THE METHOD: SCOPE, MEASURE, LOCATE, REPAIR, PROVE
AGL troubleshooting is not "light off, replace light". Ask what failed, where, why, what the operational impact is, and how the repair is proved. The chain to follow is CCR, primary cable, isolating transformer, L-823 connector, secondary lead, fixture, lamp/LED/driver/optics, then photometric output.
| Scope | Likely domain |
|---|---|
| One light | Local: secondary, transformer, connector, fixture |
| Small cluster / section | Cable, joint, circuit allocation |
| Whole circuit | CCR, control, primary cable, open or ground fault |
| Several CCRs | Common power or control |
| Whole airfield | Main power or central control |
| Random, intermittent | Connector, moisture, mechanical, control |
Scope first
- Before touching equipment, decide: one, few, section, circuit, several circuits, airfield
- Then ask: is there a common source?
- This is a heuristic, not a substitute for measurement
- "What evidence would tell these causes apart?" is the best question in any scenario
DIAGNOSTIC AND TEST MATRICES
| Symptom | First suspicion | Evidence |
|---|---|---|
| Whole circuit OFF | CCR, control, open | CCR output |
| One light OFF | Local component | Secondary and fixture test |
| Adjacent lights OFF | Section or circuit fault | Circuit layout |
| Ground fault alarm | Insulation fault | IR, fault location |
| One light flickers | Connector, fixture | Local inspection |
| All lights dim | CCR or common source | Current, step |
| One light dim | Fixture, optics | Photometry |
| Fails in rain | Moisture | IR, pit inspection |
| Stop-bar conflict | Interlock, control | Logic and status |
| PAPI wrong | Aiming, optics | Alignment, photometry |
| ALCMS mismatch | Monitoring | Field vs control comparison |
| Problem | Useful test |
|---|---|
| Open conductor | Continuity, cable fault locator |
| Insulation failure | IR, approved fault locator |
| Low output | Photometry |
| Wrong aiming | Alignment, photometry |
| Transformer suspicion | Primary and secondary functional tests |
| Moisture | IR plus physical inspection |
| Stop-bar conflict | Functional and interlock test |
Tools: multimeter, insulation tester, clamp meter, continuity set, cable fault locator, thermal equipment, photometer, alignment gear, torque wrench, ALCMS/PLC diagnostics and circuit drawings (AC 150/5340-26C discusses test and fault-location equipment).
CCR AND CIRCUIT SCENARIOS
One light OFF, rest of circuit on
The loop is intact, so start at the light assembly, secondary lead, L-823 connector and transformer (secondary winding or a shorted primary), not the CCR or primary cable. An open transformer primary would take the whole circuit down, not one light.
Entire circuit OFF
Check CCR status, input, command, output, circuit current, then ground/open alarms. Do not open every pit. First ask whether the CCR is energizing the circuit.
CCR shows no output
Check input, control, output, then the circuit. Read the alarm first: an open-circuit shutdown points to a break in the loop, an overcurrent or short-circuit trip points to a shorted circuit. Only move to the field once the CCR itself is proved.
CCR output normal, all lights OFF
An open primary gives zero loop current and normally shuts the CCR down, so first prove the reading. Put a calibrated clamp meter on the loop: no current means an open circuit (read the CCR alarm and locate the break); current flowing with every light OFF means the loop is bypassed or the setting is wrong, so check for a short across the loop, a cutout, selector or changeover switch in the wrong position, and the brightness step. “CCR healthy” does not mean the circuit is healthy.
Current abnormally high or low
Do not adjust blindly. Verify the instrument and setting, check alarms and circuit condition, then run approved fault diagnosis. A constant-current system regulates, so a large deviation is itself a clue.
CCR trips immediately
Suspect a short circuit, multiple ground faults or severe insulation breakdown, or an internal CCR or input fault. A single ground fault normally gives a ground-fault alarm rather than a trip, because the series circuit is ungrounded. Do not repeatedly reset; protect, isolate, analyze, locate. If it trips only at the top step (highest voltage), suspect stress-dependent insulation breakdown, excess load or protection settings.
CCR equipment is covered by FAA AC 150/5345-10H.
GROUND FAULTS, OPEN CIRCUITS AND INSULATION
Ground fault alarm
Causes: cable insulation, transformer, connector moisture, fixture, joint, water ingress. Follow the approved procedure; do not cycle the circuit repeatedly. A visually normal fixture does not rule out an underground fault. One ground fault may leave the lights ON, but treat it as urgent: a second fault elsewhere can short out part of the circuit or trip the CCR.
