What is MTTR in airfield lighting?
The average time it takes to put a failed light or circuit back in service. Most of it is usually spent detecting, getting there and waiting for parts, not fixing.
MTTR, mean time to repair, is the average time taken to put a failed item back in service. In airfield lighting the useful version runs from the moment the lighting fails until it is working, tested and handed back, because that is the time operations are without it. Together with MTBF it sets availability.
1Downtime divided by failures
MTTR = total downtime ÷ number of failures. In the example, four outages of 2, 4, 8 and 18 hours add up to 32 hours, so the MTTR is 8 hours. The average hides the spread: the single 18-hour outage is more than half of the total, so look at the longest outages as well as the mean. Also decide and state when the clock starts and stops, since MTTR is used for two different things in practice.
Hands-on locating and fixing the fault, and nothing else.
Detect, get there, fetch parts, repair, test and hand back: the whole outage.
2Mostly waiting, not fixing
In the example only 1 of the 8 hours is hands-on repair. The rest is detecting the fault, getting a crew airside with a permit, and fetching spares, which is why a faster pair of hands barely changes the result while a standing permit or a spare in the van changes it a lot. Splitting each outage by stage shows where the time really goes.
| Stage | What happens | How it is shortened |
|---|---|---|
| Detect | The fault is noticed, reported and confirmed | Alarms, monitoring, regular inspection |
| Access | A crew is called, briefed and cleared to work airside | Standby crews, standing permits, agreed procedures |
| Parts | The right spare is found and brought to site | Spares held to match the failures expected |
| Repair | The fault is located and fixed | Labelled circuits, as-built records, plug-in connectors |
| Test and hand back | Checks are made and the lights are returned to operations | Standard test sheet, clear handback to control |
3MTTR and MTBF give availability
Availability is roughly MTBF ÷ (MTBF + MTTR). With an MTBF of 2,000 hours, an MTTR of 8 hours gives about 99.60% availability and 35 hours down a year, while 4 hours gives about 99.80% and 17.5 hours. Halving MTTR halves the downtime, and it is often cheaper than doubling the MTBF, because it needs spares, crews, permits and records rather than new equipment. The formula is an approximation that works best when MTTR is small compared with MTBF.
- Define start and end. From the moment of failure to tested and handed back.
- Log every outage. Times of failure, report, arrival, repair and handback.
- Add up the downtime. Total hours lost across all the failures.
- Divide by the failures. That is the MTTR for that definition.
- Split by stage. Detect, access, parts, repair, test and hand back.
- Attack the longest stage. Usually waiting, not the repair itself.
| Term | Meaning |
|---|---|
| MTTR | Mean time to repair or restore: total downtime ÷ number of failures |
| Active repair time | The hands-on time spent locating and fixing the fault |
| Restoration time | Everything from failure until the lights are working, tested and handed back |
| Logistic delay | Time spent waiting for a crew, a permit, access or a spare |
| Downtime | Time the function is not available |
| MTBF | Average operating time between failures, covered in Entry 27 |
| Availability | Share of time the lighting is able to do its job, covered in Entry 26 |
Frequently Asked Questions
Q1What is MTTR in airfield lighting?
Mean time to repair, or restore: the average time taken to put a failed item back in service. It is total downtime divided by the number of failures.
Q2How is MTTR calculated?
Add up the downtime of every failure and divide by the number of failures. For example, four outages of 2, 4, 8 and 18 hours total 32 hours, so the MTTR is 8 hours.
Q3When does the MTTR clock start?
That depends on the definition, so state it. For availability the clock should start when the lighting fails, because that is when operations lose it, and stop when the lights are working, tested and handed back. Starting it when the crew arrives measures only part of the outage.
Q4Why is most of an outage not repair time?
Detecting the fault, getting a crew airside with a permit, and fetching the right spares often take longer than the hands-on work. In the example, 1 of the 8 hours is actual repair.
Q5How do MTTR and MTBF combine?
Availability is roughly MTBF divided by MTBF plus MTTR. With an MTBF of 2,000 hours, an MTTR of 8 hours gives about 99.60% availability, and halving the MTTR to 4 hours gives about 99.80%.
Q6What is a good MTTR?
There is no single figure. It depends on how critical the lights are and the operation they serve, and it is set by the airport's own maintenance objectives and target availability. Lights that must be restored quickly need faster detection, standby crews and spares on hand.
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