CORRECTIVE VS PREVENTIVE
Two opposite maintenance philosophies for AGL systems — corrective maintenance is performed after a defect or failure is identified, while preventive maintenance is planned and performed before failure to reduce the probability of breakdown. In short: Corrective = fix the problem. Preventive = prevent the problem.
At a Glance
| Feature | Corrective Maintenance | Preventive Maintenance |
|---|---|---|
| Main purpose | Restore equipment after defect/failure | Prevent failure |
| Trigger | Failure, defect, or abnormal condition | Time, usage, or planned schedule |
| Timing | After problem is identified | Before failure |
| Planning | May be unplanned | Planned |
| Equipment condition | Defective or degraded | Usually still operational |
| Downtime | Can be significant | Usually planned/minimized |
| Cost | Can be high if failure is severe | More predictable |
| Example | Replace failed runway light | Scheduled cleaning/replacement |
| Main objective | Restore operation | Maintain reliability |
Corrective Maintenance
Corrective maintenance asks: "What failed or became defective, and how do we restore it?"
EQUIPMENT OPERATING
↓
FAULT / DEFECT
↓
INSPECTION
↓
DIAGNOSIS
↓
REPAIR / REPLACEMENT
↓
TEST
↓
RETURN TO SERVICE
AGL example — a taxiway light goes out
TAXIWAY LIGHT ✕
↓
CHECK FIXTURE
↓
CHECK CONNECTOR
↓
CHECK ISOLATION TRANSFORMER
↓
FAULT IDENTIFIED
↓
REPLACE FAILED COMPONENT
↓
TEST
↓
LIGHT RESTORED ✓
That is corrective maintenance.
Preventive Maintenance
Preventive maintenance asks: "What maintenance should we perform now to reduce the chance of future failure?"
EQUIPMENT OPERATING
↓
PLANNED INTERVAL
↓
INSPECTION / SERVICE
↓
CLEAN / ADJUST / REPLACE
↓
TEST
↓
CONTINUE OPERATION
AGL example — a runway light due for scheduled service
RUNWAY LIGHT ✓
↓
SCHEDULED PM
↓
CLEAN OPTICS
↓
CHECK ALIGNMENT
↓
CHECK CONNECTIONS
↓
TEST
↓
RETURN TO SERVICE ✓
No failure was necessary to trigger the maintenance.
Same Failure, Two Different Paths
Suppose an LED taxiway light fails. Compare how each strategy responds to that same fixture over its life.
LIGHT FAILS
↓
FIND CAUSE
↓
REPLACE LED/DRIVER/
OTHER FAILED PART
↓
TEST
LIGHT STILL WORKING
↓
SCHEDULED SERVICE
↓
CLEAN
INSPECT
CHECK ALIGNMENT
CHECK CONNECTIONS
↓
TEST
Trigger Is the Main Difference
FAILURE / DEFECT
↓
MAINTENANCE
TIME / USAGE / SCHEDULE
↓
MAINTENANCE
Easy memory: Corrective = fix the problem. Preventive = prevent the problem. This is the easiest way to remember the difference.
How It Applies Across the System
| Equipment | Corrective Maintenance | Preventive Maintenance |
|---|---|---|
| Runway light | Replace failed fixture | Scheduled cleaning/alignment |
| Taxiway light | Replace failed LED module | Periodic inspection/service |
| PAPI | Repair failed unit | Scheduled functional/optical checks |
| CCR | Repair failed power component | Scheduled inspection/service |
| Isolation transformer | Replace failed transformer | Periodic electrical testing |
| Connector | Replace damaged connector | Inspect and maintain connections |
| AGL cable | Repair cable fault | Periodic testing/inspection |
| Control system | Repair failed module | Scheduled functional checks |
Planned Outage vs Unplanned Disruption
Corrective
FAILURE ↓ LIGHTING UNAVAILABLE ↓ FAULT RESPONSE ↓ REPAIR ↓ RESTORATION
Corrective maintenance can create unplanned operational disruption.
Preventive
PLANNED MAINTENANCE
↓
SCHEDULED OUTAGE
↓
SERVICE
↓
RESTORE
The work can be planned around airport operational requirements.
Predictable vs Unpredictable Cost
Corrective
Cost can be unpredictable:
MINOR FAILURE
↓
LOW COST
MAJOR FAILURE
↓
EMERGENCY WORK
+
SPARES
+
LABOUR
+
OPERATIONAL IMPACT
↓
HIGHER COST
Preventive
Costs are generally more predictable:
MAINTENANCE PLAN
↓
SPARES PLANNED
↓
LABOUR PLANNED
↓
SCHEDULED WORK
Preventive maintenance can also result in replacing or servicing components before they actually fail.
Corrective Maintenance Is Not Always Bad
Corrective maintenance can be appropriate when:
- Failure has little operational consequence.
- Repair is simple and inexpensive.
- The equipment is non-critical.
- Scheduled replacement is not economical.
- The system is designed to tolerate the failure.
But for critical AGL equipment, relying exclusively on corrective maintenance can increase operational risk.
How Inspection Connects the Two
A routine inspection can identify a defect that then triggers corrective maintenance.