Two grounds on a series circuit
Per 26C, current through two insulation breaches can heat or arc, which thermal equipment and a cable fault locator can help localize.
Open circuit
Locations: CCR output wiring, cable, joint, transformer primary, connector. Locate the open, repair, then continuity, IR, energize. After excavation, suspect cable damage and review as-builts.
Rain-related fault
Dry OK, wet low, drying improves: moisture ingress in pit, transformer, connector, joint or fixture. Do dry and wet investigation with IR trends.
| Continuity | IR | Meaning |
|---|---|---|
| PASS | FAIL | Conductor continuous, insulation compromised (moisture, cable, joint, transformer, connector) |
| FAIL | PASS | Open path: cable break, joint, disconnected component |
Insulation resistance is measured with the CCR isolated and locked off, at the test voltage in the approved procedure, and compared with the baseline record. A static IR test is not complete proof of dynamic integrity. An alarm with good IR may be intermittent, moisture-related, multiple-point or a monitor problem; use the approved fault-finding method.
TRANSFORMER, CONNECTOR AND CABLE
Transformer
Primary healthy but fixture OFF: test primary, secondary, L-823, fixture. Prove the fault before replacing. A pass on IR does not prove turns ratio or load response. After replacement, check type, wiring, polarity and connections before blaming the new part.
Wrong wattage
A 45 W fitting on the wrong transformer risks abnormal loading and premature failure. Verify fixture wattage, primary current, transformer rating, secondary and connectors.
L-823 connector
Intermittent, hot, dim or repeated failures: insertion, contact, moisture, corrosion, assembly, cable movement. AC 150/5345-26E. Three replacements in six months means find the root cause.
L-824 cable
Low IR: moisture, damage, joint, ageing, termination, wet pit. Isolate, segment as approved, locate, repair, retest. AC 150/5345-7F. A cable locator narrows an open between pits.
Repeated transformer failures in one wet pit point to drainage, sealing or construction, not the transformer. Two circuits failing at one location suggest a local environmental or mechanical cause; one circuit failing at several locations suggests a common source.
PHOTOMETRY SCENARIOS AND CALCULATIONS
ON but fails photometry
Current correct, lamp visible, 58% output: not healthy. Look at contamination, optics, ageing, orientation, damage.
Before and after cleaning
68% to 91%: contamination. 68% to 66%: cleaning did not fix it, so continue to lamp/LED, driver, optics, alignment, transformer.
Bright but wrong direction
Not acceptable: intensity, beam and orientation all matter. Check mounting, base, displacement, survey, then retest.
Wrong colour
Colour is its own acceptance parameter: fixture type, lens, LED colour, assembly. Do not start with IR.
One step low
Step 3 low, others fine: suspect CCR step or control, not the fixture.
Unstable or expired
Unstable photometer: check instrument, calibration, geometry, repeat. Expired calibration: no high-confidence final report. Same fixture 90% then 73%: control conditions and repeat.
| Calculation | Result | Reading |
|---|---|---|
| 4,500 of 10,000 cd | 45% | Below ICAO 50% individual-light criterion: unserviceable |
| 7,200 of 10,000 cd | 72% | Close to FAA 70% maintenance target: investigate and trend |
| 30 failed of 400 | 92.5% serviceable | Still check distribution |
| 500 total, 20 failed | 96% | Formula: serviceable ÷ total × 100 |
| Output 78% | 22% degradation | 100 − output % |
Remember
- One light low: local. Every light low: common source
- Photometry cannot measure a dead fixture: do visual and electrical diagnosis first
- Photometry is a snapshot; a new electrical defect can appear afterward
- New does not mean compliant
PATTERN, ADJACENCY AND INTERLEAVING
Serviceability percentage alone never decides operational acceptability. Two adjacent failures can matter more than two isolated ones, and failures in a critical zone matter even when overall figures look high (97% overall with all failures in the inner approach is not acceptable by number alone).
| Pattern | Assessment |
|---|---|
| ● ● X ● ● X ● ● | Distributed failures |
| ● ● X X ● ● ● ● | Adjacent failures: damaged pattern |
| A B A B (A fails) | Interleaved: distributed dim pattern |
| A A A A A B B B (A fails) | Non-interleaved: large dark section |
Check where the failures are, whether they are adjacent, which element is affected, the pattern, and the airport LVP criteria. ICAO low-RVR provisions address adjacent failures for taxiway centreline and stop bars; curved sections have tighter spacing, so a curve failure needs priority checking. A dark section prompts the question: are those lights on the same circuit? Use circuit drawings. See Note 18.
