Rotable Parts

What are rotable parts?

A rotable part (known as “a rotable,” a “serviceable part,” or a “replaceable unit”) is a part that wears over time but can be repeatedly restored to a functional condition. 

Rotable parts are commonly found in the aviation industry. Some examples include:

• Avionics units
• Landing gear (and components like actuators)
• Airframe
• Valves
• Fuel pumps
• Air filters

An original equipment manufacturer (OEM) will recommend that a part is removed from service after a specific amount of time.

After that, an approved workshop overhauls the part and returns it to its original design tolerances. 

Once the part has been “recertified”, the owner can reuse the part for another time period before the overhaul process is repeated.

Table of Contents

What industries use Levels of Service
Why go through the trouble of defining LoS?
Common criteria used to define Levels of Service
Defining Levels of Service in your business
Understanding LORA

How rotable parts are used

• Organizations often move rotable parts between different assets. Each rotable has a data plate with a unique serial number attached to it. This ensures asset traceability and captures the correct hours or cycles operated.

  Two primary attributes make rotable parts different from spare parts:

  • They can be repeatedly returned to a fully serviceable condition
  • A data plate and serial part number to uniquely identify the par

Which industries use rotable spare parts?

While rotable parts exist in many industries, it is easier to find them in safety-critical fields:

• Aviation & aerospace
• Nuclear power
• Rail
• Chemical
• Oil & gas

These industries operate under stringent quality systems to ensure the safety of workers and the public.

Best practices for using rotable parts

It’s crucial that safety-critical parts, like aircraft parts used on commercial aircrafts, have cradle-to-grave traceability.

This ensures:

• Operators don’t go over the approved time-in-service
• Owners know the location of each part at all times
• Overhauls are done only by approved workshops
• Components used for overhaul are of good quality
• Owners can track a rotable’s the status of the service
• In-service history, reliability, and other issues can be accurately tracked

With good record-keeping, those investigating an accident can identify who overhauled a rotable, the parts fitted, their source, and the test results post-overhaul. 

They can also interrogate its in-service history to identify any previous failures.

An example of a rotable component in action

A fuel pump on the reciprocating engine of a small single-engined aircraft is a great example of a rotable part. 

When new, this component will have an operating life of 2,400 hours. 

At 1,125 hours, the aircraft’s fuel pressure starts falling, so the maintenance personnel removes the pump and checks for a partially open bypass valve. 

The pump has a tag attached, which records:

• The date and time of removal
• The aircraft it was removed from
• Who removed it 
• The reason for the removal
• The number of hours the pump had operated
• The model, part, and serial numbers of the pump

A workshop will complete 3 tasks to put the part back in service:

1. Strip and repair the unit, replacing the bypass valve mechanism
2. Test the pump for operating pressure and record the results
3. Recertify the pump for continued use

The pump will return to stock with a serviceable tag showing 1,125 hours of use.

Maintenance technicians can now fit the pump onto another aircraft, with 1,275 hours to run until it must be removed and overhauled.

The maintenance planners will schedule the pump’s removal as it approaches 2,400 hours. Again, it will be tagged and sent for overhaul. 

This process can continue almost indefinitely, with some aircraft rotables built in the 1940s and 50s still operating today.

Managing rotable parts

Rotables are expensive assets that need rigorous management and control.  

Using CMMS to track rotable parts across their lifecycle

As the operating hours and cycles of the parent equipment are updated, CMMS software automatically updates the data for all attached rotables. 

Each rotable has its cycles or time life recorded by the CMMS, with alerts generated as the time for removal approaches. 

The CMMS will track the removed part through the maintenance store, the overhaul locations, and register its return to stock. 

The rotable’s life is reset in the CMMS, and the component awaits refitment for the cycle to begin again. The CMMS retains all maintenance, repair, and overhaul (MRO) data as a part of the rotables service history.

Participating in rotable pooling to reduce costs

Rotable pooling allows each participating business to optimize its inventory and lower its holding costs.

Rotable pooling occurs when a third-party organization, or several competing companies, sets up a centralized inventory of rotable components for use by multiple businesses. 

Each business pays a standing charge to belong to the rotable pool, and then pays for all costs, hours, or cycles associated with the component’s use — based on a contractually agreed formula. 

With each business facing the same issues, rotable pooling has become standard practice. 

Rotable FAQ

Here are 4 common questions maintenance professionals have about rotable parts.

How does a rotable differ from a repairable component?

Repairables are similar to rotables, with some even having serial numbers. Yet the repairable is less critical and less complex than the rotable. Once it wears out, technicians fit a new item, and the store sends the “core” to the supplier or OEM for a credit. 

The supplier or OEM will repair the removed item and place it back in stock for the next customer to purchase.

What’s the difference between rotable and consumable spares?

A consumable is a part, fitting, or liquid made for one-time use. 

A technician discards it once removed, with a new item fitted. Examples of consumables include seals, o-rings, oil, grease, split pins, lock wire, lubricants, and shims. 

Do rotable parts depreciate?

As a general rule, if you hold rotable parts for sale, they must be treated as inventory and are not depreciable. 

However, if the business acquires rotables to ensure the serviceability of internally-used equipment, they can be treated as depreciable assets.

Can rotables be used beyond their approved service life?

The operation of an item beyond its approved life breaches legislation and may expose the operator to legal sanctions. 

After reaching its mandated service life, the owner must overhaul the unit at a certified workshop before further use. 

In many industries, rotables are a safety-critical item, meaning their use is regulated by agencies like the FAA. 

There are exceptions where legislators can approve continued rotable use beyond the mandated life, known as “on-condition” use. This formal approval from the regulator will require component replacement once operation or wear reaches identified limits.