Available Time
Understanding Available Time
Available time is a concrete, measurable quantity: the number of hours (or minutes) a system was in an operational state during a defined period. Unlike availability percentage, which is a ratio, available time has units and can be directly compared with contractual requirements. A radar system required to provide 8,751 hours of operation per year (99.9% of 8,760 hours) can be verified by summing the logged operational hours and comparing against the requirement.
The definition of "operational" matters significantly. A system may be powered on but degraded (reduced detection range, lower data rate). Whether degraded operation counts as available time depends on the contract definition. Many specifications define "fully mission capable" (FMC) as the threshold: the system must meet all specified performance parameters. A radar with one redundant receiver offline might be "mission capable" but not "fully mission capable," and the hours would be counted differently in each case.
Available Time Formulas
Tavailable = Ttotal − Tdowntime
Downtime Components:
Tdowntime = TCM + TPM + Tadmin + Tsupply
CM = corrective maintenance (unplanned repairs)
PM = preventive maintenance (scheduled service)
admin = authorization, paperwork delays
supply = waiting for spare parts
Availability from Available Time:
A = Tavailable / Ttotal × 100%
Hours per Year:
8,760 hr (standard year), 8,766 hr (avg including leap)
Availability Types and Downtime Included
| Availability Type | Corrective | Preventive | Logistics | Administrative | Use Case |
|---|---|---|---|---|---|
| Inherent (Ai) | ✓ | ✗ | ✗ | ✗ | Design specification |
| Achieved (Aa) | ✓ | ✓ | ✗ | ✗ | Factory/lab testing |
| Operational (Ao) | ✓ | ✓ | ✓ | ✓ | Field deployment, contracts |
Frequently Asked Questions
How is available time calculated?
Available time = Total time − Downtime, where downtime includes corrective maintenance, preventive maintenance, administrative delays, and supply waits. Over 8,760 hours/year, 4.38 hours downtime yields 8,755.62 available hours (99.95%). Different availability definitions include/exclude different downtime categories: inherent excludes PM and logistics; operational includes everything.
What is the difference between inherent, achieved, and operational availability?
Inherent (Ai) counts only corrective maintenance (theoretical max). Achieved (Aa) adds preventive maintenance. Operational (Ao) adds logistics and admin delays (real-world). Ao is always the lowest and represents field performance. A system may have Ai = 99.99% but Ao = 99.9% due to parts delays and scheduled maintenance.
Why is available time important for RF system contracts?
Military and commercial contracts specify minimum available time with financial penalties for non-compliance. A radar contract requiring 99.9% Ao means 8,751.2 operational hours/year. The contractor must design the system, provide logistics, and plan maintenance to meet this target. Available time tracking provides the compliance data and identifies reliability improvement opportunities.