Approved Aircraft Modifications

When an aircraft is originally delivered from its manufacturer, it is in full compliance with its type certificate. This means that the configuration of the aircraft fully complies with the design specifications and the aircraft itself is airworthy in that respect. However, eventually, there will be a need to modify the plane for a variety of reasons. These could include simple things, like the change of sign and sticker language, to more complex modifications such as cabin reconfiguration or the installation of additional entertainment systems or avionics equipment. In this post we will take a look at how such modifications can be designed and approved in order for the aircraft to keep compliance with its type certificate and remain airworthy.

Approved Aircraft Modifications

Under EASA regulations it is, generally, not allowed to modify an aircraft without proper approvals. Parts can only be installed if they are in compliance with the IPC requirements, and technical procedures can only be followed if they come from approved maintenance manuals. In practice, this means that even what might appear as a very minor modification (change of seat covers, replacement of stickers, installation of a magazine holder) is illegal unless the operator is in possession of approved maintenance data. There are three most common ways to legalize all required aircraft modifications.

Service Bulletins issued by the Aircraft Manufacturer

Each aircraft manufacturer issues special maintenance instructions in the form of service bulletins. They are grouped into alert, mandatory and non-mandatory categories (although some manufacturers like to use more sophisticated categorizing systems). Mandatory service bulletins most often fix a design problem of the aircraft, and alert service bulletins can be expected to be required by an airworthiness directive shortly after they become published.

What we’re interested in for the modification discussion are service bulletins (SBs) which are not mandatory. Those are SBs which often describe the modification of an aircraft which is related rather to the operators convenience, cost savings, maintenance reduction, etc. A non-mandatory SB will not have a serious impact on the airworthiness of an aircraft.

The manufacturer is likely to issue (on their own) SBs related to improvements in the overall aircraft design. This could include component replacements (for example a new, improved fuel pump may appear on the market – normally an airline is not allowed to install such fuel pump unless there is an SB which permits the installation) or the change in some maintenance procedures for increased effectiveness.

However, the operator can also ask the manufacturer to create an issue a service bulletin for specific requirements of that operator. That service is of course not free, but it gives the operator the possibility to modify the aircraft to suit their needs. It is also the easiest way to get a modification approved as no one will ever question the authority of a manufacturer release service bulletin.

Modifications issued by an Approved Part-21 Design Organization

For specific airline requirements, such as changing the cabin layout of the aircraft, installing a different type of seats or even a different kind of carpet, the operator must apply to a Part 21 organization for an approved modification.

A Part 21 Design Organization (often referred to as DOA – Design Organization Approval) has the right to design the modification and issue appropriate airworthiness documentation. In particular, in most cases the airline will receive a service bulletin outlining how the modification has to be performed, a maintenance manual supplement to show how to perform maintenance on the newly installed equipment and an IPC (Illustrated Parts Catalogue) supplement which lists all the part numbers which can be used with the new aircraft configuration. The airline may also receive an ICA (Instructions for Continued Airworthiness) which is a document outlining required maintenance tasks which need to be performed after the modification is complete. Those requirements should be included in the approved maintenance program of the airline.

Based on the documents received from a Part 21 DOA, the airline may perform the modification. Full documentation of the mod must be present in the aircraft historical records to show compliance with airworthiness regulations. It has to be noted that after such a modification the original manufacturer documentation will not show the mod. Therefore it is crucial that the operator keeps track of all third party mods and ensures that all instructions and requirements are closely followed despite them not being included in the typical maintenance manuals.

Supplemental Type Certificates

For significant modifications, which alter the aircraft’s compliance with the original manufacturer’s type certificate (for example modifications of some avionic systems, like the TCAS) will require a supplement to the type certificate. This is called – you guessed it – the Supplemental Type Certificate or STC.

An modification requiring an STC is also designed by a Part 21 DOA. However, as much as the design organization has an authority approval to design and issue modification which do not alter the Type Certificate compliance, an STC needs to be separately approved by EASA itself. After approval, EASA has a full list of all approved STCs, which can be found on their website.

The actual application of the STC does not vary from a regular Part 21 mod. The operator will still receive all the maintenance manual and IPC supplements, instructions for continued airworthiness etc. However, the aircraft has now an STC embodied, which needs to be clear to the operator and all future operators of the aircraft. The STC becomes a significant part of the aircraft’s compliance with design requirements.

In-House Modifications

Airlines often struggle with the modification requirements, as they are pretty strict towards modifications which, on a common-sense basis, seem not to have any influence on the aircraft’s airworthiness. Those are really minor changes such as the installation of a magazine or newspaper holder in the passenger cabin. In fact, this is a modification which should be properly approved as it interferes with the aircrafts interior structure (holes need to be drilled into the galley wall and this may or may not have an influence on the strength of the wall as well as on its burning properties).

To overcome the Part 21 requirements airlines sometimes perform something they call an in-house mod. The approval for such mods is sometimes questionable, but the procedure for performing such mods should at least be contained in the airlines CAME and/or MOE if the operator is also approved as an MRO.

During lease transitions such “in-house” modifications are removed and the aircraft is being brought back to its original state. There is more to mod removal, but I will go through that in the section below.

Removing Aircraft Modifications

How easy is it to remove an aircraft modification? One may thing that it will be simple enough – just undo whatever you have done while modifying the aircraft. Unfortunately, this is not a legal and correct approach, as you will see in a minute.

