I've been through it personally.

I had an airplane until 2018 that optionally shipped with a rather unique air conditioning system. Rather than the compressor being engine powered like most other cars and small piston engined airplanes, there was a second alternator on the engine used to drive an electric motor which in turn powered the compressor.

This was all done mostly for weight distribution. The design of the airplane was somewhat nose heavy because the factory added a turbo system with a cast-iron manifold. So they moved the A/C system to the tail with the electric motor for power, and put it all on a shelf behind the baggage compartment in the back.

Since the airplane was built in the mid 1990s the factory is out of business, they went bankrupt in the early 2000s. The supplier of the electric A/C system components is still in business but they only deal with helicopters now, they don't have anyone left at the company who knows about the airplane units they used to make. Only about 50 of the make/model of airplane with the turbocharger option were built, so there's no motivation to keep them going for anyone else who might make parts. The market is too small.

I found an aftermarket A/C system very similar to the one the factory built in the 1990s and with the maker of that system went to the FAA to get approval for it. The aftermarket system is actually simpler electrically. Rather than two alternators with a relay system it uses one of the largest alternators you can buy, on all of the airplane's existing electrical circuit. You just have to beef up the battery cables and the circuit breaker on the alternator to handle the extra amperage. It took us 6 months and ultimately a suggestion that we have a particular engineering firm look at our plans. They were basically requiring us to re-approve the entire system, even though the system is approved in other airplanes with identical engines. All we needed approval for was a larger alternator (minimal change to weight/balance) and the repurposing of a material made by Boeing for the rear shelf (we told them we intended to use the floorboard material that Boeing puts in 737s, it's a fiberglass sheet reinforced internally with an aluminum honeycomb mesh, that can bear about twice the weight per square inch that we needed to put on it).

They made us have that engineering firm submit stress and load test data on the shelf material that Boeing already uses for the 737.

The only reason we jumped through all the hoops was the fact that the aftermarket A/C system's installer has a relative that was a 30 year Embraer engineer. He would just help us do the drawings and calculus for free as long as his son's maintenance shop was going to get the installation job. Absent that, the ~$25,000 job would've been a $50,000 job, which just isn't worth it for an airplane that's worth less than $175,000 to start with.

When we were willing to turn over the copyright of the plans to the engineering firm we were suggested to hire to "review" the data for the requested approval, including them copying and pasting Boeing's stress and load test data for the floor material Boeing already uses, suddenly the floodgates opened and everyone was happy. The paper was all signed and we got permission to put a marginally different air conditioner in one make and model of airplane (of which only 50 exist in the wild).

From initial sales call to cool air in my airplane: about 8 months.

The time to install it for two guys was about a week of shop time. After all, they put it in dozens of other makes/models and it's all the same stuff.

----------

If you want another example, you can go down the rabbit hole of a local (to me) company called Navworx, which was aggressively pursued and ultimately closed by the FAA, because the FAA changed the ADS-B spec rendering their ADS-B receiver worthless, after the spec had been in the field for about 3 or 4 years.

The ADS-B system has a "sleep mechanism" on the network, in which a data relay tower for a particular air traffic control sector will turn itself off if no ADS-B equipped airplanes are in its service area. The initial spec allowed a generic ID to report its presence in the sector and wake up the tower.

After Navworx had been making ADS-B receivers for old non-GPS equipped airplanes for about 3 - 4 years, the FAA pulled the rug out from under them by changing the spec and requiring any airplane requesting that the relay tower "wake up" have an FAA approved GPS unit installed. The initial spec only required an approved GPS for send + receive units, not receive-only units. After changing the spec with existing units already in the field, Dallas FSDO then promptly ordered Navworx to cease and desist manufacturing their previously approved ADS-B receive-only unit for older airplanes.

In effect, people who had a traffic display in an old airplane (an obvious safety benefit) were ordered to take their traffic display out and throw it away, and go back to flying without a traffic display unless they were willing to pay $10,000 for a GPS.

It just so happens that the Dallas FSDO was also the FAA office that approved Garmin's ADS-B boxes (which were technically inferior and more costly than Navworx's ADS-B boxes on initial release), and at the time Garmin was the only aftermarket supplier of approved GPS units. There was another maker of standalone ADS-B receivers with integral GPS units also in the Dallas area dealing with the Dallas FSDO, which was sold to a private equity fund that buys DoD / CIA / NSA contractors.

I leave it to you to ask yourself whether all these facts are relevant to who the FAA decided to close down and who the FAA decided to protect the market of...

-------

This business model of the FAA guaranteeing the work for legal and engineering consultants in pervasive across the entire spectrum of things they license.

Wanna buy a King Air 350 and charter it to dentists going to Aspen for the weekend when you're not flying it to lower the ownership cost? You have to hire an aviation specialized attorney and probably another consultant or two (read: someone who will sell you their paperwork) to represent you as the FAA goes through a manual review of your standard operating procedures (pilots, training, maintenance, the whole shebang) just like you were the first air charter service to ever buy a King Air 350 in the world.

It doesn't matter if you are literally copying and pasting another charter license approval for an identical airplane. Someone from the FAA office demands to physically show up at your hangar and look over the paper, pretend like he's never seen it all before, order a few changes, and then maybe approve or deny your request for a charter operator license after the second visit.