Defect Reports have been raised on aircraft alternators. These reports detail such problems as bearing failures, faulty wiring to the alternator and stators rubbing on rotors.
In the past, AAC articles have been issued to advise Certificate of Approval holders, Certificate of Registration holders, and maintenance organisations/personnel of these problems. As the lessons learnt from history appear to have been forgotten, the information contained in these articles is being, in part, reproduced.
Power failure can occur when the battery positive terminal or field terminal wire breaks. The break usually occurs at the tail of the terminal lug, with the larger gauge battery cable breaking more often than the field wire. Inspection of recent problems involving cable fractures indicate that failure was probably due to metal fatigue caused by vibration of the alternator and cable loom. Incorrect cable stripping during manufacture may also contribute to defects caused by the cable insulation not being crimped and resulting in flexing of the wire strands. Terminals should be inspected at regular intervals and reterminated if any doubt exists as to their condition.
Maintenance manuals state torque values for the drive retaining nuts to prevent them coming loose during operation. If these torque values are overlooked or ignored it could result in the vent fan loosening. Not only could this cause alternator failures but could also cause damage to fittings and equipment.
Whenever the body screws become loose, the end housing moves thus resulting in bearing failure or the rotor rubbing on the stator winding. In either case, the end result is an alternator failure. In an endeavour to overcome this problem, it is recommended that a check of the alternator body screws be performed during every 100 hourly or annual inspection. It should be noted that alternators received from overhaul facilities carry a warning to tension the through bolts every 100 hours.
The majority of the reports received have concerned failures of the rear bearings on gear-driven alternators. Most of the reports detail metal contamination of the engine, which have then required major overhaul.
Further investigations have shown that excessive vibration and side loading, originating at the drive coupling and propagating to the rear bearing, can cause the design limits of the bearings to be exceeded. This problem may be magnified by increased rotor mass for larger, higher output alternators.
Several overhaul facilities have reported extreme balance problems in new units being serviced for the first time, and on replacement rotors. Side loading and vibration can be reduced by ensuring that the rotor is correctly balanced.
Another concern is that during drive coupling installation the correct drive nut tension is not being observed. Correct tension of the drive nut tension should be strictly adhered to as over tensioning the nut can displace the copper washer.
It should be noted that not all bearings supplied are greased. Some are only supplied with an inhibitor and require cleaning and packing, with the recommended grease, prior to installation.
Careful attention to correct procedures during alternator maintenance/replacement will help reduce the incidents of premature alternator failures. Following correct procedures will also assist in avoiding costly major engine repairs due to contamination by metal fragments from the alternator drive gear.