Rotable Pool Sizing Calculator: How Many Spares Do You Need?
A rotable pool is the inventory of repairable parts that circulates between aircraft and repair shops. Too few spares and aircraft wait on the ground, too many and capital is tied up. This calculator gives a first estimate from fleet size, removal rate, repair loop time and service level.
Your numbers
Number of this part fitted to each aircraft.
From your reliability data. Do not use the manufacturer MTBF.
Removal to serviceable back in stock: transit, repair, test and return.
Capital, insurance, storage and obsolescence.
Your results
A simplified provisioning estimate that treats removals as a Poisson process. It ignores beyond-economic-repair scrap, no-fault-found returns, pooling across fleets and lead times to buy new units. Use dedicated provisioning software for contract commitments.
Pool size vs repair loop time
Sweeps Repair loop time from 5 to 150, holding your other inputs fixed.
Want pool sizing from your real removal data?
We can outline a read-only analysis of removals and repair loops from your ERP, run on your own servers.
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How to use the calculator
Enter fleet size, the quantity of the part on each aircraft, annual flight hours and the unscheduled removal rate from your own reliability data. Then enter the repair loop time, which is the full time from removal to a serviceable unit being back in stock, including transport and queueing. Pick a service level, which is the chance that a spare is available when a removal happens. Read the pool size and the investment, and use the chart to see how a shorter repair loop reduces the number of spares required. Repeat for each part number rather than averaging across a family.
The method
Expected removals per year equal installed units multiplied by flight hours divided by the mean time between unscheduled removals. The pipeline is the number of units that are in repair or transit on average, which equals removals per year multiplied by the loop time in years. Because removals are random, the pool adds a safety margin using a Poisson approximation: the pipeline plus the z score for the service level multiplied by the square root of the pipeline. This is a simplified version of the approach used in standard provisioning models.
- Removals = aircraft x quantity per aircraft x FH per year / MTBUR
- Pipeline = removals x loop days / 365
- Pool = ceil(pipeline + z x square root of pipeline)
Reading the result
The pipeline is the minimum: with fewer spares than the pipeline you will run out on average. The safety spares are the cost of the service level. Raising the service level from 95 to 99 percent often adds a spare or two on small pools, which can matter for an expensive part. The chart usually shows the biggest saving in shortening the loop time, so compare the cost of a faster repair contract with the cost of holding another unit. Also compare the carrying cost with the cost of an aircraft on the ground.
Common mistakes
The most common mistake is using the manufacturer's MTBF instead of your observed unscheduled removal rate, which can be much shorter or longer. The second is using the shop's repair time instead of the full loop, which also includes transit, customs and queueing before and after repair. Third, beyond-economic-repair scrap reduces the pool over time, so a pool needs replenishment. Finally, no-fault-found returns inflate removals without adding real failures. Use the calculator to compare scenarios rather than as an absolute answer.
Frequently Asked Questions
What service level should a rotable pool target?
It depends on how much an aircraft on the ground costs against the cost of holding a spare. Critical, no-go items often use 95 to 99 percent. Items with minimum equipment list relief can use less. Compare the cost of carrying an extra spare with the expected cost of delay.
Why use a Poisson model?
Removals are discrete, random events, and a Poisson model is a standard, simple way to describe them. It works well for low-volume parts. It is an approximation, and real provisioning tools model repair stations, pooling and scrap more fully.
What is repair loop time?
It is the full time from removing a unit to having a serviceable unit back in stock. It includes transport to the shop, the shop's turn time, return transport and any customs or waiting time. Use the whole loop, not just the repair time quoted by the shop.
Does this include scrap and no-fault-found rates?
No. This version assumes each removed unit returns to the pool. If part of your removals are scrapped, add the scrapped units to the pool over time. No-fault-found removals inflate demand, so a reliability program that reduces them can shrink the pool.
If you want removal rates and loop times pulled directly from your ERP and maintenance records, we can build a read-only reporting layer on your own servers. Netray works with SAP, Infor, Oracle and Dynamics.
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