LVP, CAT III, STOP BARS AND POWER
RCL failure during LVP
Identify the fixture, check adjacent lights and circuit, assess serviceability, inform ATC/operations, repair, retest, restore. The engineer does not decide alone that CAT III can continue; that belongs to the approved airport procedure.
Stop bar faults
Two failed lamps are not automatically acceptable: check adjacency, local criterion, valid stop indication, interlocking and LVP permission. Treat as critical during LVP.
Stop bar ON and centreline beyond ON
Interlock or control logic failure, potentially more serious than a lamp fault because the commands conflict.
Stop bar will not go OFF, or lead-on will not come ON
Trace ATC command, ALCMS, PLC/controller, output relay, circuit, fixture; for lead-on, selective switching and the centreline circuit.
UPS fault, DG in manual, LVP active
Lights being ON is not enough. Ask what happens if mains fails now: UPS, critical load, DG, ATS, changeover, airport criteria.
3 s outage on CAT III critical lights
Not automatically acceptable. ICAO CAT II/III has 1 s switch-over for several critical elements; take the criterion from the governing design.
A fault at 2 AM during LVP is operationally critical, not routine. Priority: CAT III RCL, stop-bar interlock, whole approach lighting and UPS failure in low visibility are critical; remote single edge light is assess; photometric degradation planned or urgent by criterion; cosmetic and documentation items lower.
CONTROL, ALCMS AND PAPI SCENARIOS
| Scenario | Direction |
|---|---|
| ALCMS healthy, field OFF | Monitoring error, communication, field failure, feedback; compare status with physical reality |
| Field ON, ALCMS OFF | Feedback, remote I/O, mapping, communication |
| ALCMS command works, field does not | ALCMS, communication, PLC, output, CCR/controller, circuit |
| Wrong runway lights operate | Mapping, PLC logic, circuit numbering, labelling, wiring, database: a configuration fault |
| Whole circuit flickers | Common source: CCR, control, supply, loose connection |
| One fixture flickers | Local connector, transformer, lead, driver |
| PAPI wrong indication | Aiming, unit elevation, optics, displacement, output; electrical or mechanical? |
| PAPI units at different heights | Check approved design, survey datum, tolerances |
| PAPI looks right, photometry fails | Optical degradation, beam distribution: appearance is not compliance |
Only taxiway lights out: start with the taxiway CCR, circuit, control and cable. Only stop bars out: stop-bar circuit, controller, interlock, communication, field power. Do not blame main power when other lights work.
ROOT CAUSE, DOCUMENTATION AND FAULT DATA
Repeated faults at one location call for trend and root-cause analysis. Example 5-why: transformer failed, water ingress, pit flooded, drainage blocked, poor pit maintenance, no scheduled drainage inspection. The root cause is a maintenance system failure, not the transformer.
Fault classes
Control, CCR, primary cable, transformer, connector, secondary, fixture, photometric, mechanical, environmental.
Root-cause categories
Electrical, mechanical, optical, environmental, control, installation, maintenance, component quality.
Frequency analysis
Of 100 faults, 35 transformer and 25 connector: focus on quality, installation and moisture, not just fixtures.
Record
Date, location, fixture and circuit ID, symptom, tests, root cause, action, replacement, retest, photometry, restoration time, technician, approval.
Documentation accuracy is part of troubleshooting. If the drawing says CCR-03 and the field label says CCR-04, verify by as-builts, tracing, CCR output and schedules. Never guess which circuit feeds a light.
After repair: continuity, IR, functional, photometry, control, monitoring, release. Repair fixes the component; restoration proves the system is functional, optically acceptable, controlled and serviceable.
SAFETY, MISTAKES AND RULES
Never
- Reset a tripping CCR repeatedly without diagnosis
- Assume "lamp OFF = lamp failed"
- Use uncalibrated instruments for final compliance
- Ignore grounding or moisture, or bypass interlocks casually
- Change CCR settings without authorization
- Close a defect without retest, or treat percentage alone as serviceability
Final 10 rules
- Never guess; identify scope; check the common source
- Measure before replacing; follow series-circuit logic
- Check transformer, connector and fixture together
- Use photometry for optical problems; check distribution
- Retest after every repair; document the root cause
QUICK REVISION
Symptom to cause
- Light OFF: CCR, cable, transformer, L-823, fixture
- Light dim: CCR step, transformer, connector, optics
- Whole circuit OFF: CCR, control, open or ground fault
- Low IR: insulation or moisture; no continuity: open circuit
- Good electrical, low photometry: optics, contamination, alignment
- Repeated failure: root-cause analysis
References
- FAA AC 150/5340-26C: maintenance, fault-finding, photometry
- AC 150/5345-10H CCR, 5345-26E L-823, 5345-7F L-824
EXAM / INTERVIEW SCENARIOS
One runway edge light is OFF, others on. First step?
