The number of persons holding a maintenance licence has to be appropriate to the nature of the work carried out, A line engineering set up would need a high proportion of licenced staff but a company that did a lot of major repair work is likely to need fewer licenced staff who just cover the B1 release to service of the whole aircraft, the component parts of the aircaft being released by non licenced ( but EASA 145) approved staff within the scope of the company’s repair approval. In much the same way the avionincs B2 release is likely to come from a person contracted in and approved by the company.
The system is quite flexible and within reason only confined by the lack of foresight of the persons writing the company exposition.
My guess is that the OP’s question was really based around what exactly is needed to do it himself.
I know the FAA process, because I have designed a lot of my own stuff (example) and got it all approved properly.
On G-regs, one sees a lot of stuff done by freelance guys, and it isn’t clear what exact certification process is being followed. I know that basically anybody who can solder (actually anybody who can’t, too) can work using the approval of a company in whose hangar they are working, so presumably they are working under the “supervision” of an EASA66 person based there, and that person will sign the work. That process would be similar to the same freelancer working under A&P supervision on an N-reg.
And that “freelancer” could be the aircraft owner! I think that is the key.
But the obvious Q would be: how can the owner do it totally himself?
On an N-reg you would need to become an A&P. That is I believe possible wholly in Europe. Then you could do Minor mods. To do Major mods (337 etc) an A&P can do it but you need to dig out an IA to sign it off (potentially twice if doing a field approval). There are rumours that one can’t do an IA in Europe anymore – have to go to the USA. However IMHO there are plenty of A&Ps/IAs in Europe and it would be far less effort to build a relationship with one of those.
On a G-reg you would have to become an EASA66 and then likewise you could do Minor mods – but EASA needs to decide whether a mod is minor or not… so I don’t know how exactly that works. I also think an EASA 21 company can in some cases determine if a mod is Minor but maybe then they have to do the work, or generate a work pack (£££)? In practice a lot of smaller stuff is done off the books. But for sure you will enjoy the EASA66 electrical/electronics exams – the biggest pile of absolute crap I have ever come across.
Why would a company want to be responsible for someone who they don’t know, didn’t followed the basic courses etc?
I don’t think it works like that, not in UK as well.
But the obvious Q would be: how can the owner do it totally himself?
A typical aircraft owner who makes a living with something else than aircraft maintenance, that won’t be possible on a certified aircraft. Someone who doesn’t work in aircraft maintenance won’t be able to meet the minimum required experiance to apply for a license, not for A&P nor for Part 66.
Afterwards it would require a type rating under Part 66 for EASA, and sufficient experiance to keep current, which will be impossible when you don’t work in aircraft maintenance.
If you take the Part 21 route for designs, you sure will pay a lot more than having the work performed by a shop.
If you want to do all work on your aircraft yourself, you should buy an experimental aircraft (Even then you would require avionics and pitot static testing by certified avionics staff for Dutch experimental aircraft)
In practice a lot of smaller stuff is done off the books.
Why I don’t understand??? Nice to get in trouble during an ARC renewal.
it would be far less effort to build a relationship with one of those.
For sure it would be far less expensive to just use the service of any maintenance shop (for airframe, powerplant and avionics) and get a good relationship with them. They can help you with approvals, have all the required tools, nice hangar with heating, 2nd hand for cowling etc.
What is the exact requirement for a person to install N-reg avionics?
I understand the FAA Part 145 (N-reg) or EASA Part 145 (EASA-reg) company avionics shop angle. That is clear, although the company does need to have the aircraft type on its list of approved types, AIUI, and that is the case for any maintenance.
But it cannot be just that, because loads of avionics work is not done by any 145 company.
In Europe, one bumps into 3 kinds of freelance people:
I believe EASA has some sort of an “avionics approved installer qualification” but I have never heard of that for FAA where AFAIK that work can be done under A&P supervision, but most A&Ps cannot do wiring so does the wireman need any formal qualification?
Peter
In EASAland the avionic certification is done by a B2 licence holder with any mechanical aspects and the final aircraft release to service being covered by a B1 licence holder.
Depending on the aircraft status this my require B1/B2 licence holders issuing the certification under a 145 approval.
But basicly if you can get a guy with B1&B2 licences and an approved modification for the aircraft & kit being fitted he can fit the kit and release the aircraft to service provided EASA 145 maintenance is not required.
As for the FAA system, I dont know enough to comment.
I have never heard of that for FAA where AFAIK that work can be done under A&P supervision, but most A&Ps cannot do wiring so does the wireman need any formal qualification?
The avionics installer needs an A&P certificate only, and can supervise others in exactly the same way as for mechanical or structural work. There is no separate qualification for avionics and wiring. However, my observation is that (mostly) FAA A&Ps who do avionics do avionics only and have learned a lot through practical experience. An IA is required for Form 337s, again the same AFAIK as for other aircraft work.