An aircraft modification, regardless of how much it interferes with the aircraft’s actual structure, cannot be “just de-modified”. After a modification has been completed, the current (modified) status of the aircraft is now its approved status. It, once again, cannot be simply changed. What this means in practice is that a de-modification is really another modification even if it brings the aircraft back to its original state. In other words – the whole modification process has to be followed once again to change to aircraft back to the old configuration.

As much as some people have a problem with accepting that fact, please keep in mind that a modification may have required changes to the aircraft which cannot be removed. You cannot “un-drill” a hole, for example. Therefore, a new modification procedure is required to do something about such issues and, if nothing can be done and the aircraft still remains airworthy, to show that the airworthiness is not compromised.

Posted in Aircraft Maintenance Management, Aircraft Maintenance Regulations, Airworthiness, Airworthiness Management, Airworthiness of Aircraft, Airworthiness Regulations, Maintenance, Maintenance of Aircraft
17 comments on “Approved Aircraft Modifications
  1. Justyna says:

    Considering the breadth of topics covered here, you can clarify every complicated subject and these procedures and requirements become easily accomplishable.

  2. Mike says:

    Thank you, I’m glad I can be of help 🙂

  3. Craig Thomson says:

    Can I suggest you mention the possible need for a Flight Manual Supplement as part of a modification.

    • Mike says:

      Hi Craig, thank you for this! Of course, if the modification alters the way the aircraft is operated by the flight crew, an AFM supplement will be necessary. Which is another item that sometimes “gets lost” when the aircraft switches operators, and after several new AFM revisions have been issued by the OEM. Excellent point!

  4. John says:

    Hi Mike,

    Please can you explain a bit more on the in house modification approvals? Surely the operator / 145 performing them need some sort of an approval??
    Also any part 21 approved organisation designing a modification regardless of it being minor or major, do they need to inform the original equipment manufacture? (example airbus) or only the local national aviation authority or easa? Does the part 21 organisation get approval from the local naa or easa for each minor modification they produce even if it doesn’t alter its type certificate? I can see from your post any major mod STC will require something from easa!

    Hope my questions make sense!


    • Mike says:

      Hi John, thanks for your comment!

      For in house modifcations the operator does need an approval. This would typically be done by approving an MOE procedure to say what mods the operator can do, how they do it, who approves them, etc.

      For the second question – this is a grey area I think. The Part 21 does not need to inform the TC holder. But … the operator should make sure that the TC holder (like Airbus) knows what mods have been done to an aircraft. Having said that, a Part 21 should provide maintenance procedures for their mods, so the TC holder won’t have to.

      • Mike says:

        Sorry, I meant a CAME procedure, not MOE.

      • John says:

        Hi Mike can you categorically confirm the above is correct regarding part 21 design organizations not having to inform the TC holder? This is because I am on a part 66 course at the moment and the text is as follows in one of the books, which contradicts what you say, so I’m a little confused..
        “The Supplemental Type Certificate will not be issued unless the type certificate holder has been informed and has expressed no technical objections to the proposal and has agreed to collaborate with the STC holder in matters affecting continuing airworthiness.”


        • Mike says:

          Hi John.

          Despite in some situations it could be interesting having NTO (no technical objection) to your Part 21 STC, it is not a must.

          Example 1. Regardless replacing tourist seats by other manufacturer P/N is a major change, and therefore it involves an STC, you do NOT need any data from TC Holder. In this particular example, substantiations are really simple (aisle width, (E)TSO seats, …).

          Example 2. Installing a new wide cargo door requires a deep analysis of the aircraft structure, pressurisation loads, … You will probably like to receive a NTO from TC Holder, but it is not strictly necessary.

          I hope this answers your question.

  5. Vui Chon says:

    Hi, i would like to ask if an airline made change to its seating configuration, for example , reducing some business class and adding some eco seat, does this consider as major mod ? will this require a STC ?

  6. Hello Vui.
    I think you do not need an STC for cabin layout changes. All you need is a to rewieght the aircraft and change the LOPA as required. Of course you will need TSO compliant seats.

  7. Yuliia says:

    Hi Mike,
    Can you explain such example.
    If operator approved by Part-145, wants to make minor change (removing old carpets and installed new one with new materials). What approvals does they need? Additionaly operator applies for TC minor change approval, with airworthiness requirements for flammability 25.853 what procedures the operator should have done to get the approval from national authority? As Part-145 organization the operator have been already approved to install new carpets with new materials.


  8. Prakash says:

    I would like to know, whether change in aircraft seating layout say for example from 5 seat configuration to litter configuration, is considered as modification. It yes, whether it is a major modification of minor modification. What supporting documents will be required for compliance and obtaining approval from regulator.

    Please help.


  9. Alvi says:

    HI Mike
    Can you briefly explain the difference between Modifications issued by an Approved Part-21 Design Organization and STC. Isn’t both are same.You categorize them differently. Also, Does STC embodied means that next time when manufacturer issues MPD , it would include that mod. Or You have to fill up COC.
    Please reply

  10. Keith Anderson says:

    Hi Mike,
    Good explanation thanks. I have a biz jet, client requires new carpets and a seat re-upholstered, providing the material specs are the same as original (colour changing), and the burn certs are in order, then can we not write the EO and apply for the minor change?

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