Airbus A320 APU Starter Contactor Failure During Ground Operations
An Airbus A320 experienced repeated APU start failures during overnight ground operations at a remote station. Maintenance troubleshooting identified progressive electrical damage inside the APU starter contactor assembly caused by repeated start attempts under degraded aircraft battery conditions. The event ultimately resulted in starter motor overheating and loss of independent APU start capability.
Event Summary
During overnight turnaround operations, maintenance personnel reported multiple unsuccessful APU start attempts on an Airbus A320-214. The aircraft was operating from a remote airport position with limited ground support availability. Flight crews observed intermittent starter disengagement, slow spool acceleration and several automatic start aborts during successive start cycles.
Operational Symptoms
Recorded maintenance observations included: • Delayed APU acceleration during start sequence • Intermittent starter disengagement • Abnormal voltage drop during cranking • Repeated ECAM advisory messages related to APU start faults • Increased starter current draw during successive attempts • Burnt electrical odor detected near avionics bay inspection area The aircraft remained safely on external ground power throughout troubleshooting.
System Description
The A320 APU starting system uses an electrically driven starter motor controlled by the Electronic Control Box (ECB). During start initiation, the starter contactor connects aircraft electrical power to the starter motor, allowing the APU to accelerate to self-sustaining speed before automatic disengagement. Stable battery voltage and proper contactor operation are essential to prevent excessive current draw, contact arcing and starter overheating during high-load start cycles.
Maintenance Inspection
Initial troubleshooting focused on aircraft battery condition, starter electrical supply and ECB fault history. BITE data confirmed multiple interrupted start sequences associated with unstable starter current characteristics. Following removal of the starter assembly, technicians identified: • Severe electrical pitting on starter contact surfaces • Localized thermal discoloration inside the contactor housing • Elevated resistance across contactor terminals • Heat-related damage on sections of starter wiring insulation • Early-stage armature overheating inside the starter motor No faults were detected within the ECB control logic.
Root Cause Analysis
Engineering investigation determined that repeated APU start attempts under degraded battery conditions generated excessive current flow through the starter contactor assembly. Progressive electrical arcing increased contact resistance and produced localized overheating during each subsequent start cycle. The unstable electrical supply eventually caused incomplete starter disengagement characteristics and abnormal thermal loading within the starter motor assembly.
Corrective Actions
The following maintenance actions were completed before return to service: • Replacement of APU starter motor assembly • Replacement and inspection of starter contactor components • Aircraft battery capacity verification • Inspection of starter power wiring and terminal connections • ECB operational test and fault reset • Successful APU start test using aircraft electrical power • Follow-up operational test under pneumatic and electrical load Subsequent testing confirmed stable starter engagement and normal APU acceleration characteristics.
Engineering Lessons
This case demonstrated how degraded battery performance can progressively damage high-current APU starting components during repeated start attempts. Monitoring starter current trends, voltage stability and abnormal cranking behavior provides valuable early indication of contactor deterioration before complete operational failure occurs. The incident also highlighted the operational impact of temporary GPU dependency during remote ground operations where external support equipment availability may be limited.