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.

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.