# Rotable Pool Sizing Calculator: How Many Spares Do You Need?

> Source: https://netray.co/tools/rotable-pool-sizing-calculator
> Publisher: Netray (netray.co), AI and integration engineering for ERP and manufacturing systems.
> Section: Free Tools

## Short answer

Estimate rotable spares from fleet size, removal rate, repair loop time and service level using a Poisson safety margin.

## Category

aerospace-defense

## Industry

Aerospace MRO

## Audience

- Materials planner
- Fleet maintenance planner
- Pool manager
- Inventory analyst

## Tool

**Kind:** calculator

**Currency:** $

**Inputs:**

#### Aircraft in fleet

**Key:** aircraft

**Type:** number

**Default:** 20

**Min:** 1

**Step:** 1

**Unit:** aircraft

#### Quantity per aircraft

**Key:** qpa

**Type:** number

**Default:** 2

**Min:** 1

**Step:** 1

**Unit:** units

**Help:** Number of this part fitted to each aircraft.

#### Flight hours per aircraft per year

**Key:** fh

**Type:** number

**Default:** 3000

**Min:** 0

**Step:** 50

**Unit:** FH

#### Mean time between unscheduled removals

**Key:** mtbur

**Type:** number

**Default:** 12000

**Min:** 1

**Step:** 100

**Unit:** FH

**Help:** From your reliability data. Do not use the manufacturer MTBF.

#### Repair loop time

**Key:** tat

**Type:** slider

**Default:** 45

**Min:** 5

**Max:** 150

**Step:** 1

**Unit:** days

**Help:** Removal to serviceable back in stock: transit, repair, test and return.

#### Target service level

**Key:** service

**Type:** select

**Default:** 0.95

**Options:**

##### 90%

**Value:** 0.9

##### 95%

**Value:** 0.95

##### 98%

**Value:** 0.98

##### 99%

**Value:** 0.99

#### Unit value

**Key:** price

**Type:** number

**Default:** 85000

**Min:** 0

**Step:** 1000

**Unit:** $

#### Annual carrying cost rate

**Key:** carry

**Type:** number

**Default:** 15

**Min:** 0

**Step:** 1

**Unit:** %

**Help:** Capital, insurance, storage and obsolescence.

**Outputs:**

#### Expected removals per year

**Key:** removals

**Expr:** aircraft * qpa * fh / mtbur

**Format:** decimal

**Description:** Installed units multiplied by flight hours, divided by MTBUR.

#### Average units in the repair loop

**Key:** pipeline

**Expr:** removals * tat / 365

**Format:** decimal

**Description:** Mean demand during the repair loop.

#### Service level z score

**Key:** z

**Expr:** normsinv(service)

**Format:** decimal

#### Recommended pool size

**Key:** spares

**Expr:** ceil(pipeline + z * sqrt(pipeline))

**Format:** integer

**Primary:** true

**Description:** Pipeline plus a Poisson-style safety margin, rounded up.

#### Of which safety spares

**Key:** safety

**Expr:** spares - ceil(pipeline)

**Format:** integer

#### Pool investment

**Key:** pool_value

**Expr:** spares * price

**Format:** currency

**Primary:** true

#### Annual carrying cost

**Key:** carry_cost

**Expr:** pool_value * carry / 100

**Format:** currency

**Chart:**

#### Pool size vs repair loop time

**Type:** line

**X:** tat

**From:** 5

**To:** 150

**Steps:** 30

**Series:**

- spares

**Disclaimer:** 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.

## Content

**Intro:** 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.

**Sections:**

#### How to use the calculator

**Body:** 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

**Body:** 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.

**Bullets:**

- 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

**Body:** 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

**Body:** 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.

**Cta:** 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.

**Faq:**

#### What service level should a rotable pool target?

**Answer:** 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?

**Answer:** 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?

**Answer:** 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?

**Answer:** 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.

## Search terms

rotable pool sizing calculator, aircraft spare parts pool size, rotable spares calculation, mtbur spares calculator, component pool sizing, airline spares provisioning, repair loop time spares, rotable inventory optimization

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