A maintainability prediction is read by four people who want four different things from it, and only one of them wants the headline number.
The four figures, stated together
| Figure | What it sizes |
|---|---|
Mct | How long one repair takes: the number a specification usually names |
Mmax(90) or Mmax(95) | The slot: how long a repair may take before the plan breaks |
M̄ | Mean active maintenance time over corrective and preventive actions together; never a repair time |
MMH/OH | Maintenance man-hours per operating hour: the manning and the support price |
Quoting the mean without the percentile leaves the reader to assume the distribution, and the assumption is always wrong in the same direction, because repair times are skewed and people imagine them symmetric. The worked example's percentile is 1.9 times its mean; a maintenance window sized on the mean is overrun by more than a third of the work.
State with both: the level of repair, the maintenance concept they assume, and the procedure used. Two predictions that differ in any of the three are not comparable and are routinely compared anyway.
And state the level. The same failures give 55 minutes at organisational level, 62 at intermediate and 210 at depot on the worked example: a factor of four across the range. The figure that belongs in an availability model is the one for whichever action returns the equipment to service, which is the organisational number wherever repair is by exchange, and is the in-place repair time where the equipment stays down while the item inside it is changed. Taking a workshop or depot figure that runs in parallel with the equipment operating is the error the level is stated to prevent.
The contribution list, which is what a designer can act on
Sorted by λ·t, and sorted again by activity. The first list names the items whose repair should be made easier or whose failure rate should come down; the second names the design discipline that owns the problem.
| If the activity sum is dominated by | The finding belongs to |
|---|---|
| Disassembly and reassembly | Mechanical design: access, fasteners, connector placement, item boundaries |
| Isolation | Testability: the diagnostic cannot name the item |
| Alignment | Systems design: a part that needs setting up after fitting is a part designed to be fitted twice |
| Checkout | The test procedure, and whether the equipment can prove itself without a full sequence |
That table is the reason the analysis is worth running early. All four findings are cheap while the design is a drawing and expensive afterwards, and none of them is visible in a single MTTR figure.
What it feeds
| Consumer | What it takes |
|---|---|
| Availability | Mct as the repair term, with the logistic delay added separately |
| Support planning | MMH/OH, corrective and preventive, with crew sizes in it, and the skill mix behind it |
| Spares | Removal rates per item, which the ambiguity groups can make larger than the failure rates |
| The contract | The predicted mean and percentile the demonstration will be measured against |
What the prediction does not support
- Precision. Individual task times are judgements; a factor of two on any one is unremarkable. The mean is a planning number, not a measurement, and quoting it to a tenth of a minute is a presentation error the worked example commits deliberately so the arithmetic can be followed.
- A downtime claim.
Mctis active maintenance time. What the operator experiences isMDT, which includes waiting for the part, the permit and the person, and on most fleets those exceed everything this analysis computes. - Whether the preventive tasks are worth doing. The prediction costs the scheduled work, and on the worked example that is three quarters of the labour, but whether each task earns its place is a reliability-centred maintenance question and not this one.
- The technician you will actually get. Every time assumes trained, equipped and unhurried. The demonstration is where that assumption meets a night shift.
- An availability improvement. Halving
Mcton a system with an 8,000-hour MTBF moves availability in the fifth decimal place. Maintainability prediction is a labour-and-logistics tool; when it is sold as an availability lever it disappoints precisely.