The repair station (part 145) becomes involved when the avionics work includes a transponder (91.413) – or touches the static system (91.411). – (edit: actually, I think an A&P can do work on the static system too)
An A&P can do the work but you need an FAA 145 firm to re-test the static system after the work was done – AIUI.
I was not aware transponders can be installed only by a 145 company…
I could be wrong. Hopefully NCYankee will be along soon to answer definitively. 
An A&P can work on a static system and inspect it. See the text of the appendix quoted below and note that the tasks in section (a) do not specify they must be performed by a repair station, while section (b) requires the altimeter tests to be performed by an appropriately rated repair facility (AKA a repair station).
Appendix E—Altimeter System Test and Inspection
Each person performing the altimeter system tests and inspections required by Sec. 91.411 shall comply with the following:
(a) Static pressure system:
(1) Ensure freedom from entrapped moisture and restrictions.
(2) Determine that leakage is within the tolerances established in Sec. 23.1325 or Sec. 25.1325, whichever is applicable.
(3) Determine that the static port heater, if installed, is operative.
(4) Ensure that no alterations or deformations of the airframe surface have been made that would affect the relationship between air pressure in the static pressure system and true ambient static air pressure for any flight condition.
(b) Altimeter:
(1) Test by an appropriately rated repair facility in accordance with the following subparagraphs.
A transponder is an appliance. An A&P may not repair it, but if one is repaired with the proper documentation or a replacement unit is installed, they may sign off the work.
Sec. 65.85 Airframe rating; additional privileges.
(a) Except as provided in paragraph (b) of this section, a certificated mechanic with an airframe rating may approve and return to service an airframe, or any related part or appliance, after he has performed, supervised, or inspected its maintenance or alteration (excluding major repairs and major alterations). In addition, he may perform the 100-hour inspection required by part 91 of this chapter on an airframe, or any related part or appliance, and approve and return it to service.
Sec. 91.411 Altimeter system and altitude reporting equipment tests and inspections.
(a) No person may operate an airplane, or helicopter, in controlled airspace under IFR unless—
[(1) Within the preceding 24 calendar months, each static pressure system, each altimeter instrument, and each automatic pressure altitude
reporting system has been tested and inspected and found to comply with appendices E and F of part 43 of this chapter;
(2) Except for the use of system drain and alternate static pressure valves, following any opening and closing of the static pressure system, that system has been tested and inspected and found to comply with paragraph (a), appendix E, of part 43 of this chapter; and]
(3) Following installation or maintenance on the automatic pressure altitude reporting system of the ATC transponder where data correspondence error could be introduced, the integrated system has been tested, inspected, and found to comply with paragraph (c), appendix E, of part 43 of this chapter.
(b) The tests required by paragraph (a) of this section must be conducted by—
(1) The manufacturer of the airplane, or helicopter, on which the tests and inspections are to be performed;
(2) A certificated repair station properly equipped to perform those functions and holding—
(i) An instrument rating, Class I;
(ii) A limited instrument rating appropriate to the make and model of appliance to be tested;
(iii) A limited rating appropriate to the test to be performed;
(iv) An airframe rating appropriate to the airplane, or helicopter, to be tested; or
(v) deleted
(3) A certificated mechanic with an airframe rating (static pressure system tests and inspections only).
(c) Altimeter and altitude reporting equipment approved under Technical Standard Orders are considered to be tested and inspected as of the date of their manufacture.
(d) No person may operate an airplane, or helicopter, in controlled airspace under IFR at an altitude above the maximum altitude at which all altimeters and the automatic altitude reporting system of that airplane, or helicopter, have been tested.
Note that if a transponder is repaired or replaced, but the signal lines to the encoder and the encoder have not been mucked with, then correspondence error would not be introduced and the integrated test would not need to be performed. If the encoder was changed, then the integrated correspondence test would need to be performed by a repair station.
Sec. 91.413 ATC transponder tests and inspections.
(a) No persons may use an ATC transponder that is specified in 91.215(a), 121.345(c), or Sec. 135.143(c) of this chapter unless, within the preceding 24 calendar months, the ATC transponder has been tested and inspected and found to comply with appendix F of part 43 of this chapter; and
(b) Following any installation or maintenance on an ATC transponder where data correspondence error could be introduced, the integrated system has been tested, inspected, and found to comply with paragraph (c), appendix E, of part 43 of this chapter.
(c) The tests and inspections specified in this section must be conducted by—
(1) A certificated repair station properly equipped to perform those functions and holding—
(i) A radio rating, Class III;
(ii) A limited radio rating appropriate to the make and model transponder to be tested;
(iii) A limited rating appropriate to the test to be performed;
[(iv) deleted]
(2) A holder of a continuous airworthiness maintenance program as provided in part 121 or Sec. 135.411(a)(2) of this chapter; or
(3) The manufacturer of the aircraft on which the transponder to be tested is installed, if the transponder was installed by that manufacturer.