Do not replace the fixture. Check fixture, secondary lead, transformer and L-823, because the rest of the series circuit is working.
CCR current is normal but all lights are OFF. What next?
An open primary gives zero loop current and normally shuts the CCR down, so first prove the reading. Put a calibrated clamp meter on the loop: no current means an open circuit (read the CCR alarm and locate the break); current flowing with every light OFF means the loop is bypassed or the setting is wrong, so check for a short across the loop, a cutout, selector or changeover switch in the wrong position, and the brightness step. CCR health does not prove circuit health.
Two taxiway centreline lights are OFF. Which matters more, count or pattern?
Location and pattern. Adjacent failures can compromise continuous guidance, especially in low visibility.
A CAT III system is 95% serviceable. Operational?
Not automatically. Check failure location, adjacency, critical elements, pattern and the LVP procedure.
Continuity passes, IR fails. Meaning?
The conductor is continuous but insulation is compromised: moisture, cable, joint, transformer or connector.
IR passes, light does not work. Why?
IR shows insulation condition only. Check transformer operation, connector, secondary, fixture, lamp/LED, driver and current.
Photometry 68% before cleaning, 91% after. Conclusion?
Optical contamination was the main issue, not necessarily an electrical failure.
Photometry stays at 66% after cleaning. Next?
Continue diagnosis: lamp/LED, driver, optics, alignment, transformer, electrical condition. Do not keep cleaning.
Output is 4,500 of 10,000 cd. Result?
45%, below 50% of the specified value: individual light photometrically unserviceable under the ICAO criterion, though release also needs system and procedural checks.
Output is 72%. Is it perfect?
No. It is near the FAA 70% maintenance target, so investigate and trend. It is not an overall acceptance limit.
Fault only in rain?
Suspect moisture ingress: pit, joint, transformer, connector, fixture. Use dry-weather tests, wet investigation and IR trends.
Same pit keeps killing transformers?
Root-cause analysis: drainage, sealing, groundwater, pit construction, not repeated replacement.
Stop bar ON and centreline beyond it ON?
An interlock or control logic failure with conflicting commands.
ALCMS healthy but the field light is OFF?
Compare status with reality: monitoring, communication, feedback or a real field failure.
UPS fault during LVP while lights are on?
Ask what happens if mains fails: check UPS, DG, ATS, changeover and LVP criteria.
Which test first for a completely OFF light?
Visual and electrical diagnosis. Photometry cannot measure a dead fixture.
Ultimate scenario: CAT III, LVP, RCL OFF, adjacent RCL OFF, CCR normal, high overall serviceability.
Treat RCL as critical and the adjacent pair as more significant than a count; confirm exact fixtures and circuit; notify ATC/operations per procedure; inspect transformer, L-823, secondary and fixture; repair; run electrical, functional and, as applicable, optical and alignment checks; confirm the pattern and serviceability are restored; record the event and restoration.
FINAL STUDY TAKEAWAY
The best troubleshooter does not replace components fastest. They find the fault correctly, understand its operational significance, remove the root cause and prove the system is safely restored.
Scope it, measure it, locate it, repair it, prove it, document it.
ENGINEERING & EXPERT NOTES
Say Your Sequence Out Loud
Exam TechniqueScope, measure, locate, repair, verify: stating the order shows structured thinking in an interview and prevents skipped steps on site.
Use Symptoms to Narrow, Not to Conclude
MethodA pattern of dark lights suggests where to look; only measurement confirms what failed.
Ask What Else Could Explain It
JudgementBefore committing to a cause, name one alternative and say how you would rule it out.
Mention Safety Before Touching Anything
SafetyIsolation, permits and proving dead belong in every answer. It is also how a competent technician works.
REFERENCES & FURTHER READING
These notes are a study aid. Verify against current sources and your approved site procedures.
| # | Source | Use it for |
|---|---|---|
| 1 | FAA AC 150/5340-26C | Maintenance, fault-finding, safety, photometry |
| 2 | FAA AC 150/5345-10H, 26E, 7F | CCR, L-823 and L-824 equipment |
| 3 | ICAO Annex 14 Vol. I | Serviceability and adjacency criteria |
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