The maintenance of aircraft components is a story of its own, and in this introductory post, I will try to go through some of the basics and the general idea of what an aircraft component actually is and who can do maintenance work on it.
Let’s start with the second question – who can do maintenance on aircraft components? A Part 145 approved maintenance organization, with an A-class approval (for aircraft) can carry out maintenance on aircraft. However, an aircraft is – quite obviously – made up of hundreds or thousands of components. So how can deep can an aircraft maintenance organization go? It’s quite obvious that we can remove an engine from an aircraft. But can we remove a fuel pump from the engine? And if so, are we allowed to replace the driving shaft of that fuel pump, would this still be aircraft maintenance?
Maintaining components as part of the aircraft
The answer, as usual, lies within the aircraft technical documentation and relevant legislation, Part 145 in this case, for our EASA example. The legislation lists what maintenance manuals should be used with reference to aircraft maintenance, and those include mainly the AMM (aircraft maintenance manual) and IPC (illustrated parts catalogue). However, also the CMM (component maintenance manual) is listed as possible aircraft maintenance data. So what can we do?
In fact, this is actually quite simple. The AMM will tell you what you are allowed to do on an aircraft and hence, what technical actions are considered aircraft maintenance rather than component maintenance. It is possible to perform maintenance tasks in accordance with a CMM provided the AMM refers to it and tells you to perform those tasks. The regulation itself allows for component maintenance in accordance with a CMM under A category approval provided the component does not need to be removed from the airframe in order to undergo such maintenance unless it is being removed only for the purpose of obtaining better access. This may sound complex, but it really isn’t – the general idea is that you can do component maintenance under the aircraft category in two cases:
- When the AMM tells you to do so by providing appropriate instructions or by pointing to a relevant CMM
- When the maintenance of the component does not require the unit itself to be removed from the airframe except for the purpose of improving technical access. This means that you cannot perform any component maintenance which requires shelf tests or mechanical finishing, for instance.
Furthermore, the IPC which I mentioned earlier, gives you a list of all aircraft parts and components. This means that all the part numbers which are listed in the IPC can normally be replaced on the aircraft. In other words, if the IPC lists a component as one part number and does not list the specific pieces which make up a given component, it should generally be considered as a standalone unit, which needs to be maintained under a component maintenance approval.
So now we know what component maintenance is not. So how do we go about fixing the components which are being removed from the aircraft?
Maintaining components on their own
The ability to fix components requires a different kind of Part 145 approval – an approval within the C category. There are several of those categories, numbered C1, C2, C3 .. and so on and they are more or less structured by ATA chapters (so you get categories like hydraulics, or landing gear). This approval allows you to do something, that is not allowed when you maintain components in the A category as described above – it allows you to issue a component release certificate in the form of an EASA Form One or equivalent (like the FAA Form 8130).
The components release certificate is a document stating that the component you have just performed maintenance on is ready for release to service within the limits prescribed in relevant documentation or set forth by your limitations (workshop limitations). The EASA Form One is mandatory for operators (your customers) to be able to fit the component to their aircraft.
Typical component maintenance events
There are three main component maintenance events which are generally performed by specialized shops:
- Aircraft Component Overhaul
- Aircraft Component Repair
- Aircraft Component Modification
All of those have their own specifics and require a slightly different approach from both the operator / airworthiness provider and the workshop. Let’s look at them in slightly more detail.
The overhaul of aircraft components
Even though modern aircraft strive away from hard time maintenance, and condition monitoring is becoming the main approach for maintenance steering groups, many components still require regular overhauls once in a while.
An overhaul is sometimes called a major repair, although personally I don’t like this term as it is highly confusing. In most cases, when an overhaul is required, the operator will provide the workshop with a component which is generally serviceable except for the fact that it has reached it’s hard time limit. This means that there will be no defect syndromes to take care off and no defect description as such.
Overhauls are defined either in component maintenance manuals (CMM) or in specific overhaul manuals (OM) although the latter is generated mainly for major components such as engines, propellers or landing gear. Whatever its name, the technical documentation specifies exactly what needs to be accomplished during an overhaul of the given aircraft component. Such tasks generally include:
- Dismantling of the component to given detail
- Thorough detailed visual inspections and special inspections (such as NDT) of selected parts
- The discarding and replacement of parts which have life limits within the components or need to be replaced whenever the component is torn down (such as seals)
- Cleaning, fixing of known and detectable defects
- Thorough bench testing of the component
As a result, we get a component which is treated as new without additional limits imposed. What’s worth noting, however, is that especially for major components (like landing gear or engines) there are life limited parts within a components, and their life limit is often longer than the TBO (time between overhaul) for the component itself. As a result, we are allowed to place a “used” part into a component during overhaul. In order for this to be possible, the repair shop needs to obtain from the operator or airworthiness provider accurate and properly signed life limit details on the part – this is the workshops only source of information regarding the parts inside the overhauled component. Without this, they will not be able to release the component to service and hence will have to replace all parts with brand new ones, which is generally quite expensive and an unnecessary cost as such.
Aircraft component repair
The repair of aircraft components is slightly different from a complete overhaul, as the workshop needs only to determine what causes a given defect, rectify that defect, run some bench tests and release the component back to service.
The trouble shooting procedures are generally available in the CMM or the CRM (component repair manual). Therefore, in most common scenarios, the procedure is as follows:
- Check the documentation for a trouble shooting (T/S) procedure applicable to the given defect symptoms
- Carry out all tests in accordance with the procedure
- Determine the fault
- Eliminate the fault in accordance with an appropriate procedure from the CMM/CRM
- Do reassembly tests and release the unit
As simple as this may sound, there is something that every operator or airworthiness provider needs to understand and remember:
Whenever a component is torn down (“opened up”) in a workshop, it is the workshop responsibility to assess its general state (not only with respect to the reported defect, but as a whole). This means, that a simple defect may end up requiring even a complete overhaul if, after the components is opened, it turns out that several parts and subassemblies need replacing due to excessive wear and tear. This always increases costs and is very difficult to plan for because, for parts which are not hard time controlled, it is almost impossible to assess their condition without bench testing them at least.
Aircraft component modification
Quite often, components need to be modified. Sometimes the modification is intended to improve the usability of a given component and is voluntary, at other times a modification is required for instance during every overhaul.
Modifications can have several forms, from introductions of new, modified component sub-assemblies to the replacement of software in modern, electronic aircraft components.
A modification is rarely done on its own – most often it is performed during an overhaul or a repair (generally speaking – during a shop visit) of the component. It is either ordered by the operator or airworthiness provider if it is voluntary, or performed by the workshop automatically if it is mandatory.
The fact that a modification has been applied to a given component must be made clear on the release document (EASA Form One) and recorded in the operator’s airworthiness record system.
I hope that this brief post was somewhat helpful. If you have any ideas, questions or anything you would like to share – please leave a comment below or email me!