What is MTBF in airfield lighting?
The average operating time between one failure and the next. It shows how often to expect failures, and it is a measure of failure frequency rather than a lifetime.
MTBF, mean time between failures, is the average operating time between one failure and the next for an item that is repaired and put back in service. In airfield lighting it is used to compare equipment, to predict how many failures to expect, and to size spares. It is an average over many items, not a promise about how long any one item will last.
1Operating time divided by failures
MTBF = total operating time ÷ number of failures. In the example, 200 fixtures each ran 8,760 hours, which is 1,752,000 fixture-hours, and 8 failed, so the MTBF is 219,000 hours. Turned round, the failures to expect are operating hours ÷ MTBF. Count only the hours the equipment is energised, since a fixture that is switched off is not exposed to electrical failure in the same way, and decide first what counts as a failure: a light out, a circuit dark, or a call-out.
Average time to the one failure of an item that is replaced, such as a lamp.
Average time between failures of an item that is repaired and returned to service.
2An average, not a lifetime
The mistake to avoid is reading MTBF as a life. When the failure rate is steady, the chance of running a full MTBF without failing is only about 37%, and roughly 10% will have failed by a tenth of it. An MTBF of 219,000 hours does not mean a fixture lasts 25 years. It means that across a large group, failures arrive at about one per 219,000 operating hours. MTBF also says nothing about wear-out, which is why it is only meaningful in the steady phase.
| Phase | What happens | Does MTBF apply? |
|---|---|---|
| Early life | Faults from manufacture, wrong installation or bad joints show up soon after energising | No: the failure rate is falling |
| Useful life | Failures are random and the rate is roughly steady | Yes: this is where MTBF is meaningful |
| Wear-out | Ageing, corrosion, seal and insulation breakdown push the rate up | No: the failure rate is rising |
3Many parts, shorter MTBF
When any one failure counts, failure rates add. If 100 identical elements each have an MTBF of 1,000,000 hours, the group as a whole has an MTBF of about 10,000 hours, which is under one and a half years of continuous running. This is the chain idea from Reliability in numbers: every joint, connector and fixture adds to the total failure rate. Because an isolation transformer keeps the loop closed when a lamp fails, one light out and a circuit dark have very different MTBFs, so calculate each against its own definition of failure.
- Define a failure. A light out, a circuit dark, or a fault needing a visit.
- Count operating hours. Energised hours, added up across all items in the sample.
- Count the failures. From maintenance records, not memory.
- Divide hours by failures. That is the MTBF for that definition of failure.
- Check like for like. Same type, same conditions, same phase of life.
- Update as records grow. More hours and failures make the figure steadier.
| Term | Meaning |
|---|---|
| MTBF | Mean time between failures of a repaired item: operating time ÷ number of failures |
| MTTF | Mean time to failure of an item replaced when it fails, such as a lamp |
| Failure rate | Failures per unit of operating time; in the steady phase it is 1 ÷ MTBF |
| Operating hours | Hours the item is energised and working, not calendar hours |
| Fixture-hours | Operating hours added up across all the fixtures in the sample |
| Useful life | The steady middle of an item’s life, between early failures and wear-out |
| MTTR | Average time to repair or restore, covered in Entry 28 |
Frequently Asked Questions
Q1What is MTBF in airfield lighting?
Mean time between failures: the average operating time between one failure and the next for an item that is repaired and returned to service. It is total operating time divided by the number of failures.
Q2How is MTBF calculated?
Add up the hours all the items were operating, then divide by the number of failures in that time. For example, 200 fixtures running 8,760 hours each is 1,752,000 fixture-hours, and 8 failures gives an MTBF of 219,000 hours.
Q3Does an MTBF of 100,000 hours mean it will last 100,000 hours?
No. MTBF is an average across many items, not a guaranteed life. If the failure rate is constant, only about 37% of items run a full MTBF without failing, and about 10% have failed by a tenth of it. Wear-out is not described by MTBF.
Q4Why is the MTBF of a whole circuit lower than that of its parts?
Failure rates add up. If 100 identical elements each have an MTBF of 1,000,000 hours, and any one failing counts as a failure, the group has an MTBF of about 10,000 hours. More parts means more chances to fail.
Q5What is the difference between MTBF and MTTF?
MTBF is for items that are repaired and put back to work. MTTF, mean time to failure, is for items that are replaced when they fail, such as a lamp. The arithmetic is the same, and some suppliers use MTBF for both.
Q6How is MTBF used with availability?
Together with MTTR, the mean time to repair, it gives availability as roughly MTBF divided by MTBF plus MTTR. A longer MTBF means fewer failures, and a shorter MTTR means each one is put right sooner.
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