ROUTINE INSPECTION
↓
DEFECT FOUND
↓
CORRECTIVE MAINTENANCE
↓
REPAIR
Example
INSPECTION
↓
CRACKED LIGHT LENS
↓
REPLACE LENS
↓
CORRECTIVE MAINTENANCE
The maintenance is corrective because an existing defect was found and corrected.
Scheduled Doesn't Always Mean Preventive
"If maintenance is planned, it is automatically preventive."
Consider a light that failed yesterday, with the repair scheduled for today. Even though the repair was scheduled, it is still corrective maintenance, because the failure already occurred.
LIGHT FAILED YESTERDAY
↓
REPAIR SCHEDULED FOR TODAY
↓
REPAIR
The classification depends on why the maintenance was initiated, not simply whether the work was scheduled.
Building a Complete AGL Maintenance Program
A practical AGL maintenance program may combine both strategies:
AGL SYSTEM
│
┌──────────┴──────────┐
▼ ▼
PREVENTIVE CORRECTIVE
MAINTENANCE MAINTENANCE
│ │
Planned service Defect / failure
│ │
▼ ▼
Reduce failure risk Restore operation
And predictive maintenance can be added:
PREVENTIVE
+
PREDICTIVE
+
CORRECTIVE
↓
COMPLETE MAINTENANCE STRATEGY
Side-by-Side Parameters
| Parameter | Corrective | Preventive |
|---|---|---|
| Trigger | Failure/defect | Planned interval |
| Timing | After problem | Before failure |
| Planning | Often reactive | Proactive |
| Objective | Restore operation | Prevent failure |
| Downtime | Potentially unplanned | Usually planned |
| Cost predictability | Lower | Higher |
| Failure risk | Higher if used alone | Reduced |
| Spare planning | Reactive | Planned |
| Suitable for critical AGL | Limited as sole strategy | Generally important |
Quick Recall
PROBLEM ↓ FIX ↓ RESTORE
SCHEDULE ↓ SERVICE ↓ PREVENT
Corrective maintenance reacts to an existing problem. Preventive maintenance acts in advance to reduce the chance of a problem.
Corrective vs Preventive Maintenance
Corrective maintenance is performed after a defect or failure is identified, while preventive maintenance is planned and performed before failure to reduce the probability of breakdown.
For critical AGL equipment, a maintenance program that relies exclusively on corrective maintenance carries higher operational risk. A combined strategy — preventive maintenance for reliability, corrective maintenance for the failures that still occur, and predictive maintenance layered on top where condition data is available — is generally the more complete approach.
Failed runway light → corrective maintenance. Scheduled runway-light cleaning/alignment → preventive maintenance. A trend showing deteriorating CCR condition → predictive/condition-based maintenance.
Engineering & Expert Notes
Mix Strategies by Asset Criticality
PlanningCritical lights justify preventive attention; low-impact items may suit run-to-failure. Decide asset by asset.
Use Corrective Work Records to Improve the Preventive Plan
LearningRepeat failures show which tasks and intervals should change.
Keep Spares for the Corrective Work You Expect
SparesStock the items that fail most often so repairs finish at the first visit.
Audit That Planned Tasks Are Actually Done
AssuranceA preventive plan that is skipped gives false confidence.
Frequently Asked Questions
Corrective maintenance is performed after a defect or failure is identified, while preventive maintenance is planned and performed before failure to reduce the probability of breakdown. The trigger is the key difference: failure vs schedule.
No. If a light failed yesterday and the repair is scheduled for today, it is still corrective maintenance because a failure already occurred. Classification depends on why the maintenance was initiated, not whether it was scheduled.
No. Corrective maintenance can be appropriate when a failure has little operational consequence, the repair is simple and inexpensive, the equipment is non-critical, or scheduled replacement is not economical. It becomes a risk mainly when relied on exclusively for critical AGL equipment.
Yes. A routine inspection can find an existing defect, such as a cracked light lens, and the repair that follows is still classified as corrective maintenance because it corrects a defect that was already present.
Predictive maintenance uses condition or trend data — for example, a trend showing deteriorating CCR condition — to schedule service based on actual equipment condition, sitting alongside preventive and corrective maintenance as part of a complete strategy.
Preventive maintenance is generally important for critical AGL equipment because it reduces failure risk and allows planned outages. Relying exclusively on corrective maintenance for such equipment can increase operational risk.
Governing Documents
For an actual project maintenance program, always verify the current edition of the applicable documents.
FAA AC 150/5340 Series
Airport lighting equipment and systems circulars, including maintenance-related guidance for AGL fixtures, circuits and control equipment.
ICAO Annex 14, Volume I
Aerodrome Design and Operations — general requirements for the serviceability and maintenance of visual aids.
DGCA / AAI Requirements
For Indian airport projects, also verify applicable DGCA requirements, AAI maintenance manuals, airport-specific technical specifications and O&M contract requirements.
Final Engineering Note
Do not design a maintenance program from a generic web diagram. The final maintenance plan should always follow the governing standard, manufacturer recommendations, and the airport's approved maintenance manual.
Corrective maintenance is not a failure of planning, and preventive maintenance is not a guarantee against failure — each plays a different role in maintaining a reliable AGL system.
Compare the Systems. Then Learn How to Engineer Them.
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