What is availability in airfield lighting?
The share of time the lights are ready to do their job. It depends on how rarely they fail and on how quickly a failure is put right.
Availability is the share of time a system is able to do its job when it is needed. For airfield lighting that means the right lights are on, at the right intensity, whenever operations need them. It counts both how often the lighting fails and how long it stays failed, so it joins reliability with speed of repair.
1Uptime over total time
Availability is uptime ÷ (uptime + downtime). A year has 8,760 hours, so each hour of downtime costs about 0.011% of availability. In the example all three cases fail four times a year, yet restoring in 2 hours gives 99.91% while restoring in 72 hours gives 96.71%. The failures were equally frequent; only the repair time changed.
How rarely it fails. Judged by the time between failures.
How much of the time it works. Also counts how long each failure lasts.
| Availability | Downtime in a year | Downtime in a 30-day month |
|---|---|---|
| 99% | 87.6 hours (about 3.7 days) | 7.2 hours |
| 99.9% | 8.8 hours | 43 minutes |
| 99.99% | 53 minutes | 4.3 minutes |
| 99.999% | 5.3 minutes | 26 seconds |
2Two levers: fail less, restore faster
Failing less is the job of reliability: fewer joints and connectors, redundancy, and isolating transformers so that one failed lamp does not open the series circuit. Restoring faster is a separate job, and often the cheaper one. It covers detecting the fault quickly through alarms and monitoring, getting a trained crew airside with permits already in place, holding the right spares, and designing repairs to be quick: plug-in connectors, labelled circuits, accessible pits and current as-built drawings. Restoration time covers finding, reaching, fixing and testing, and the waiting is often longer than the fixing.
3Available against what?
Availability only means something against a stated requirement. A circuit with one light out may still be available if the lighting still meets what the operation needs, while a dark runway-edge circuit is not. Because each fixture has its own isolating transformer, one failed light and a dark circuit are very different events and should not be counted the same way. Planned maintenance also takes lighting out of service, and whether it counts as downtime depends on how the target is written, so state it.
- Define what counts as available. Which lights, how many must work, and for which operations.
- Set a target. A percentage over a period, agreed with operations.
- Record downtime. Time each outage from failure until restored and tested.
- Find the biggest losses. Sort outages by cause and by total hours lost.
- Fix the biggest cause. Reduce how often it fails or how long it takes to restore.
- Review and repeat. Compare with the target and adjust spares, staffing and design.
| Term | Meaning |
|---|---|
| Availability | Share of time the system can do its job when it is needed |
| Uptime | Time the function is available |
| Downtime | Time it is not, from failure until the lighting is restored and tested |
| Restoration time | Detect, reach, repair and test: the whole time the lights are out |
| Serviceability | Share of individual lights working at a given moment |
| MTBF | Average time between failures, covered in Entry 27 |
| MTTR | Average time to repair or restore, covered in Entry 28 |
| Availability in averages | MTBF ÷ (MTBF + MTTR), explained in the next entries |
Frequently Asked Questions
Q1What is availability in airfield lighting?
The share of time the lighting is able to do its job when operations need it. It reflects both how often failures happen and how long it takes to put them right.
Q2How is availability calculated?
Uptime divided by uptime plus downtime, usually shown as a percentage over a year. A year has 8,760 hours, so about 88 hours of downtime gives 99.0% and about 8.8 hours gives 99.9%.
Q3How is availability different from reliability?
Reliability is how rarely failures happen. Availability also counts how long each failure lasts. Two systems that fail equally often can have very different availability if one is repaired in hours and the other in days.
Q4Why does repair time matter so much?
Downtime is the number of failures multiplied by the time taken to restore each one. Cutting restoration time, through quick detection, spares on site, trained crews and easy-to-replace parts, lowers downtime as surely as having fewer failures, and is often cheaper.
Q5What does 99.9% availability mean in practice?
About 8.8 hours of downtime in a year, or roughly 43 minutes in a 30-day month. Each extra nine cuts the allowed downtime to a tenth, and usually costs much more to achieve.
Q6Does one failed light make a circuit unavailable?
Not necessarily. Each fixture is fed through its own isolating transformer, so one failed light does not open the series loop. Whether the circuit counts as available depends on the requirement, such as how many lights must work and where the unserviceable ones are.
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