Availability is the property that will not stay inside the design office. This chapter takes eight systems, one from each industry the platform serves, and works the availability question on each of them end to end: which analysis technique the system actually calls for, why that one and not another, what the ledger counts as downtime, what the denominator should be when clock time is the wrong measure, the formulas, the arithmetic, and what the resulting number tells someone to buy, schedule or rewrite. The arithmetic is the same in all eight. Where the downtime comes from, and therefore which technique can see it, changes sharply.
The eight systems are the same in every concept topic of this row. The aircraft electrical system worked here for its dispatch and its logistics tail is the one worked in reliability for its failure rates, in maintainability for its repair times, in safety for its catastrophic failure condition and in testability for its built-in test coverage. Read a column and you learn one property across eight industries; read a row and you watch one machine reveal its five RAMS faces. Every page ends by handing you sideways to the same machine's next property. All numbers are illustrative: invented for teaching, chosen to be plausible and to make the arithmetic legible, and none of them is field data from any real product.
The technique is a choice, not a default
The single most useful thing this chapter does is refuse to apply one model everywhere. A = MTBF/(MTBF + MTTR) is right for a system with independent items and dedicated repair, and it is wrong the moment a spare has to travel, a queue forms, a function is dormant, or the product is measured in something other than hours. Each page names its technique and justifies it, then says what a different technique would have given and why it was rejected.
| Industry | System | Technique this system calls for |
|---|---|---|
| Defence and aerospace | Transport aircraft AC electrical power | Ai-to-Ao decomposition with a Monte Carlo sparing model for the outstation tail, plus λT/2 for the dormant RAT |
| Railway | Wayside level-crossing controller | Steady-state series RBD availability in unavailability currency, with the possession wait as administrative delay |
| Space systems | Satellite attitude control | Service-clock outage accounting on the imaging denominator, with no repair term to put in the formula |
| Automotive | Electric vehicle brake system | Vehicle-off-road event accounting on a user-demand denominator, with a first-visit-fix multiplier |
| Energy and resources | Gas compressor train | Production availability accounting, planned and unplanned ledgers multiplied, plus Markov for the pump pair |
| Medical devices | Infusion pump fleet | Discrete-event Monte Carlo of a queue, with the M/M/1 formula as the closed-form check |
| Electronics and high tech | Storage array controller pair | Markov model of the repairable pair, budgeted in minutes per year |
| Nuclear | Emergency diesel generator train | On-demand unavailability accounting: the three-line standby ledger and λT/2 |
What the eight rows have in common
Read down the column and the same shape appears eight times in eight disguises: the inherent number was the smaller and easier half of the answer, and the operational number was decided by something that is not a component. The aircraft's 8.7 missing hours were a spare in the wrong place; the crossing's seven hours per event were a track possession; the vehicle's 0.9 days were a distribution network; the pump fleet's twelve hours were a queue running at 94% of capacity. In three of the eight, active repair was under 7% of the event, and in the storage array it was zero by design, hot-swapped out of the arithmetic and replaced by detection and dispatch.
The other half of the pattern is bookkeeping, and it is where availability differs most from the properties either side of it. Every page had to choose a denominator before it could quote a number, and every page had to decide what counted as downtime: the compressor's turnaround is twice its unplanned loss, the array's 20-minute change window is four years of its budget, the satellite's answer moves by a factor of two on the definition of an imaging opportunity, and the diesel's surveillance test causes unavailability while removing six times more. Three of the eight carry a dormant function whose λT/2 unavailability never appears on any uptime record and is, in two cases, the least available thing in the system.
That is the argument for reading the other four columns. Availability is the only property in this book that is assembled rather than designed, and its inputs are made elsewhere: the failure rates in reliability, the restoration times in maintainability, the detection split that decides which failures are announced and which sit latent in testability, and, for every dormant protective function in this chapter, the demand-side reckoning in safety.