You have perhaps heard about an A1 rated Maintenance Organisation, or a B1 AMO or a C7 and eventually, even about a D1.

These are just some of the possible ratings that can be assigned to a Maintenance Organisation at the time of its approval.

Such ratings simply identify what the Organisation is approved for and while A, B and C refers to authorization to carry out maintenance on aircraft, engines and components respectively, the D rating is authorization to carry out and release in an EASA Form 1 “Non Destructive Tests” (NDT) only.

Let us delve in some more detail:

Category “A” class rating


Let’s start by saying once more again that a class rating “A” refers to the possibility (for the Organisation with such a rating), to carry out maintenance on aircraft.
So this is the class rating hold by many Organisations on the need to carry out maintenance at the airport’s tarmac for example (which is called “Line Maintenance”) or within Hangars (which is called “Base Maintenance”).

If any component (e.g. an engine, a landing gear a structural part), can be maintained whilst it is still fitted to the aircraft then, the A rated Organisation can also carry out maintenance on such component.

If the component needs instead to be removed in order to be maintained, then it should be sent to a specific shop (e.g to an Engine shop or to a component shop), unless it falls within the exceptions below described.

The manuals allowed to be used for a Category A Organisation are the Aircraft Maintenance Data only, like AMM (Aircraft Maintenance Manual), SRM (Structural Repair Manual) and Service Bulletins applicable to the aircraft, just to mention a few.

In some exceptional case, the competent authority may agree the use of a Component Maintenance Manual, in accordance with an agreed procedure which normally aims to ensure that the A-rated Organisation has the capability to carry out something supposed to be done at a specific shop.

The rating A1 refers to large aircraft (above 5700 Kg), A2 refers to aircraft equal and below 5700 kg, A3 refers to helicopters and A4 refers to aircraft others than A1, A2 and A3.

Category “B” class rating

(Engines & APUs)

The Category B Organisation is logically very similar to the one above described (Category A). Indeed, a category B class rating means that the approved maintenance organisation may carry out maintenance on the uninstalled engine and/or APU and engine and/or APU components, in accordance with engine and/or APU maintenance data (e.g. Engine Manual) or, if agreed by the competent authority, in accordance with component maintenance data, only whilst such components are fitted to the engine and/or APU (unless exception below described).

The rating B1 refers to turbine engines, B2 refers to piston engines and B3 refers to APU

Possible exceptions and peculiarities for the “A” and “B” classes

Besides the already mentioned possibility to use the CMM if agreed by the competent authority, there is also the possibility to temporarily remove a component (from the aircraft or from the engine/APU) in order to improve access to that component. Like for the use of the CMM, maintenance on removed components can only be done in accordance with a written procedure to be agreed with the competent authority.

Several considerations might be made by the competent authority for what concern the possible agreement related to such exceptions and we can here only make some speculation.

For example, we may already guess that some of the conditions that the Authority will check when considering to grant the use of the CMM for a component, is the absence of conditions that can only be met at the specific shop for that component.

While, about the possibility to maintain components off the aircraft or the engine, the Authority might check how the applicant of such a privilege will ensure the safety in terms of availability of expertise (trained personnel), specific tools, maintenance data and facility requirement.

Category “C” class ratings


A category C class rating means that the approved maintenance organisation may carry out maintenance on uninstalled components (excluding engines and APUs) intended for fitment to the aircraft or engine/APU.

Currently is possible to obtain 22 different ratings (C1, C2, C3, C4, C5. C6. C7 ….C22) depending applicable ATA chapter.

Just few examples:

C1: ATA 21 -> Air Cond & Press
C2: ATA22 -> Auto Flight

C7: ATA 49,71,72,73,74,75,76,77,78,79,80,81,82,83 -> Engine & APU (components)

C14 : ATA 32 -> Landing Gears

C20: ATA 53, 54, 57.10, 57.20, 57.30 -> Structural

Category “D” class rating

Non Destructive Testing (NDT)

At the moment, the D1 is the only available class rating and it is a self contained class rating not necessarily related to a specific aircraft, engine or other component.

Indeed it refers to the possibility to carry out and release (in an EASA Form 1) NDT tests on any component, structure or area.

Such a D1 — Non Destructive Testing (NDT) rating is only necessary for an approved maintenance organisation that carries out NDT as a particular task for another organisation. A maintenance organisation approved with a class rating in A or B or C category may carry out NDT on products it is maintaining subject to the maintenance organisation exposition containing NDT procedures, without the need for a D1 class rating.

Reference for this article is the Appendix IV to the EASA Part M — Class and Ratings System to be used for the Approval of Maintenance Organisations referred to in Annex I (Part-M) Subpart F and Annex II (Part-145) / Regulation EU 2015/1088